CHAPTER X.

THE WONDERFUL EFFECTS OF HEAT—(_concluded_).

The young philosopher had now investigated the effects produced by an elevation of temperature, not only upon the _bulk_, but upon the _form_ of different bodies. He had found, first, that heat increased the size of certain substances, without destroying the cohesion among their constituent particles; and, secondly, that it loosened the attraction between the atoms of other substances, and rendered them free to move: so that solids became converted by it into liquids, and liquids into vapours, while the heat which was absorbed and disappeared during the production of such changes he had ascertained not only to exist between the molecules of the resulting liquid or vapour in a _latent_ or insensible state, but to be again evolved in a sensible form when the vapours became condensed or the liquids solidified.

The next step, therefore, was to study the circumstances regulating the _ignition_ and _combustion_ of bodies.

That there is an intimate connexion between the principles of light and heat, Humphry had little doubt. Indeed, it was plain to him that the two are mutually disposed to produce each other. He had, however, as yet considered only the laws of heat, divested of luminosity; but, at present, he was about to examine the one in connexion with the other; the laws of ignition and combustion being those of the production of artificial heat, accompanied with light _for the time being_.

Whether substances, when _merely warm_, are capable of emitting rays of light, it is impossible to determine; “but,” said the lad to himself, “the slightest increase of temperature is perhaps accompanied with some kind of luminous power that our sense of vision is incapable of perceiving, since it is only when the temperature of bodies is raised to a high point that they acquire the property of becoming luminous to our eyes.”

It is extremely difficult to ascertain the precise temperature at which bodies, when heated, acquire the property of giving out light; for the result is greatly modified, not only by the sensitiveness of the eye of the observer, but also by the clearness of the atmosphere at the time of making the experiment.

The amount of heat necessary for producing luminosity, however, certainly exceeds 650°, since this is the temperature at which quicksilver boils; and though Humphry heated the metallic fluid to ebullition in a dark room, it did not become, so far as he could detect, in the least degree luminous.

Subsequent experiments, however, induced him to place the degree at which heated bodies begin to emit light in _the dark_ at 810°; though the investigations which have since been made in connexion with the subject lead to the conclusion that the first gleam of light which is given out from a heated platinum wire occurs at a temperature of about 865°. The luminous rays emitted at this heat, however, are not red, but of a _lavender-grey_ colour (similar to those which exist in the solar spectrum beyond the violet band), and seem to be the first transition from darkness to ordinary light.

At the temperature of about 1000° the light emitted by the heated body becomes _visible in daylight_, and is then of a dull-red hue.

At 1200° the tint of incandescence brightens into a vivid crimson, or “cherry-red,” as it is termed.

Then, as the temperature increases, the light emitted by the glowing body assumes partly a _yellow_ colour; so that at 1700° an “_orange_ heat,” as it is called, is produced.

At length, however, when the heat rises to the highest point, the light emitted acquires such brilliancy as to be painful to the eye; the incandescent substances then appearing no longer tinted, but positively colourless in the fire. This constitutes what is denominated a “_white_ heat,” and occurs at no less a temperature than 3000°.[37]

At this intense temperature a remarkable change is found to occur in the character of the heat itself, for it has been before shown that the heat-rays emanating from an ordinary fire are stopped by glass; so that while the light emitted by the burning coals passes freely through plates of glass, and is capable of being reflected by glass mirrors, like the light of the sun itself, the _heat_ radiated by them—_unlike that of the solar beams_—has neither the power to traverse the transparent substance, nor is it susceptible of being concentrated into a focus by reflexion from a glassy surface.

Artificial heat, however, _when at a very high temperature_, is found to have all the properties of solar heat. Not only does it then admit of being focussed by burning-glasses in the same manner as the sunbeams, but the light emitted by it darkens solutions of silver as effectually as the light of day; so that (as more recent experiments have proved) a photographic portrait can be taken as well by the rays from coke at a white heat, as they can by the rays of the sun itself.

* * * * *

But only those substances are capable of being rendered incandescent, which have power to sustain the high temperatures requisite for ignition, without being vaporized or decomposed by the heat. Many bodies, however, are either dissipated or destroyed long before they attain this intense temperature; while, on the other hand, those termed combustibles, when heated in the air, _burst into flame_, and undergo what is termed _combustion_.

“Now what _is_ combustion?” said Humphry to himself, as he thought over the subject. “What are the phenomena which occur when substances burn with the evolution of flame?”

The boy knew, that formerly it was supposed bodies owed their combustibility to the presence of a certain principle called “phlogiston,” which during combustion, said the philosophers, escaped from them, producing light and heat; whereas when the bodies had lost their phlogiston—and had become “dephlogisticated,” as it was termed—they ceased to be combustible.

Phlogiston, however, Humphry was well aware, was a purely imaginary principle, of whose existence no proof had been given, and which had been invented merely to explain a process that appeared to be otherwise incomprehensible.

Moreover, Humphry had learnt from the books he had already read upon the subject, that the metals were increased in weight after being burnt; so that it was impossible to attribute the combustion in such cases to the escape of phlogiston, since it was inconceivable how a body could be rendered _heavier_ by _losing_ something which had previously been combined with it.

Nevertheless, the belief in this visionary phlogiston had continued for nearly half a century; and it was only in the year 1775 that more correct views had been propounded concerning the process.

At the time of young Davy’s commencing the study of this subject, Lavoisier’s new theory of combustion had been in existence but a few years, and the boy having obtained from Mr. Tonkin the loan of the treatise in which the more correct views were originally propounded, had eagerly perused the volume, being not a little delighted with the precision of reasoning and the boldness of speculation contained in it.

Still Humphry was not satisfied with merely reading and acquiring the ideas of others. He criticised the theoretical speculations of the great French philosopher, doubted, and rejected, and advanced speculations of his own, while speculation led him to experiment.[38]

Humphry began the investigation of the phenomena of combustion by an experiment, to prove that the air in which combustibles are suffered to burn till they are extinguished undergoes a very remarkable change.

For this purpose the lad put a little water in a soup-plate, and on it he placed a small piece of candle, so that it might swim on the surface. Having lighted the wick he covered it over with a large tumbler, and found that the candle then burnt only for a short time, whilst immediately the flame was extinguished the water rose in the tumbler considerably above its level in the soup-plate.

Hence it was evident, that the portion of the air which was necessary for combustion had been removed by the burning candle from the atmosphere confined within the tumbler, and that, therefore, it was no longer capable of sustaining the flame.

“But maybe,” thought Humphry, “the candle, in burning, gives off some gas, which is prejudicial to combustion.”

So, to satisfy himself whether such were the case or not, the boy burnt some charcoal in an old iron saucepan, that he had previously drilled full of holes, in order to admit the air. Then, having fitted a tin tube into the lid of this, he, by means of the chimney so formed, conducted the gas evolved by the burning charcoal into a wide-mouthed bottle, that he had previously filled and placed with its mouth downwards, on a perforated stand in a pail of cold water; so that as the combustion went on the gas produced kept bubbling up in the pail from the end of the tube, and displacing the water as it rose into the inverted bottle that stood immediately above it. The arrangement, however, will be more readily comprehended by reference to the subjoined engraving:

[Illustration]

As soon as sufficient gas had been collected Humphry removed the tube from the pail, and corked the bottle under water; then having set the bottle of gas on a table, he attached a piece of candle to the crooked end of a long wire, and lowering this, while alight, into the gas, found, to his astonishment, that the flame was immediately extinguished.

“So then,” cried the delighted boy, “here is a kind of air that I can neither see, nor feel, nor smell, and yet it extinguishes burning bodies like water.”

But Humphry was too eager to examine the properties of the gas he had collected to wait to reflect upon the curious results it afforded him. Accordingly he procured a tall glass jar, and having placed a piece of burning candle at the bottom of this, he proceeded to empty the gas from the bottle into the jar, when to his surprise he discovered that he could pour out the heavy air that had come from the burning charcoal as though it had been a liquid, while, immediately it fell upon the lighted candle at the bottom of the jar, the flame disappeared as suddenly as if so much water had been showered upon it.

After this the boy amused himself by decanting the gas backwards and forwards from one vessel to the other, and ultimately found that it was instantaneously fatal to animals, destroying sentient life as rapidly as it extinguished burning substances.

Humphry’s next step was to discover what substance was capable of readily absorbing this gas, and after many trials he found that lime-water did so with great facility.

Accordingly he added about an ounce of quick-lime to a quart of water in a glass bottle, and corking it up closely he shook it several times, so as to dissolve as much of the lime as possible; after which he allowed it to settle, and then decanted off the transparent and colourless liquid into a clean bottle with a glass stopper. This transparent solution of lime in water he then poured into the glass jar containing the gas from the burning charcoal, and having corked the vessel tightly up, he shook it about, and immediately perceived that the lime-water was rendered turbid by the gas, being no longer clear and transparent as before, but changed to an opaque milky white; then having filtered the turbid water, and so separated from it all the white particles that had rendered the solution opaque, he dried and weighed the sediment, and found that the quantity of lime which had been dissolved by the water had become nearly doubled in weight by the gas which it had absorbed.

The youth had now learned how to remove the products of combustion, and he was consequently in a position to determine whether the air, after a substance had been burned in it, really had or had not been deprived of anything during the process.

Humphry therefore placed a small quantity of lime-water at the bottom of a wide-mouthed bottle, and through the cork of this he passed one end of a long wire, while to the other end of it he attached a small piece of wax taper. This he lighted, and then lowered down into the air that stood above the lime-water in the vessel. The cork was now forced tightly into the mouth of the bottle, and in a minute or two the taper was extinguished. After this the jar was shaken well up, when the youth beheld, to his great delight, the lime-water rendered turbid by the gas evolved during the burning of the taper.

The next step was to discover whether the air which remained in the bottle (and from which the products of the burning taper had been removed by the lime-water) was still capable of sustaining combustion.

Accordingly another lighted taper was lowered into it, but this was as rapidly extinguished as the one had been by the gas from the burning charcoal itself. It was afterwards found, too, that that part of the air which remained after combustion was as destructive of animal life as even the charcoal gas had been discovered to be.

“How wonderful!” exclaimed the boy, “that the atmosphere round about us should be made up of two different kinds of air—one that enables combustibles to burn and animals to live in it, while the other immediately extinguishes flame and destroys sentient life! How can I collect that portion of the air which supports combustion and maintains life, _apart_ from that which puts an end to it? I should like to see what it would do by itself, and whether substances would burn brighter in it alone; for surely such must be the case, since in the atmosphere it is mixed with another kind of air that extinguishes flame and destroys living creatures, so that the one must constantly be counteracting the effects of the other.”

Humphry racked his brains for a long time for the means whereby to separate the two kinds of air from each other. At last he remembered one of Lavoisier’s experiments in connexion with the subject, and immediately set to work to repeat it.

With this view the lad obtained some “calcined mercury,” for this substance he knew to have been produced merely by burning metallic mercury for a long time in a tube exposed to the air, so that the portion of the atmosphere which supported combustion (instead of being evolved in a gaseous form, as in the case of the burning charcoal) had become _fixed, or rendered solid_, in the “_calx_” which resulted from the process. The boy was therefore anxious to see whether it were not possible, by burning the calcined product once more, to drive off that portion of air which had been taken up by it during the previous burning, and so to discover what are the peculiar and distinctive properties of the air which had been absorbed. Consequently, he submitted some of this calcined mercury to a red heat in a retort, and collected the gas that was given off from it in a wide-mouthed bottle from which he had cut off the bottom. This, having corked, he filled with water, and stood on a perforated ledge in a pail—the gas being collected as before described. When the water had all been displaced from the bottle, and it was consequently full of gas, Humphry slid it, while under the water, off the ledge into a soup-plate, and then, removing it to a table, proceeded to investigate its properties.

Here, then, he had a jar-ful of the gas (named _oxygen_ by chemists) that maintained the combustion of bodies in the open air, and separate, too, from the other gas, which tended rather to retard their burning in the atmosphere.

Humphry’s first experiment was to introduce into the gas thus obtained a lighted taper, placed at the end of the wire as before, and the boy was enraptured as he beheld the flame immediately enlarge (instead of diminishing, as when confined in a jar of mere atmospheric air), and become intensely bright, while the combustion proceeded at so rapid a rate that the piece of taper itself was soon consumed. Then another piece of taper was used, but this was blown out immediately after being lighted, so that the wick was merely glowing on its introduction into the gas. On being plunged into the jar, however, it was instantly rekindled, and burst into the same vivid flame as before.

Next, the combustion of _sulphur_ was tried in the gas. This substance burns in the open air, as is well known, with a small blue flame. On placing a small piece of lighted sulphur, however, in a copper capsule attached to the end of a long wire, it was no sooner lowered into the jar than it began to burn with a beautiful purple or lilac-coloured light, the flame becoming suddenly enlarged, and the sulphur itself appearing to dissolve in the gas. At the conclusion of the experiment the water in the soup-plate, in which the jar stood, was set carefully on one side, for after-examination.

After this the lad tried the combustion of _phosphorus_ in another jar of gas, in the same manner. Humphry knew the combustion of this to be very vivid, even when inflamed in the atmosphere; so, to prevent accidents, he used in the jar a piece not larger than a pea: but even this, when lowered alight into the gas, produced so intensely white a flame that he could scarcely bear to look at it, while clouds of white flaky matter were evolved from it like smoke; the heat, too, was so great, that he was afraid the jar would crack: and so it would have done, had he not, luckily, employed a very large one.

The young experimentalist was overjoyed at the splendour of the combustion of these substances, and longed to see whether it were possible to burn the _metals_ by such means. So, having made another jar-ful of the same gas, and placed it over some water in a soup-plate, he took a piece of watch-spring, and when he had affixed the sulphur tip of a match to the end of this, he lighted the match and plunged the whole into the gas. He was soon well repaid for his pains; for in a short while the metal burst into vivid combustion, throwing off a shower of the most brilliant sparks, which played around it like a fountain of fire, whilst goutes of the white-hot metal fell hissing through the water, and lay beneath it for some time, red hot upon the plate, the glaze of which was afterwards found to have been even fused at the points where the molten iron had fallen upon it.

But the boy’s rapture on beholding the wonder of combustible iron was not altogether unmingled with fear; for the heat produced by the burning of the metal was so intense, that he grew nervous lest the glass jar should break during the experiment. He was wise enough, however, to hold it in his hand, so as to allow a little of the gas to escape, as well as to prevent the jarring of the glass on the plate beneath.

Humphry was now nearly exhausted with his labours, and it was time to reflect upon all that had occurred.

In the first place, then, it was certain that a considerable quantity of the gas used in these experiments had disappeared during the combustion, for the water had each time risen in the jar above the level of that in the soup-plate. “What, then, had become of the lost gas?” he asked himself. There was but one answer—_It had combined with the burning body, and formed a new substance with it_. In the case of the burning sulphur, the water that had been in the soup-plate below the jar was found, on examination, to be sour to the taste, and to redden vegetable-blue colours; so that here the gas had combined with the combustible and produced an _acid_ that was soluble in water. Again, with the burning phosphorus, the white flakes that had been evolved during the combustion had been ultimately dissolved by the water, which likewise tasted sour, while it stained vegetable colours in the same manner as the sulphur product did; whereas, in the case of the burning iron, the metal appeared to have been rusted, for the particles remaining at the bottom of the soup-plate were found, after the experiment, to have lost their metallic nature, and to have assumed all the character of a “_calx_,” or _rust_.

“Well, then,” said the lad, “it seems that during combustion one part of the air combines with the burning bodies, and so either _rusts_ or _acidifies_ them.” In confirmation of this view, he recollected “that the gas evolved from the burning charcoal also gave a slightly sour taste to the water it passed through.”

“Still,” mused Humphry, “if a part of the air really _does_ combine with the combustible burning in it, the result, of course, should be, that the combustible, after being burnt, should be _heavier_ than before—even as the lime with which I absorbed the gas from the burning charcoal became greatly increased in weight by it.”

The youth was not long in putting this part of the matter to a practical test. Having accurately weighed a small quantity of calcined mercury (which, as we said before, he knew to be a rust of the metal), he set to work again to make it red hot, and to drive off the air with which it had previously been made to combine while burning. This gas he collected in a small glass jar, open at the bottom, and having a stop-cock at the top of it. Then the boy took a thin hollow ball of glass, which had also a stop-cock fitted to it. Having screwed this on to the metal plate of his air-pump, he exhausted the glass ball as entirely of air as he could, and then closing the cock he detached it from the pump, and proceeded to ascertain the weight of the ball now that it was divested of air. This done, Humphry screwed the stop-cock of the glass ball on to that of the glass jar in which he had collected the gas from the calcined mercury; then, turning on both the cocks, the gas rose from the jar into the ball, and when the jar itself was full of liquid, and all the gas had consequently been removed from it, he closed the stop-cock once more, and, unscrewing the glass ball from the jar, proceeded to ascertain how much the ball had gained in weight, now that it contained the whole of the gas evolved from the calcined mercury.

The next step was to weigh the mercury itself. This, however, was no longer the red powder that it was before the gas had been driven off from it, but had now become “reduced” into so much bright liquid metal; and on being put into the scales it was found to have lost just as many grains in weight as the gas, which had been collected from it, required to balance it in the scales.

But this was not enough to satisfy the cautious young experimentalist, for he still desired to see whether, if the same quantity of metallic mercury were burned in the same quantity of gas, the resulting compound of the metal and the air would weigh exactly as much as the air and the metal did separately.

Accordingly, Humphry proceeded now to burn the metallic mercury in the gas, and so to cause them to combine once more. By keeping the metal at a red heat with the gas above it, the combination was at length effected, and then, on weighing the red “calx,” or rust, that resulted from the process, it was ascertained to be precisely as heavy as the metal and the gas had weighed when separate.

Here, then, it was manifest that substances by _burning were increased in weight, and that they were just as much heavier after combustion as the weight of the quantity of air which had been absorbed by them during the process._

“Is it true, therefore,” mused the boy, “that the candle and the coals, which appear to us to be destroyed by combustion, become positively increased in weight by it?”

The experiment which Humphry had already performed in collecting the gas from burning charcoal assured him that such was positively the case, for he knew that this gas, though invisible, had an absolute weight, being so much heavier than the atmosphere that it admitted of being poured, like water, from one vessel to another. The experiment with the calcined mercury, moreover, told him, that if he had weighed the charcoal before it was burnt, as well as the quantity of air which it had consumed while burning, he would have found the whole of the gas which resulted from the combustion would have been precisely as heavy as the air and the charcoal added together.

For the same reason, if the gases evolved from a burning candle were to be collected, they, likewise, would be found to be heavier than the candle itself; and just as much heavier, too, as the quantity of air which had disappeared during the combustion.

_Combustion, therefore, was merely the rapid combination of a portion of the air with a combustible body, accompanied with the evolution of heat and light._

* * * * *

Still Humphry could not quit the subject without examining the conditions which were necessary to produce such a combination. _The principal requisite was manifestly elevation of temperature._

Some substances, however, inflame at ordinary temperatures—immediately on entering the atmosphere—as, for instance, the gas called “_phosphuretted hydrogen_.” This was a new discovery in young Humphry’s time, and the boy delighted to produce the gas by heating a small quantity of phosphorus in a retort completely filled with a moderately strong solution of caustic potash—the heat being carefully applied until the solution boiled, while the beak of the retort was kept under the shelf of a water-bath. Upon coming into contact with the air, Humphry saw the bubbles of gas, as they left the surface of the water, suddenly inflame, with a slight explosion; and as the atmosphere was still, each bubble, on bursting, produced a beautiful expanding ring of white smoke.

It is this gas which gives rise to the production of those lights in the air which are known by the names of “_ignes fatui_” (“_will-o’-the-wisps_,” or “_Jack-o’-lanterns_,”) and “_corpse-candles_”—the former appearing over marshes, and the latter being seen to rise from recent graves—but both alike proceeding from the decomposition of organic matter.[39]

Again, _phosphorus dissolved in sulphuret of carbon_ produces a spontaneously inflammable solution; so that if a small quantity of the liquid be poured on a piece of paper it evaporates rapidly, and leaves the phosphorus behind, which immediately bursts into flame.

The same phenomenon of spontaneous combustion also occurs with the substance called “_pyrophorus_.” This is generally formed of _powdered alum_ heated with an equal weight of brown sugar or honey. After the materials have been melted and well mixed in an iron ladle, they are made red hot in a phial coated with clay, and placed in a crucible of sand—the heating process being continued until a blue flame appears at the mouth of the bottle; this is allowed to burn for about five minutes, when the phial is well stopped and removed from the fire.

The compound, on being cooled and exposed to the air, is spontaneously combustible.

_Sulphate of potassa_, likewise, when heated to redness with half its weight of lamp-black, forms a compound, which takes fire immediately on exposure to air.

Again, _tartrate of lead_, heated to a dull red in a glass tube, forms, when cool, a very perfect pyrophorus, which immediately inflames on being shaken out into the atmosphere. Further, when _iron is in a state of extreme mechanical division_—such as very fine powder—its affinity for the oxygen of the atmosphere is such that it heats, and even ignites, on exposure to the air. This is the case with the finely-divided metal as obtained by the action of hydrogen gas upon red-hot iron-rust, so that, when suffered to cool in this gas, the iron is as spontaneously oxidizable as even _potassium itself_.

Moreover, if a small piece of _spongy platinum_ be held in a jet of hydrogen, issuing from a small tube into the atmosphere, the platinum immediately becomes red hot, while the gas itself bursts into flame.

_Platinum wire_, or _foil_, if the surface be perfectly clean, acts so rapidly at common temperatures on a mixture of oxygen and hydrogen gases (mixed in the proportion of 1 to 2), that it often becomes red hot on being introduced into a vessel containing them, and kindles the mixture. Handling the platinum, however, wiping it with a towel, or exposing it to the atmosphere for a few days, suffices to soil the surface of the metal, and so to prevent its action.

Finally, a piece of the metal called _potassium_ (procured from _potash_) has so strong an affinity for oxygen, that when thrown upon water, at ordinary temperatures, the metal decomposes it the instant it touches the liquid, and so much heat is disengaged that the potassium is inflamed, and burns vividly while swimming on the surface. The same spontaneous combustion ensues, indeed, with _ice_—so that the cold body appears to heat the metal even to inflammation.

But a still more curious instance of spontaneous inflammation is to be found in the sudden explosion of a mixture of _Chlorine and Hydrogen gases when exposed to sunshine_; for though the two gases, when mixed together in equal volumes, may be preserved without change in a dark place for any length of time, nevertheless, immediately they are submitted to the direct solar rays, the whole mixture becomes suddenly inflamed, and a violent explosion ensues.

Next to those substances which are spontaneously combustible comes _phosphorus_, which inflames, when perfectly dry, at the low temperature of 60°. Indeed, such is its tendency to combine with the air, that, if free from all moisture, it takes fire by the heat of the hand alone. Slight friction, as when rubbed upon a piece of coarse paper, also produces the same result. It is very difficult, however, to light a piece of paper by the flame of phosphorus, for the paper becomes coated with a crust of the solid _phosphorous acid_, which is produced by the combustion, and serves to protect it from the flame.

There is, likewise, a gas (for the knowledge of which we are indebted to the after-discoveries of Davy himself), called _protoxide of chlorine_, which requires so slight an elevation of temperature to decompose it, that even the heat of the hand is sufficient to cause it to explode with the evolution of heat and light. This gas is produced by the action of hydrochloric acid on chlorate of potash and water, and it is so explosive that it frequently detonates violently in being transferred from one vessel to another. It should, therefore, be dealt with by none but experienced chemists. A small piece of phosphorus let up into it instantly takes fire, and burns with much brilliancy. Sulphur likewise decomposes it with violent detonation, and even a piece of blotting-paper introduced into the gas is sufficient to cause it to be suddenly resolved into its elements.

The gas termed _Binoxide of Chlorine_ (called also the _Peroxide_) is even more explosive than the Protoxide. It detonates violently when heated to 212°, emits a strong light, and undergoes a greater expansion than the simple oxide above described.

Again, the _Binoxide of Hydrogen_ (or _Peroxide_, as it is sometimes denominated), when heated to 212°, gives off oxygen so rapidly as to cause an explosion, while the rusts (oxides) of some of the metals act upon it with such energy, that, when dropped into it, a violent detonation immediately ensues, and the glass tube on which the experiment is conducted becomes red hot.

Further, the gas called _Binoxide of Nitrogen_, when combined with _sulphurous acid_ gas, produced a compound called _Nitro-sulphuric Acid_, which is so prone to decomposition that it cannot be collected in a separate state, and the salts of which are held together with such slight affinity that even a little charcoal powder, or spongy platinum, is sufficient to cause a violent evolution of gas, while at a temperature only a few degrees above that of boiling water, an explosion ensues.

Moreover, the _Bisulphuret_ (called, also, the _Persulphuret_) _of Hydrogen_, which is a yellow oil-like liquid, has its elements so feebly united, that at a heat short of 212° it is instantaneously resolved into _sulphur_, and the simple _Sulphuretted Hydrogen_ which is evolved in the form of gas, with almost explosive violence. The same effect is produced by the mere contact of most substances—especially the metals, flint, and even the earths in powder—while the oxides of gold and silver are “reduced” by it with such energy that they are rendered instantaneously red hot.

These binary compounds of oxygen or sulphur have most of them been discovered since Davy’s time. They are, however, remarkable in possessing kindred affinities, and being severally decomposible at a temperature of 212°.

After these, in the order of ready decomposibility, come the compounds of _Nitrogen_.

The peculiar black powder, called by chemists _iodide of nitrogen_, which is produced by pouring some strong ammonia upon a very small quantity of iodine, is so explosive, that it detonates violently as soon as it is dried; and the slightest pressure, even when moist, produces a similar effect. If put into pure ammonia, it explodes when lightly pressed in that liquid. Heat and light are emitted during the detonation, which is merely a species of instantaneous combustion. So dangerous is this compound, that the most experienced chemists seldom operate on more than a few grains of iodine at once.

The yellow oil-like liquid, called _chloride of nitrogen_, which results from the action of chlorine gas upon sal ammoniac, also enters into instantaneous combustion at very low temperatures; so that, when it is heated to a little above 200°, it detonates with tremendous violence, a vivid flash of light being produced at the same time, while the vessel—which, to prevent accidents, is covered with a wire cage—is broken to atoms. This compound is so dangerous, being one of the most explosive substances yet known, that in dealing with it the face is always protected by a mask, and only a small globule of it, no larger than a mustard seed, experimented upon. Dulong, the French chemist who discovered it, lost an eye and the use of a finger whilst operating with it; and Davy himself, in after-life, was wounded in the face by the effects of its detonation. The mere contact of this substance with certain combustibles causes it to explode violently, even under water, at ordinary temperatures. If touched with phosphorus, India rubber, common oil, turpentine, caustic potash, or even soap, it detonates so violently as to break to pieces the vessel containing it, and to scatter the water in which it is immersed in a shower all around.

_Bromide of nitrogen_, again, is said to be even more easily decomposed than the chloride. It is a dark-red oily liquid, having a fœtid odour, and giving off a vapour that is very irritating to the eyes. This compound, when touched with phosphorus, or even a small piece of arsenic, detonates with tremendous violence.

Next to the above remarkable compounds of Nitrogen, the _fulminates of the precious metals_ (into the composition of which, however, Nitrogen also enters) must be ranked in the order of ready combustibility; for these likewise explode at very low temperatures, with the production of heat and light.

First come the _Nitrurets of Mercury and Silver_—that is to say, compounds of those metals with Nitrogen. These are formed by the action of Ammonia on the oxide of Mercury or Silver. The Nitruret of Silver explodes with tremendous violence when gently rubbed or heated, and the Nitruret of Mercury when struck with a hammer, or acted upon by strong oil of vitriol.

_Fulminating Gold_, when suddenly heated to about 290°, detonates with great force and a vivid flash; and if exploded upon platinum foil, the metal is torn at the point of contact. Friction with hard bodies, or an electric shock, also explodes it. The more it is washed and dried, the more explosive this compound becomes; and if long retained at the temperature of boiling water, so as to become perfectly dry, the slightest friction causes it to detonate immediately and violently. If it be moist, however, it does not explode on the application of heat till dried, and those portions which first become dry explode the soonest; so that, in such a case, a succession of detonations is produced.

_Fulminating Mercury_ requires a temperature of 300° to cause it to explode, which it then does with a bright flame. It also detonates by friction, so that the greatest caution is required in preparing and dealing with it. This compound has even been known to explode in a moist state, and in the most careful and skilful hands it cannot be touched without considerable danger.[40] This is the substance used in the percussion caps; it is introduced into the caps moistened with a little tincture of benzoin, so as to be dropped into them, and then carefully dried. Howard, the discoverer of the compound, endeavoured to substitute it for gunpowder, but the explosion was found to be so sudden that it burst the gun without expelling the shot.

_Fulminating Silver_ likewise explodes, with the evolution of light and heat, at nearly the same low temperature. A grain, or merely half a grain of this substance, detonates with great violence, when heated or when touched with any hard body. On being placed upon a piece of rock crystal, and rubbed in the slightest manner by another crystal, it explodes with great force. It has sometimes exploded upon the contact of a glass rod, even under water; so that merely the feather of a common quill is generally used to collect it. It is dangerous to keep it in a cork-stoppered phial, for serious accidents have arisen from its unexpected explosion in a confined state. In short, persons cannot be too careful in meddling with it, and its use for detonating balls and other purposes of amusement is highly perilous and reprehensible.

_Fulminating Platinum_, on the other hand, explodes at a temperature of 420° with a loud report.

There is likewise a _fulminating powder_, composed of a mixture of 3 parts _nitre_ with 1 of _sulphur_, and 2 of dry _carbonate of potash_. This substance explodes with much violence at the low temperature of 330°; so that if a little of the compound be heated up to that point upon a metallic plate, it blackens, fuses, and detonates with great force.

Again, a mixture of 3 parts _chlorate of potash_ and 1 of _sulphur_ detonates loudly when struck upon an anvil with a hammer, and even sometimes explodes spontaneously. If 2 or 3 grains of chlorate of potash be reduced to powder in a mortar, and some very fine flour of brimstone be then added to it, the two substances, when rubbed together, will detonate with a smart noise, like the cracking of a whip. A mixture of _chlorate of potash_ and _sulphuret of antimony_ takes fire by gentle trituration, and deflagrates with a bright puff of flame and smoke. Chlorate of potash was proposed by Berthollet (the French chemist) as a substitute for nitre in gunpowder. The attempt was made at Essone, in 1778; but no sooner was the mixture of the chlorate with the sulphur and charcoal submitted to trituration than it exploded with violence, and proved fatal to several persons.

With _phosphorus_ and _chlorate of potash_ the explosion is dangerously violent: 1 grain of phosphorus with two of the chlorate, if placed in a small piece of paper and struck with a hammer upon an anvil, will immediately explode, and the phosphorus be thrown about in an inflamed state. Gunpowder, again, if mixed with powdered glass and struck with a heavy hammer upon an anvil, almost always explodes.

Moreover, a mixture of _oxygen_ and _hydrogen gases_, suddenly submitted to violent mechanical compression, unite with a vivid flash of light and produce water.

Next to Phosphorus and the Fulminates, _Sulphur_ is the most easily kindled. This body enters into combustion at about 500°. It is the comparatively low temperature at which sulphur bursts into flame, that makes it so important an ingredient in gunpowder, matches, &c. The easy combustibility of sulphur may be well illustrated by propelling a small quantity of it in powder into the current of hot air issuing from the glass chimney of a gas lamp, when it will be seen to take fire at a considerable height above the flame.

_Wood_, _cotton_, _paper_, &c. require, on the other hand, their temperatures to be raised much higher than that required for the inflaming of sulphur, in order to be made to enter into combustion.[41] Paper merely becomes brown or scorched at the heat of 440°, nor can it be lighted at a red heat, though the temperature of this is 1000°. Cotton or tow, however, when greased with oil, occasionally absorbs air so rapidly, and produces so much heat during the process, that spontaneous combustion frequently occurs from this cause. The same effect sometimes arises from _hay_ being stacked before being perfectly dry; the moisture sets up a kind of fermentation in the interior of the stack, and this evolves so much heat that the temperature becomes raised to the point required for the material to enter into rapid combination with the atmosphere, so that spontaneous combustion is the result.

Again, there are certain mixtures of gases which are explosible at a red heat, while others cannot be made to enter into combustion at that temperature, but require the presence of flame in order to fire them.

_Carburetted hydrogen (coal gas)_, _sulphuretted hydrogen_, and _carbonic oxide_, can be made to inflame in air by red-hot iron or charcoal; and a mixture of _oxygen_ and _hydrogen_ gases can be exploded by a red heat visible in daylight, whereas a dull red heat only causes the two gases to combine silently without detonation. _Fire-damp_, however, (which is light carburetted-hydrogen gas, and the same as that which rises from stagnant pools on disturbing the mud at the bottom), cannot be inflamed by the strongest red heat; so that a fire made of charcoal, that will burn without flame, may be blown up to whiteness without exploding a mixture of this gas with air. A piece of iron also, at the highest degree of red heat, and even at an ordinary white heat, does not inflame an explosive mixture of _fire-damp_, but when brought to its _highest point of white_ heat, iron immediately causes the fire-damp to combine with the air with a violent detonation.

The knowledge of this fact, which we owe to the researches of Davy himself, was of immense importance in the construction of the safety-lamp, and, once discovered, it was treasured in the brain for after use, for the preservation of life and the mitigation of suffering.

Again, there are some substances which are so readily inflammable that they take light even at the approach of flame. These bodies consist of highly vaporizable liquids, such as _alcohol_, _ether_, _naphtha_, _sulphuret of carbon_, _oil of turpentine_, &c. From the tendency of these combustible liquids to pass into vapour, the surrounding atmosphere (immediately on opening any vessels containing them) becomes charged with their fumes, so that an almost explosive mixture is formed with the air; and as this extends to some distance from the liquid itself, the approach of a lighted body instantaneously causes the whole volume of vapour to pass into one sheet of flame—an effect which is occasionally attended with the most disastrous results.

On the other hand, some combustible liquids require their temperatures to be highly raised before they can be inflamed; such is the case with the common fixed oils—as lamp-oil, and others. For this purpose a cotton wick is usually employed in burning them, so that, by the capillary attraction of the fibres, a small portion of the liquid may be raised above the level of the rest, and the oil thus be brought into connexion with the flame by minute quantities at a time; the consequence being that, as each small portion of the liquid is heated, it is converted into vapour, or gas—and this, by the high temperature maintained by the burning wick, is made to enter into combustion with the surrounding air, and so to be continually inflamed above it. Any porous substance which is a bad conductor of heat (such as Bath-brick or sandstone), will, if cut to a fine edge, answer all the purposes of the ordinary cotton lamp-wick; for if a light be applied to this, so as to raise its temperature sufficient to convert into gas the film of oil at the summit, the fluid will be readily inflamed, and continue burning until such an incrustation of charcoal ensues at the tip as to prevent the oil being heated any longer by the flame.

Further, some substances cannot be made to burn at ordinary temperatures in the open air, though, on being confined in a vessel of oxygen gas, they readily enter into combustion when their temperature is raised. This is the case, as we have seen, with iron, and some of the other metals.

* * * * *

The next problem to be resolved was, Whence come the light and heat that are emitted during the process of combustion?

“All cases of combination with oxygen,” mused the youth, “such as the rotting of wood, and the gradual rusting of metals in the open air, are, probably, attended with the evolution of heat; but in such instances the process is so slow, that the heat evolved is unobserved, and dissipated without accumulation. When the combination, however, takes place in a shorter time, as in the production of vinegar, the heat becomes proportionately sensible; and when the combination with the oxygen is so rapid that the whole of the heat is evolved in a much more limited period, as during combustion, the increase of temperature is rendered considerably more intense. A pound of charcoal, for instance, combining with oxygen in the process of respiration, gives off the same amount of heat as it does when in a state of ignition, and takes up precisely the same quantity of the gas. In the one case, however, the combination is spread over 30 hours, whereas in the other it occupies but as many minutes.”

Humphry reflected for a long time as to the origin of the light and heat evolved during the burning of substances.

“Are the light and heat,” said he to himself, “originally imprisoned, as it were, in the combustible, and set free during the burning of it? or are they merely the result of the rapid and energetic combination of the oxygen of the air with the burning substance?”

The former assumption the boy knew to be Lavoisier’s theory of the subject, but such an explanation appeared to him to be inconsistent with the facts.

The _products of combustion_ are not always the same. In some cases they consist of _gases_, as in the burning of Charcoal and Sulphur, &c.; in others, _liquids_ are produced, as by the combustion of oxygen and hydrogen gases, the product in that case being merely water; while in others, again, a _solid_ product is the result—as when phosphorus is burnt a white solid, called “phosphorous acid,” being then formed: and so with zinc, the combustion of which produces a solid white rust, or oxide.

“Now, if the heat evolved during combustion,” thought Humphry, “proceeds from the liberation of the _latent_ caloric, or that which previously existed in the substances in an _insensible_ state, it would follow that such heat should be given off only when the combustibles pass from a _rarer_ to a _denser_ form; as, for instance, when water is produced by the burning of its two constituent gases, or when the oxygen of the atmosphere becomes fixed in some solid product, as in the oxides of zinc or mercury.

“But by combustion,” the boy went on, “many solid bodies are converted into gases; and in such cases, according to Lavoisier’s theory, instead of heat being evolved, it should be _absorbed_, and positive _cold_ produced by the process of burning.

“This, however, is not the case,” added Humphry. “The explosion of _Gunpowder_, for example, is attended by immense heat, and yet the ingredients composing it, in passing from the solid to the gaseous state, expand some hundred-fold, having their volume increased, it is said, no less than 250 times. So, again, the gas called _Protoxide of Chlorine_, at the instant of decomposition evolves light and heat with explosive violence, and yet it is known to become one-fifth greater in volume afterwards. The oily substance, too, called _Chloride of Nitrogen_, on being made to enter into combustion, is resolved into its elements with tremendous force of inflammation, expanding into more than 600 times its bulk: so that, according to Lavoisier’s theory, a prodigious degree of cold ought to be produced by such an expansion, whereas light and heat are evolved by it.”

That the heat of combustion is due rather to intense chemical action going on at such times, Humphry made many experiments to prove.

First, he generated some _Chlorine_, or green gas.[42] This he did by mixing in a retort some common salt with a little black oxide manganese, and some sulphuric acid and water. The gas which came over was of a greenish-yellow colour, and had a pungent, disagreeable smell, exciting cough and great irritation in the lungs when inhaled. Having collected some of this gas over warm water in a receiver, with a stop-cock at the top, the boy took a retort which had another stop-cock fitted to the end of it, and having introduced into this some copper-leaf, he screwed the retort on to the plate of his air-pump, and proceeded to exhaust it of air as perfectly as he could. This done, he screwed the stop-cock of the retort on to that of the receiver in which he had collected the gas; then, turning on both cocks, the chlorine rushed up into the retort, and the metal immediately became spontaneously ignited by it, and burnt in the gas with considerable energy.

The experiment was then repeated with a little powdered antimony, and the same vivid combustion ensued.

Now these Humphry knew to be cases of mere _chemical affinity_. The chlorine gas had a strong tendency to combine with the metals employed, and as these were used in the best form for promoting the combination, the union of the gas with the copper and antimony was so rapid and energetic, that combustion was the consequence.[43]

Next, the lad took some _Sulphur_, and in this he heated some shavings of iron in a close vessel, when the metal, in a short time, was seen to become intensely ignited, and to burn, as it were, in the vapour of the sulphur. This was most curious, for sulphur he had never thought to be a supporter of combustion.

Further, Humphry heated some platinum and tin-foil, and at the moment when the two metals fused into one mass they became, to his astonishment, vividly ignited.

Nor was this all: the boy slacked some recently-burnt lime in a dark place, and found that, when the water was thrown upon it, the heat rose to upwards of 500°, while a faint light was emitted.

Another substance, discovered since the date of the above experiments, affords a striking illustration of the heat produced by energetic chemical action. This is a peculiar liquid called _Peroxide of Hydrogen_ (see p. 264), consisting merely of water combined with an extra quantity of oxygen. So readily decomposible is this fluid, that at the heat of boiling water evolutions of gas are produced with such violence as to cause an explosion; and almost all the metals, when in a state of minute division, resolve it rapidly into its elements. The _peroxides_ of lead, mercury, gold, platinum, &c., act on the liquid with surprising energy, the decomposition of it being complete and instantaneous upon dropping those substances into the liquid; for oxygen gas is then given off with such force as to produce a detonation, while the temperature becomes so intense that the glass tube in which the experiment is conducted grows suddenly red hot.

Subsequently, Humphry amused himself by mixing a little _Chlorate of Potash_, about the size of a pea, with the same quantity of _loaf sugar_, having previously reduced each to powder. Then he placed the mixture on a piece of tile, and, dipping a glass rod into a bottle of strong _Oil of Vitriol_, let the acid drop from the rod upon the powdered Chlorate and sugar, when they were instantly kindled, and burnt with a red and blue flame. For the Vitriol immediately decomposed the Chlorate of Potash, and so produced heat enough to ignite the materials; while the oxygen given out from the Chlorate maintained the sugar in a state of vivid combustion. A little _Camphor_, mixed with _Chlorate of Potash_, and touched with a drop of _Oil of Vitriol_, may be made to inflame in the same manner. A like effect is also produced when the _Chlorate_ is mixed with _Spirits of Wine_.

After this, the lad placed a small piece of _Phosphorus_, and a few grains of _Chlorate of Potash_, at the bottom of a thin glass vessel, and then poured gently upon them some _hot_ water. The heat of the water was sufficient to inflame the Phosphorus, while the oxygen evolved from the Chlorate of Potash in connexion with it tended to keep up the combustion; so that the two burnt with a vivid and pleasing light under the water.

Now Humphry knew that the reason of these effects was, that the Chlorate of Potash contains a large quantity of oxygen, which, having a strong tendency to combine with the combustibles, enters rapidly and energetically into union with them immediately their temperatures are elevated, and so gives rise to the phenomena of heat and light which are evolved during the combustion.

The same result is produced by _Nitre_ in ordinary gunpowder, for this also contains a large quantity of oxygen gas, which serves to inflame the small particles of charcoal that are mixed with it, while the sulphur—which forms the other ingredient—being inflammable at a low temperature, renders the gunpowder capable of being united by a mere spark.

_Oxygen_ thus appeared to Humphry to be the main “supporter of combustion,” while the body burnt seemed to be the “combustible.”

But Humphry had seen that bodies burnt in _Chlorine_ gas as well as in oxygen: this, therefore, was another “supporter of combustion.” The same effect, again, he had found to be produced in the vapour of _sulphur_. Sulphur, however, was a “_combustible_ body;” so that, in this instance, the same substance was both a combustible and a supporter of combustion.

The division of all bodies, therefore, into these two classes, as propounded by the French chemists, appeared to have no foundation in nature.

Nevertheless, Humphry was determined to put the matter to the test of experiment, and to see whether the light and heat evolved during combustion proceeded from the combustible itself, or from the combination of it with the air during the burning.

Accordingly, the lad inflamed a jet of hydrogen gas in a vessel of oxygen, when the light and heat certainly appeared to proceed from the jet of _hydrogen_, while the oxygen seemed to act merely as the _supporter_ of the combustion.

On reversing the experiment, however, and causing the oxygen gas to issue from the jet, Humphry found that he could inflame it in a vessel of hydrogen, and that then the light and heat seemed to be evolved from the burning _oxygen_, while the hydrogen appeared only to keep it inflamed.

It was evident, therefore, from the last experiment, that even oxygen itself might be ranked among the _combustibles_, and hydrogen be considered as a _supporter of combustion_; but Humphry now saw that the real truth of the matter was, that the heat and light evolved during combustion came from _neither one substance nor the other_, but arose simply from the rapid and energetic chemical union of the _two_: for even two metals, at the moment of their union, evolve heat and light, as shown in the experiment of the fusion of platinum with tin-foil.

Combustion, therefore, is simply the consequence of the _rapid_ chemical action of one body upon another; and the reason why it is necessary to elevate the temperatures of certain bodies before they can be inflamed, is merely because _heat_ promotes the chemical action of substances upon each other.