CHAPTER VIII.
THE WONDERFUL EFFECTS OF HEAT.
The effects of heat are manifold.
In the first place, an increase of temperature _expands_ or enlarges almost all bodies, while a decrease causes them to _contract_ or become diminished in bulk.
Secondly. Heat _changes the form_ of bodies, converting solids into liquids, and liquids into vapours; while cold, on the other hand, condenses vapours into liquids, and causes liquids again to solidify or congeal.
Thirdly. Heat causes _ignition_; that is to say, it changes dark opaque substances to a bright transparent red, rendering them capable of giving out light, when their temperatures are raised to a high degree, and, when increased to the highest point, causing them to become even white in the fire, and then endowing them with the properties of the solar beams, so that their rays have the same power of traversing plates of glass, and of producing chemical changes, even as the rays of the sun itself.
Fourthly. _Combustion_, or the burning of bodies, with the evolution of flame, is another effect of heat. There is also a species of combustion called _slow_ (_erema-causis_ is the chemical term for it), which is unaccompanied with flame—as in the rotting of wood and other organic tissues, the rusting of metals, and even the breathing of animals and ourselves. In each of these processes there is the same combination of a combustible body with the oxygen of the atmosphere—but at a much _slower_ rate—than in the more rapid and energetic forms of combustion; and hence but slight increase of temperature (if any) is discernible, while no flames or luminous gases that are perceptible to our senses are evolved under such conditions.
Fifthly. _Phosphorescence_ is likewise produced by heat. During combustion and ignition, bodies become _temporarily_ luminous; but in states of what is called phosphorescence they are _permanently_ so; and there are many substances—such as the compact phosphate of lime, the dark-blue kind of Derbyshire spar, several varieties of heavy spar, and powdered quartz—which acquire the property of shining constantly in the dark after having been made nearly red hot.
Sixthly. _Electricity_ is induced by heat; for it has been discovered that if a bar of the metal called antimony be heated at one end, while the other is kept cool, an electric current ensues.
Lastly. Heat promotes both _vegetable_ and _animal life_. For not only is intense cold destructive of organic existence, but the increased warmth of the summer invariably calls into being an infinite number of plants, flowers, insects, and the many forms of organised nature that give variety and grace to the earth. Moreover, heat produces in ourselves, and other sentient animals, a _feeling of warmth_, and the absence of it a sensation of cold; by which we are enabled to measure—though hardly with perfect accuracy—the different changes of temperature occurring in the substances around us, and also, by the agreeable impression which we derive from warmth, induced to seek that degree of heat which is best fitted for the promotion of our health and development of our faculties.
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Humphry began by studying the laws which regulate the expansion of bodies under the three different forms in which they exist in nature, viz. _solids, liquids, and gases or vapours_. To determine the expansion of different solids, the youth procured short bars of the several substances upon which he had decided to experiment. The bars were all of the same length and thickness, and were accompanied with a gauge, which measured their dimensions at ordinary temperatures.
The following diagram illustrates the apparatus employed. The first step was to test the length and breadth of each bar that was to be used. This was performed first, by placing it in the gap at the upper part of the gauge, and seeing whether it exactly fitted between the notches; and secondly, ascertaining whether it was precisely of the same diameter as the hole at the bottom part of the plate. This done, the bars were successively plunged into hot water, when, on applying them once more to the gauge, they were found to be so much _enlarged_ in all their dimensions that it was impossible to make them pass through either of the apertures. After this they were severally cooled down, by immersion in a mixture of snow and salt, to the temperature of the freezing-point of water, when they were discovered to have considerably _contracted_ in bulk; so that they could be passed through both of the openings with perfect ease.
[Illustration]
It was by such means Humphry ascertained that different solids possessed different degrees of expansibility, and that metals are more susceptible of change of bulk than other solid bodies. Each solid, however, was found to have a rate of dilatation peculiar to itself. _Lead_, for instance, when heated, from the freezing to the boiling-point of water, was discovered by measurement to have expanded one-350th; _iron_, one-800th; and _glass_, one-1000th.
_Platinum_, however, was found to be less expansible than iron, and _copper_ more so. _Silver_, on the other hand, was more expansible than copper, while _tin_ was more so than silver; _lead_, again, more than tin, and _zinc_ even more than lead: so that glass was proved to be less capable of being increased in bulk by heat than the metals; whilst, among the metals themselves, platinum was ascertained to be the least expansible, and zinc the most so.
On talking over these matters with Mr. Borlase, the doctor told Humphry that the expansion of metals was a matter of considerable importance in many arts. “For instance,” said the gentleman, “coopers put the iron hoops upon their casks in a heated state, so that they may gradually contract on cooling, and firmly bind the staves together. With the same view the wheelwright heats the tire of his wheel, in order that it may, as it cools, press strongly upon the ‘felly,’ or circumference; and, for the same reason, the plates of large boilers are united with red-hot rivets, which, during their contraction on cooling, draw the sheets of metal closely and securely together.
“In the iron bridge,” continued the doctor, “which was constructed over the Severn in Shropshire, when I was a lad, it has been found that the arches are nearly one inch longer in summer than they are in winter; so that, if due allowance had not been made for the expansion of the metal, the stone piers, on which the arches rest, must have given way to the pressure long before this. The same allowance for expansion, Humphry, has to be made in the clamping together of stones in the construction of church steeples; for the changes of bulk which occur in metals at different temperatures, though comparatively small in amount, take place with irresistible force.”
The perpendicularity of the walls of the Museum of Arts and Manufactures, in Paris, it may be added, were restored by Molard, upon the same principle. In consequence of the weight of the roof, the walls were bulging outward, and, in order to straighten them, iron rods were laid across the interior of the building, their ends being made to project through the brickwork outside. These rods were then heated, and, when in an expanded state, a strong iron plate was passed over each end of them, and screwed firmly up against the exterior of the walls. As the rods cooled, they naturally contracted, and drew the walls somewhat nearer together. The bars were afterwards again elongated by heat, and again screwed up previous to their contraction; and so, by a repetition of the process, the walls were gradually brought to a perpendicular position.
Humphry was delighted with the ingenious applications of the expansion and contraction of metals by heat and cold, and Mr. Borlase, observing the interest he took in the subject, proceeded to explain to him how, by the same principle, the alterations in the length of the pendulum of a clock were “compensated,” and the instrument so made to vibrate seconds at all seasons. For a pendulum to beat exactly sixty times a minute, he told the boy, it was necessary that it should be a fraction more than 39 inches long, in the latitude of London. “If, however, the pendulum be made of metal,” he said, “it will be liable to be _longer_ in summer and _shorter_ in winter; so that the clock will be slow in the warm weather season and fast in the cold: for when the bob is let down the one-100th part of an inch the clock loses 10 seconds in 24 hours, and a change of temperature equal to 30° (which is nearly the difference between summer and winter, in our climate), will alter the length of the pendulum-rod about one-5000th part, and so occasion an error in the rate of going of 8 seconds a day.
“To counteract the expansions of the metal rod of the pendulum,” continued his preceptor, “there are many ingenious contrivances. The simplest of these, perhaps, is as follows: A compound bar of two differently expansive metals, such as steel and brass, is formed by rivetting or soldering the two metals together; for if such a bar, with the brass _uppermost_, be placed upon a heated plate, it will be found to warp or curve _downwards_, in consequence of the expansion of the brass being greater than that of the steel. If, however, on the other hand, the compound bar be placed on a plate cooled down by a mixture of snow and salt, it will be found to warp or curve _upwards_, because the brass will contract the more with the cold. Now, if two such compound bars, with the most expansible metal at top, be placed at the upper part of a pendulum-rod, one on either side of it, and firmly fixed at one end, they will, as they warp upwards or downwards, tend to shorten the pendulum-rod when it becomes lengthened by the heat, and to lengthen it when it becomes contracted by the cold.”
To make this more readily intelligible to Humphry, the doctor exhibited to him the following engravings:
[Illustration]
“Let us now,” said Mr. Borlase, as he placed his finger on the centre drawing, “suppose the pendulum, with the compensation-bars perfectly horizontal, to be vibrating seconds at a temperature of 60°, and that, some few months afterwards, the heat rises to 80°; in such a case, of course, the pendulum-rod would be _elongated by the heat_, and the longer the rod the slower the vibrations, so that it would then vibrate _less_ than sixty times in the minute. The effect of the increase of temperature, however, on the compensation-bars (the most expansible metal being uppermost), would be to warp them _downwards_ (as shown in the left-hand drawing),” said the doctor, pointing to the illustration, “and thus they would _shorten_ the pendulum-rod as much as the heat had lengthened it. In cold weather, however, Humphry, the metal rod of the pendulum would be _diminished in length_; but then the compensation-bars would warp _upwards_, and so tend to _elongate_ it, to the same extent as it had been contracted by the cold (as may be seen on reference to the picture on the right hand).”
The next day Humphry was busy making experiments concerning the expansion of _liquids_. He first took a large thermometer tube and poured into it a sufficient quantity of spirits of wine to fill the bulb at the bottom and make the fluid rise some few inches in the stem above. Then, having marked upon the glass with a file the level at which the spirit stood at an ordinary temperature, the boy plunged the instrument into a vessel of boiling water, and immediately beheld the liquid rise in the tube till it stood several inches above its former level. After this he immersed the tube in a mixture of snow and salt, and found the liquid contract, so that it fell in the stem almost down to the bulb. On removing the instrument, however, the fluid immediately commenced rising again, and so pleased was the youth with the motions that he repeated the experiment over and over again, being not a little delighted to perceive that each time, as he plunged the instrument into the hot or cold bath, the spirit invariably rose or fell to precisely the same place in the tube.
To measure the different rates of expansion among different liquids, the young chemist provided himself with a long and narrow glass tube, which was graduated into cubic inches, and into this he poured a certain quantity of the liquid he wished to experiment upon. Then plunging the graduated tube into the snow-and-salt mixture, he noted the precise volume of the fluid at that temperature; after which he immersed it in a vessel of boiling water, and then noted again how many cubic inches it occupied in the tube at the higher temperature; so that the difference told him how much the liquid had been expanded between 32° and 212°, or the freezing and boiling point of water.
It was thus the lad ascertained that 9 measures of spirits of wine at 32° become expanded into 10 measures at 212°, and 9 measures of strong aquafortis also become 10, between the same extremes of temperature. Again, 12 measures of olive oil are increased into 13, while 14 of ether and the same quantity of oil of turpentine swell each into 15, with the like increase of heat. Then 17 measures of oil of vitriol, at the freezing-point of water, are dilated into 18 at the boiling-point, and 22¾ measures of water are increased to 23¾ within the same range of temperature, while 55½ of quicksilver become 56½ when similarly treated; consequently spirits of wine is no less than 6 times more expansible than quicksilver, so that in the depth of winter 100 pints of spirits of wine are dilated into 105 in the height of summer.
While making his experiments, however, as to the rate of expansion in liquids, the boy had been astonished to perceive, when the tube contained water, that, on placing it in the mixture of snow and salt, the liquid, as it was cooled down, continued to shrink till it had attained the temperature of about 40°; and then, _instead of contracting any farther_ (as was the case with other liquids till they froze), _it began to expand slowly, and kept rising in the tube until it congealed_. He noticed, too, that, when the water was at its freezing point, or 32°, it was of the same bulk as it was at 48°; so that it expanded just as much for the 8° below 40° as it had contracted in the 8° above that point.
Humphry then tried another experiment illustrative of this remarkable property of water. Having produced two cylindrical glass vessels, he surrounded one of them at the bottom with a circular tin tray, that fitted closely to the exterior of the cylinder, and affixed to the other a similar tray, but this he placed at the upper, instead of the lower part of the cylinder as before—in the manner represented in the subjoined engraving:
[Illustration]
A thermometer then having been placed in each of the glass vessels, they were respectively filled with water at 50°, while a freezing mixture of pounded ice and salt was placed in each of the trays.
After the temperature of the whole of the water in both vessels had been reduced to 40°, it was found by the thermometer in the vessel, with the freezing mixture _at the top_, that the cooling effect would not proceed downwards, _but was limited to the surface_, where the water ultimately froze; for the ice-cold water being _lighter_ than the water below at 40°, necessarily _floated_ like oil upon the surface. In the other cylinder, however, where the cold was applied at the _lower_ rather than the upper part of the water, the effect was very different; for there, the liquid becoming lighter, as its temperature sank below 40°, _ascended_, whilst the warmer and heavier water at the top _descended_, until it was cooled, and so expanded in its turn; and thus the _whole_ of the liquid was ultimately reduced to the freezing-point; whereas in the other cylinder this effect was limited to the _surface_ only. Humphry now could see the reason why lakes and ponds froze only on the surface, and why, on breaking the ice (as he had repeatedly done when out snipe-shooting with his uncle, Leonard Millett), the water underneath was always found to be warmer than the air above.
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The lad had now but to investigate the rate of expansion among _aëriform bodies_, or _gases_, to complete this part of the subject.
Accordingly, he took the thermometer tube he had before used, and placed it, with its open end, downwards, in a glass of water, thus:
[Illustration]
The tube was of course filled with air, so he applied his palm to the bulb, and found the heat of the hand sufficient to expand the air within, and drive a stream of bubbles up through the water. On removing the source of heat, however, the volume of air began to contract, and the liquid to mount in the tube, so that he could see by the height the water rose in the stem the amount of expansion which the air had undergone.
Humphry then proceeded to ascertain the amount of expansion produced in a given quantity of air, when heated from the freezing to the boiling point of water, and discovered that 100 cubic inches of air at 32° become dilated to 137½ cubic inches at 212°. Air, therefore, at the freezing-point, expands one-480th for every degree of heat that is added to it; so that 480 cubic inches at 32° become 481 at 33°, and 482 at 34°, and so on, the volume expanding one cubic inch with each additional degree of heat. A volume of air, therefore, at 32° would be doubled at 480°, and tripled at 960°, the latter temperature being that of a dull-red heat.
Steam, and other vapours, when heated by themselves, are subject to the same law of expansion as air.
But although the expansion produced in aëriform bodies by heat is great in amount, the actual force which is thus developed is small when compared with that of solids and liquids under the same circumstances. This is owing to the extreme elasticity of aëriform bodies; so that, although air becomes tripled in volume at a red heat, vessels are easily found capable of sustaining the pressure of the expanded fluid. It is only when a portion of liquid is present, so that _volume after volume_ of vapour is added to those already generated—as in the production of steam—that, on resisting the expansion, the pressure becomes enormous, and mounts up to a dangerous point.