Part 15

In 1778 Dr. Priestley pursued his experiments on the property of vegetables growing in the light to correct impure air, and the use of vegetation in this part of the œconomy of nature. A discovery which was announced to several men of science in England previous to the publication of the same ideas by Dr. Ingenhouz.[54] Indeed from its having been communicated to M. Magellan whose pleasure and whose occupation it was, to give information of new facts to his philosophical correspondents, and of this in particular to Dr. Ingenhouz then engaged in similar researches, there is hardly a doubt but the latter knew of the experiments then pending on the subject by Dr. Priestley.

[54] Doctrine of Phlogiston established, p. 107, et. seq. The theory of the amelioration of impure air by the absorption and excretion of vegetables growing in the light, has been doubted by Dr. Darwin in his Phytologia, and opposed by Count Rumford in a paper published in the transactions of the Royal Society, for 1787: also by Dr. Woodhouse of Philadelphia, Nicholson’s Journal, for July 1802, and by Mr. Robert Harrup, Nicholson’s Journal, for July 1803.

It is painful to notice these aberrations from propriety in the conduct of men highly respectable in the philosophical world, arising from an over anxious avarice of literary fame, and an improper jealousy of the reputation of another. Not that it derogates from the character of a philosopher to wish for the applause of those who know how to appreciate his merit, or who are benefited by his exertions; such an anxiety is laudable when it does not lead to encroachments on the literary rights of others; nor is it at all desireable under the present circumstances of human nature, to expect from men of science an attention to their pursuits arising from motives of pure benevolence alone, and excluding all views, hopes, and expectations of the gratifying tribute of public approbation. I believe no man ever laboured with a more single eye to public utility than Dr. Priestley. But consideration in society, and the respectability attendant upon great talents, and great industry, successfully employed for the benefit of mankind, is a motive to useful exertion so universal, so honest, so laudable, and withal so powerful, that it is the common interest, as well as the duty of society, to bestow it liberally where it has been earned faithfully, and to concede it to those only, who have really deserved this honourable reward.

From this period Dr. Priestley seems to have attended to his pneumatic experiments as an occupation; devoting to them a regular portion of his time. To this attention, among a prodigious variety of facts tending to shew the various substances from which the gasses may be procured; the methods of producing them; their influence on each other, and their probable composition, we owe the discovery of vitriolic acid air, of fluor acid air, of vegetable acid air, of alkaline air, and of dephlogisticated nitrous air, or gazeous oxide of azote as it has been called, the subject of so many curious experiments by Mr. Davy. To these we may add the production of the various kinds of inflammable air by numerous processes that had escaped the observation of Mr. Cavendish; in particular the formation of it by the electric spark taken in oils, in spirits of wine and in alkaline air; the method of procuring it by passing steam through hot iron filings, and the phenomena of that hitherto undetermined substance the finery cinder, and its alliance to steel. To Dr. Priestley we owe the very fine experiment of reviving metallic calces in inflammable air and its absorption in toto, apparently at least, undecomposed. He first ascertained the necessity of water to the formation of the gasses, and the endless production of air from water itself.

Dr. Priestley’s experiments on this subject, to wit: the generation of air from water, opened a new field for reflection, and deserves more minute notice. No theory has yet been proposed adequate to the explanation of the facts. He had before remarked that water was necessary to the generation of every species of air, but the unceasing product of air from water had never been before observed.

In his first set of experiments he procured air, by converting the whole of a quantity of water into steam: then, to obviate the objection to the water having imbibed air from the atmosphere he put the water on mercury in long glass tubes immersed in mercury: in a third process he used no heat, but merely took off the pressure of the atmosphere. In all these cases a bubble of air was extricated from the water, which being separated by inclining the tube, another bubble was again produced on each repetition of the experiment. That this could not be air imbibed from the atmosphere appeared from this, that though the first portions were generally purer than atmospheric air, the next became less pure, and at length wholly phlogisticated.

It did not appear that the addition of acids, enabled the water to yield more air, nor did he succeed in attempting to convert the whole of a given quantity of water into air, although exposing the water confined over mercury to heat, and separating the air produced, it still continued to produce more air for twenty or thirty repetitions of the experiments. When a certain proportion of air was thus produced at any one time, no continuance of the experiment would encrease the quantity until it was separated. Hence he concludes that the longest continuance of water in the state of vapour would not convert it into air. The water used was pure distilled water previously boiled to separate any adventitious air that might have been imbibed from the atmosphere. The precautions he used, and the replies to such objections as he foresaw the experiment would be liable to, are detailed in the papers he published on the subject, to wit, a separate pamphlet published in England in 1793, and a communication in the Am. Ph. trans, v. IV. p. 11-20.

In the last mentioned paper, he proceeds also to give an account of some experiments on the property of water to imbibe different kinds of air, and the conversion of sp. of wine, into inflammable air.

This paper inserted in the American transactions, was read before that society in Feb. 1796. In Ap. 1800 another paper was read before the same society on the production of air by the freezing of water Am. Ph. trans. v. V. p. 36. In this paper he recapitulates the general result of his former experiments on the generation of air from water, namely “that after all air had been extracted from any quantity of water by heat or by taking off the pressure of the atmosphere, whenever any portion of it was converted into vapour, a bubble of permanent air was formed, and this was always phlogisticated. The process with the Torricellian vacuum (he says) I continued for some years and found the production of air equable to the last. The necessary inference from this experiment is, that water is convertible into phlogisticated air, or that it contains more of this air intimately combined with it than can be extricated from these processes in any reasonable time.”

He proceeds to state his imperfect attempts to procure air from water by freezing, until he procured cylindrical iron vessels seven or eight inches high and near three inches wide at the bottom, the upper orifice closed with a cork and cement, in the centre of which was a glass tube about one fifteenth of an inch in diameter. In this apparatus the water in the iron vessel was frozen by means of snow and salt, the vessel being immersed in mercury, and the water contained over the mercury. The quantity of water was about three ounces. The experiment was repeated nine times without changing the water, and the last portion of air procured in this manner was as great as any of the preceding; so that there remained no reasonable doubt but that air might be produced from the same water in this manner ad libitum. Having obtained near two inches of air in the glass tube, Dr. Priestley put an end to the experiment, and examining the air found it wholly phlogisticated, not being affected by nitrous air, and having nothing inflammable in it.

The inference drawn by the Doctor from those experiments is, that water when reduced by _any means_ into the state of vapour, is in part converted into phlogisticated air; and this is one of the methods provided by nature for keeping up the equilibrium of the atmosphere, as the influence of light on growing vegetables is the means of recruiting the other part; both of them being subject to absorption and diminution in several natural processes. And he thinks that they strengthen also the opinion, that water is the basis of every kind of air, instead of being itself a compound of hydrogen and oxygen according to the new theory. At all events the experiments themselves must be considered as extremely curious, as well as new.

The water and the salt thus made use of gave rise to another experiment of the most important nature to the present theory of chemistry, if it should on future repetition be ultimately verified. This experiment related by Dr. Priestley in a letter to Dr. Wiston is in substance as follows. Having repeatedly used as above mentioned a freezing mixture of common salt and snow, the experiment being finished, he evaporated the snow water in an iron vessel and recovered the salt. The salt thus recovered contained some calx of iron. He put it by in a bottle and labelled it, according to his usual practice. In October 1803, he wanted to procure some marine acid, and took the salt thus procured by evaporating the snow water, for the purpose. On commencing the distillation, he was surprized to find the receiver full of the characteristic red fumes of the nitrous acid. The vitriolic acid used for the purpose was diluted with about an equal quantity of water. On finishing the process, he took some of the acid in the receiver, and dissolved copper in it, and thus procured good nitrous air. He was himself perfectly persuaded that no nitre had been used in the freezing mixture, nor had any by accident or design been mixed with the salt. He was not unacquainted with the common mode of clearing black oil of vitriol by the addition of nitre. So that no means of accounting for this curious fact remained, but the snow or the iron: he seemed to think that should this experiment be fully verified hereafter, it would confirm the vulgar hypothesis of snow containing nitre, and account for the fertilizing quality usually attributed to snow. He had no opportunity in that winter of repeating the experiment as he died in about three months after, and his previous illness had compelled him to forsake his laboratory.

Of the almost discarded theory of Phlogiston Dr. Priestley to his death remained the strenuous advocate, and almost the sole supporter; _ipse Agmen_. Beautiful and elegant as the simplicity of the new doctrine appears, many facts yet remain to be explained, to which the old system will apply, and the French theory is inadequate. These are collected with an ingenuity of arrangement, and a force of reasoning in the last pamphlet published by the Doctor on the subject,[55] which no man as yet unprejudiced can peruse, without hesitating on the truth of the fashionable theory of the day.

[55] The doctrine of phlogiston established 1803.

Certainly, it has not yet been sufficiently explained on the new theory, what becomes of the Oxygen from the decomposed water in the solution of metals in acids; nor why inflammable air is produced when one metal in solution is precipitated by another; nor why dephlogisticated air is hardly to be procured from finery cinder, if at all; nor why this substance so abounding in oxygen according to the new theory, will not oxygenate the muriatic acid; nor why it should answer all the purposes of water in the production of inflammable air from charcoal; nor why water in abundance should be produced when finery cinder is heated in inflammable air, and none when red precipitate is exposed to the same process; nor what becomes of the oxygen of the decomposed water when steam is sent over red hot Zinc, and inflammable air is produced without any addition in weight to the Zinc employed; nor why there should be a copious production of inflammable air when hot filings of Zinc are added to hot mercury in a hot retort and exposed to a common furnace heat, which I believe is an unreported experiment of Mr. Kirwan’s; nor why sulphur and phosphorus are formed by heating their acids in inflammable air without our being able to detect the oxygen which on the new theory ought to be separated, nor why water should be produced by the combustion of inflammable air with ,47 of oxygen, and nitrous acid when ,51 of oxygen is employed, for this experiment can now no more be doubted than explained; nor why on the new doctrine the addition of phlogisticated air, should make no alteration in the quantity of acid thus obtained; nor why red hot charcoal slowly supplied with steam, should furnish inflammable air only and not fixed or carbonic acid air; nor why nothing but pure fixed air should be produced by heating the carbonated Barytes in the same way; nor why fixed air should be formed under circumstances when it cannot be pretended that Carbon is present, as when gold, silver, platina, copper, lead, tin and bismuth are heated by a lens in common air over lime water; or why the grey and yellow calces of lead should furnish carbonic acid and azote, and no oxygen; nor why the residuum of red lead when all its oxygen is driven off by heat should be either massicot or glass of lead according to the degree of heat, and not lead in its metalline state; nor why plumbago with steam should yield inflammable and not fixed air; nor why minium and precipitate per se heated in inflammable air should produce fixed air; nor why on the evaporation of a diamond in oxygen, the fixed air produced should far exceed the weight of the diamond employed, if some of the oxygen had not entered into the composition of the carbonic acid so formed; nor why there should be a constant residuum of phlogisticated air (or azote) after the firing of dephlogisticated and inflammable airs, if it be not formed in the process; nor why phlogisticated air if a simple substance, should be so evidently formed in the various processes enumerated by Dr. Priestley in the 13th section of the pamphlet of which I have made the foregoing abstract? whether the doctrine of phlogiston is still to be used as the key to the gate of chemical theory, or whether it be properly thrown aside for the elegant substitute of the French chemists, can hardly be ascertained, until the preceding difficulties are cleared up on the new doctrine, for on the old theory they are sufficiently explicable. The summary of arguments in favour of Phlogiston, published by Dr. Priestley, in 1803, are evidently too important, and too difficult of reply, to be slighted by those who adopt the opposite opinions. _Non nostri est tantas componere lites._ Should the old theory ultimately fall, it maybe fairly said of its respectable supporter, _si Pergama dextra defendi potuit, etiam hac defensa fuisset_.

This was almost the last of Dr. Priestley’s chemical publications,[56] through all which, his characteristic talent as an author has been eminently preserved, that of not only adding greatly to the existing stock of knowledge, but exciting others to exertion and reflection in the same line of pursuit. Nor can I help thinking that much of the labours of the French philosophers in this department of science would never have been undertaken, if they had not been called forth by the previous discoveries, not of Lemery, Margraaf, Bayen, Macquer, and Beaumè, but of Hales, Black, and Macbride; of Cavendish and Priestley and Scheele.[57] Would to God there were no other object of contest between the rival nations of Great Britain and France, but which should add most to the sum of human knowledge, and contribute most to the means of human happiness.

[56] To the end of this Appendix will be subjoined a list of the scattered papers on Philosophical subjects which Dr. Priestley published in periodical collections, besides those which are inserted in the Philosophical transactions.

[57] I do not mean to deny the tribute of praise to Marriotte and Venel, any more than to Brownrigg and Lane, and it is certain that Lavoisier was engaged in pneumatic experiments, previous to 1774.

It is impossible to conclude the preceding account better than by the following extract of a letter to Mr. Lindsey from a man[58] well able to appreciate the labours of Dr. Priestley; and the late testimony in favour of his discernment by Dr. Bostock. “To enumerate Dr. Priestley’s discoveries, would in fact be to enter into a detail of most of those that have been made within the last 15 years. How many invisible fluids whose existence evaded the sagacity of foregoing ages has he made known to us? The very air we breathe, he has taught us to analyze, to examine, to improve: a substance so little known, that even the precise effect of respiration was an enigma until he explained it. He first made known to us the proper food of vegetables, and in what the difference between these and animal substances consisted. To him Pharmacy is indebted for the method of making artificial mineral waters, as well as for a shorter method of preparing other medicines; metallurgy for more powerful and cheap solvents; and chemistry for such a variety of discoveries as it would be tedious to recite: discoveries which have new modelled that science, and drawn to it and to this country, the attention of all Europe. It is certain that since the year 1773, the eye and regards of all the learned bodies in Europe have been directed to this country by his means. In every philosophical treatise, his name is to be found, and in almost every page. They all own that most of their discoveries are due either to the repetition of his discoveries, or to the hints scattered through his works.”[59]

[58] Richard Kirwan, Esqr.

[59] Vindiciæ Priestlianæ, p. 68.

“This is not the only instance” (says Dr. Bostock,[60] speaking of Mr. Jurin’s opinion that azote was generated, instead of being absorbed, in the process of respiration as Dr. Priestley, and after him Mr. Davy had supposed,) “in which, after the conclusions of Dr. Priestley have been controverted by his contemporaries, a more accurate investigation of the question, has ultimately decided in his favour. The complicated apparatus, and imposing air of minuteness which characterize the operations of the French chemists, irresistibly engage the assent of the reader, and scarcely permit him to examine the stability of the foundation upon which the structure is erected. The simplicity of the processes employed by Dr. Priestley, the apparent ease with which his experiments were performed, and the unaffected conversational stile in which they are related have, on the contrary been mistaken for the effects of haste and inaccuracy. Something must also be ascribed to the theoretical language which pervades, and obscures the chemical writings of this Philosopher, in consequence of his unfortunate attachment to the doctrine of Phlogiston.”

[60] Essay on respiration, p. 208.

When the operose experiment of the French chemists on the formation of water, shall have been sufficiently repeated, and verified by other experiments to the same point, less complex, less tedious, less expensive, and easy to be repeated; when the water thus supposed to be formed is sufficiently distinguished from the water absolutely necessary to the generation of all airs, and attendant upon them[61] both in a state of mixture and combination; and when the difficulties enumerated a page or two back, as attendant on the modern theory shall be explained on the new system, as well as on that of Stahl, then, and not until then, will it be time to lament Dr. Priestley’s unfortunate attachment to the doctrine of Phlogiston.

[61] Mr. Kirwan found that common inflammable air from iron, and vitriolic-acid, contained about 2-3 of its weight of water mixed with it; which might be separated from the air by means of concentrated vitriolic-acid in a watch glass over mercury, without diminishing the quantity or altering the characteristic properties of the air thus treated.

_Of Dr. Priestley’s other Scientific Works._

The other philosophical labours of Dr. Priestley consist of his history of electricity, his history of the discoveries relating to light and colour, and his popular introductions to perspective, electricity and natural philosophy.

It appears that after the publication of his history of electricity, he intended to have pursued the plan, by composing similar histories of every branch of science: a magnificent idea, and which none but a man conscious of uncommon powers could have contemplated. Few men indeed were so capable of such an undertaking as Dr. Priestley; for independant of his habits of patient and regular industry in his literary pursuits, and the wide field of his attention to scientific objects, he had a facility of perusing, abstracting, and arranging the works of others, not commonly attendant even upon equal abilities in other respects. This great undertaking of Dr. Priestley to embrace the various departments of philosophy, appears a labour sufficient for one life; and had due encouragement been afforded, this projected series of histories would in all probability have been compleated, usefully to the world, and reputably to himself. But he proposed this undertaking laborious as it was, without designing that it should occupy the whole or the principal portion of his time, but his leisure hours only; for at no period did he postpone his professional duties, or his theological studies, to any other object whatever. The life of Dr. Priestley is almost a perpetual illustration of a seeming paradox, respecting mental energy, that men of talents, uncommonly laborious, and who appear to get through more business than one person could be supposed equal to, have usually more leisure time at their disposal, than those who have little to do: so much does the habit encrease the power of exertion. Nor was any man less averse to the innocent pleasures of social enjoyment than Dr. Priestley, or better calculated as well as more inclined to contribute to the common stock of amusing, and instructive conversation. It cannot indeed be truly said of him, as Dr. Johnson[62] once related of himself, that he had never refused an invitation to dinner on account of business but once in his life, yet no man more readily found leisure for social intercourse. This arose from his habit of dividing his time into certain portions appropriated to his respective pursuits, and determining to perform a certain quantity of literary duty, within the assigned period.