The attention of Dr. Priestley, however to these subjects was not originally excited by the works of his predecessors, but by the accident of his proximity to a brew-house at Leeds, where of course fixed air (a subject that had attracted much attention about that time) would be produced in a large way. It was thus that one experiment led to another, until the fruits of his amusements were the discoveries on which his philosophical reputation is principally founded. It is no more than justice to his character to mention in this place, that of all men living he was the freest from literary deception and the vanity of authorship. He never claims the merit of profound investigation or great foresight, for discoveries that might easily have been so stated as if they had been the pure result of those qualifications, but which were in reality the offspring of accident and circumstance. He excites others to patient labour in the field of experiment, from observing that success does not depend so much on great abilities or extensive knowledge, as on patient attention, and perseverance; and that much of his own reputation was owing to the discovery of facts that arose in the course of his pursuits, the result of no previous theory, unlooked for and unexpected. In v. 3 p. 282 of his experiments on air he says “Few persons I believe have met with so much unexpected good success as myself in the course of my philosophical pursuits. My narrative will shew that the first hints at least of almost every thing that I have discovered of much importance have occurred to me in this manner. In looking for one thing I have general found another, and sometimes a thing of much more value than that which I was in quest of. But none of these unexpected discoveries appear to me to have been so extraordinary as that I am about to relate (viz. the spontaneous emission of dephlogisticated air from water containing a green vegetating matter) and it may serve to admonish all persons who are engaged in similar pursuits, not to overlook any circumstance relating to an experiment, but to keep their eyes open to every new appearance and to give due attention to it however inconsiderable it may seem.”[49] To this candour of disposition, and the readiness with which he acknowledged his mistakes and his oversights, even those who opposed his opinions bear honourable testimony. “The celebrated Priestley himself (says M. Berthollet in his reply to Kirwan on Phlogiston p. 124 of the Eng. translation) often sets us the example, by rectifying the results of some of his numerous experiments.”
[49] See also the 1st, vol. of his early edition of experiments on air p. 29.
Numerous indeed those experiments were as well as important: far too numerous to be particularised here; though it may not be improper to call to the recollection of the reader some of the more interesting facts which we owe to Dr. Priestley, and the times of their discovery and communication.
The first of his publications on pneumatic chemistry was in 1772, announcing the method of impregnating water with fixed air, and on the preparation and medicinal uses of artificial mineral waters; a discovery that domesticated much of the knowledge that had heretofore been disclosed only in the works of learned societies; and that beautifully exemplified how much of the health and the pleasure of common life, might depend on the ingenious researches of men of science. Though this was the first publication of Dr. Priestley on the chemistry of the airs, he had certainly commenced his experiments in this branch of Science, soon after his arrival at Leeds, and as early at least, as 1768. In the year 1771 he had already procured good air from saltpetre; he had ascertained the use of agitation, and of vegitation as the means employed by nature in purifying the atmosphere destined to the support of animal life, and that air vitiated by animal respiration was a pabulum to vegetable life; he had procured factitious air in a much greater variety of ways than had been known before, and he had been in the habit of substituting quicksilver in lieu of water, for the purpose of many of his experiments. In his paper before the Royal Society, in the spring of 1772, which deservedly obtained him the honour of the Copley Medal, he gives an account of these discoveries. In the same paper he announces the discovery of that singular fluid nitrous air,[50] and its beautiful application as a test of the purity or fitness for respiration of airs generally. In the same paper he shews the use of a burning lens in pneumatic experiments, he relates the discovery and properties of marine acid air; he adds much to the little of what had been heretofore known of the airs generated by putrefactive processes, and by vegetable fermentations, and he determines many facts relating to the diminution and deterioration of air, by the combustion of Charcoal, and the calcination of metals.
[50] Honestly referring to Dr. Hales and Mr. Cavendish for any idea that might have remotely led to this discovery (See Obs. on air 1st ed. v. 1 p. 108) the discovery however was completely his own.
Dr. Priestley seems always to have thought nitrous air as convenient a substance for eudiometrical experiments as any of the later substitutes, viz. the liquid sulphurets and the combustion of phosphorus. The foundation of Mr. Davy’s substitute, muriat or sulphat of iron saturated with nitrous air, was as Mr. Davy acknowledges first discovered by Dr. Priestley himself. See Nich. Journ. for Jan. 1802 p. 41. The different states of the solutions of iron in vitriolic acid have been ingeniously applied to the analysis of mixed gasses by Humboldt and Vauquelin.
Soon after this, in confirmation of Sir John Pringle’s theory of intermittents and low fevers being generally owing to moist miasma when people are exposed to its influence, he ascertained by means of his nitrous test that the air of marshes was inferior in purity to the common air of the atmosphere.[51]
He had obtained very good air from saltpetre in 1771, but his full discovery of dephlogisticated air, seems not to have been made until June or July, 1774,[52] when he procured it from precipitate per se, and from red lead. This was publicly mentioned by him at the table of Mr. and Madame Lavoisier, at Paris, in October 1774, to whom the phenomena were until then unknown. The experiments on the production of dephlogisticated air, he made before the scientific chemists at Paris about the same time, at Mr. Trudaine’s. This hitherto secret source of animal life and animal heat, of which Mayow had but a faint and conjectural glimpse, was certainly first exhibited by Dr. Priestley, and about the same time, (unknown to each other) by Mr. Scheele of Sweden. For the honour of science, it were much to be wished that the pretensions of Mr. Lavoisier were equally well founded. He has done sufficient and been praised sufficiently for what he has done, to satisfy a mind the most avaricious of fame; he is deservedly placed in the first rank among the philosophers of his day, and he ought not to have thrown a shade over his well earned reputation, by claiming for himself the honour of those discoveries which he had learned from another.
[51] Phil. trans. v. 54 p. 92.
[52] See Doctrine of Phlog. established p. 119.