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