[These selections from Priestley's account of the discovery of oxygen and from Lavoisier's first formal presentations of his theory of acids are classical examples of scientific work which will always be worth reading. They have also the historical interest due to the fact that the discoveries they describe served as the turning-point of chemistry to the paths it has since followed. The dates of publication were respectively 1775, 1776 and 1777. We realize the progress of the century when we remember that these experiments are now among the first in an elementary course. These two papers are also representatives of two well-defined types of scientific advance; Priestley's discovery was one of the happy accidents that often reward the investigator, one of the cases where he reaps a hundred fold, while Lavoisier's work was the result of gifted insight and careful consideration of the entire range of phenomena concerned. Lavoisier had, as is shown in this paper, the faculty of giving the right meaning to the data acquired by others. The phlogiston theory is now so much a matter of antiquity that it seems proper to give the modern equivalents of some of Priestley's terms: Air is used by him in the modern sense of gas, dephlogisticated air=oxygen, inflammable air=hydrogen, phlogisticated air=nitrogen, marine acid air=hydrochloric acid gas, fixed air=carbon dioxid, nitrous air=nitric oxid (N O), dephlogisticated nitrous air=nitrous oxid (N20), vitriolic acid air=sulphur dioxid, mercurius calcinatus=red oxid of mercury.]
THERE are, I believe, very few maxims in philosophy that have laid firmer hold upon the mind than that air, meaning atmospherical air (free from various foreign matters, which were always supposed to be dissolved, and intermixed with it), is a simple elementary substance, indestructible and unalterable, at least as much so as water is supposed to be. In the course of my inquiries I was, however, soon satisfied that atmospherical air is not an unalterable thing; for that phlogiston with which it becomes loaded from bodies burning in it, and animals breathing it, and various other chemical processes, so far alters and depraves it, as to render it altogether unfit for inflammation, respiration and other purposes to which it is subservient; and I had discovered that agi-
↑From 'Experiments and Observations on Different Kinds of Air.' London, 1775.
