so, on the new theory, they were regarded as compounds of phlogiston, and, in burning, the phlogiston was supposed to escape into the atmosphere. The ease with which such metals as zinc, iron, lead, and tin burn under certain conditions was well known to the chemists of that period, and hence all metals were regarded as largely composed of phlogiston; and when it was shown that the oxides, then called calces, resulting from the burning, weighed more than the metal burned, the facts were cited to prove that phlogiston was specifically light, and therefore, when removed from a body, added to its weight.
It has been said that the increase of weight resulting from burning and other forms of oxidation was not recognized until Lavoisier introduced the balance into chemical investigations at the close of the last century; but, although such phenomena could not be formulated under a general principle until after the discovery of oxygen in 1774 (nearly simultaneously both by Priestley and by Scheele), the fact that the so-called calces resulting from the burning of the metals weigh more than the metals was well known to metallurgists from a much earlier period. Thus Lémery, who died in 1715, in his well-known treatise on chemistry, describes the increase of weight attending the calcination both of tin and lead; and Boerhaave, a famous Dutch physician and chemist of the same period, thus describes the calcination of lead: "And if, while the lead is in fusion, it be kept continually stirring with a spatula, it turns into a red powder called minium, or red lead, in which operation this is further observable that the lead augments in weight."
During the eighteenth century the theory of phlogiston became modified by the increasing knowledge of the definiteness of chemical combination. Like the other constituents of a body, it was held that the phlogiston in combustibles must be united in definite proportions. So, moreover, when, leaving the fuel in the process of combustion, phlogiston entered into union with the air, it could only be absorbed by the atmosphere up to a certain limit. Hence, a candle soon goes out if burned in a confined vessel; because, after the air is saturated with phlogiston, no more can escape from the combustible. Priestley called oxygen gas, when first discovered, dephlogisticated air, because he ascribed its wonderful power of sustaining combustion to the absence of phlogiston, which oxygen gas could therefore absorb to a proportionally great extent. On the other hand, hydrogen was called phlogisticated air; and Cavendish, when he first isolated this exceedingly light and combustible gas, thought he had discovered phlogiston itself.
As has been already intimated, Aristotle's doctrine of the chemical elements was, in some form or other, received by stu-