736 THE POPULAR SCIENCE MONTHLY. 



so, on the new theory, they were regarded as compounds of phlo- 

 giston, 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 dis- 

 covery 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 L^me- 

 ry, 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 be- 

 came 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 ; be- 

 cause, 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 won- 

 derful power of sustaining combustion to the absence of jjhlogis- 

 ton, which oxygen gas could therefore absorb to a proportionally 

 great extent. On the other hand, hydrogen was called phlogisti- 

 cated 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- 



