38 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



" 2. That the quantity of tin calcined is greater in a large 

 retort than in a small one. . . . 



" 3. That the sealed retorts, weighed before and after the 

 calcination of a portion of the tin which they contain, show 

 no difference in weight, proving that the gain in weight of 

 the metal does not come from the fire nor from anything 

 outside the flask. 



"4. . . . That in the calcination of tin, the gain in weight 

 of the metal is almost exactly equal to the weight of the 

 quantity of air admitted, proving that the part of the air 

 which combines with the metal during calcination has almost 

 the same density as that of atmospheric air " ( Works, II. 

 118-119). 



Having thus shown that air was made up of two parts, 

 one of which was absorbed by tin, whilst the other was not 

 acted on, Lavoisier set himself to obtain separately the 

 part of the air which combines with metals and enables 

 substances to burn. He was able to imagine what 

 the properties of this gas must be : it must support com- 

 bustion, and probably substances would burn in it with great 

 ease. He tried to obtain it from metallic calces in which 

 he knew it was present, but was unable to set it free by 

 any of the methods at his disposal. The recovery of this 

 lost air, which Lavoisier required to complete the proof of 

 his theory of combustion, was, however, accomplished within 

 a few months by the independent work of Priestley in 

 England and of Scheele in Sweden. 



B. THE DISCOVERY OF OxvGEN. 1 



Priestley (1774) discovers a gas richer than common 

 air. The facts which Lavoisier required in order to com- 

 plete the proof of his theory of combustion were supplied 



1 The discovery of oxygen followed that of most of the gases 

 described in Chapter V. It is described here in order to complete 

 the story of the calcination of metals. 



