46 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



weight, the gain in weight of the tin must have been accom- 

 panied by the disappearance of an equal weight of air. When 

 the flask was opened a quantity of air rushed in, the weight of 

 which was almost equal to that gained by the tin ; this showed 

 that the part of the air absorbed by the tin (about ) did not 

 differ greatly in density from the air outside. 



Joseph Priestley, in 1774, isolated the active part of the air 

 by heating the red calx of mercury, and showed that it was a 

 brilliant supporter of combustion and maintained respiration 

 much longer than ordinary air. He regarded it as air minus 

 phlogiston, and called it " dephlogisticated air." Lavoisier 

 then carried out quantitative experiments on the calcination of 

 mercury, in which he showed that the volume of gas set free 

 when the calx was heated was identical with the volume of air 

 absorbed in its preparation, whilst the weight of the gas set 

 free was equal to the loss in weight of the calx when reconverted 

 into metal. 



Since sulphur and phosphorus were converted into acids by 

 combination with the active part of the air, Lavoisier gave to it 

 the name oxygen, i.e., acid-producer, the same idea being- 

 implied in the expressive German name, sauerstoff. 



Oxygen was also prepared independently about 1774 by 

 Carl Wilhelm Scheele, who obtained it first by heating nitric 

 acid and nitre. He proved in many ways that air was a mixture 

 of an active and an inactive constituent, and showed that 

 ordinary air could be reproduced by adding " fire-air," or 

 oxygen, to the " foul air " remaining after rusting or burning 

 had taken place in it. 



The chemical changes described in this chapter may be 

 represented by the following equations : 



Calcination of tin . . Sn + O 2 = SnO 2 (white calx). 



(Stannic 

 oxide.) 



2Sn + O 2 = 2SnO (black calx). 



(Stannous 

 oxide.) 



lead . 2Pb + O 2 =2PbO (yellow calx). 



(Litharge.) 



-. 2Cu + O 2 = 2CuO (black calx). 



(Cupric 

 oxide.) 



