156 THE HISTORY OF SCIENCE 



he ascribed to latent heat. The first discovery (1754) 

 was the result of the preparation of quicklime in the 

 practice of medicine ; the second (1761) involving 

 experiments on the temperatures of melting ice, boil- 

 ing water, and steam, stimulated Watt in his improve- 

 ment of the steam engine. In 1766 Joseph Priestley 

 began his study of airs, or gases. In the following year 

 observation of work in a brewery roused his curiosity 

 in reference to carbonic acid. In 1772 he experi- 

 mented with nitric oxide. In the previous century 

 Mayow had obtained nitric oxide by treating iron 

 with nitric acid. He had then introduced this gas 

 into ordinary air confined over water, and found that 

 the mixture suffered a reduction of volume. Priestley 

 applied this process to the analysis of common air, 

 which he discovered to be complex and not simple. 

 In 1774, by heating red oxide of mercury by means 

 of a burning-glass, he obtained a gas which sup- 

 ported combustion better than common air. He in- 

 haled it, and experienced a sense of exhilaration. 

 " Who can tell," he writes, " but in time this pure air 

 may become a fashionable article in luxury ? Hith- 

 erto only two mice and myself have had the privilege 

 of breathing it." 



The Swedish investigator Scheele had, however, 

 discovered this same constituent of the air before 

 1773. He thought that the atmosphere must consist 

 of at least two gases, and he proved that carbonic 

 acid results from combustion and respiration. In 

 1772 the great French scientist Lavoisier found that 

 sulphur, when burned, gains weight instead of losing 

 weight, and five years later he concluded that air 

 consists of -two gases, one capable of absorption by 



