230 EIGHTEENTH CENTURY, PT. ill. 



he even melted solid minerals in acids and tested them in 

 the same way. 



One of the first uses that he made of his tests was to try- 

 Black's * fixed air.' When he heard of this gas he suspected 

 that it must be an acid, because it joined itself to lime, 

 which is an alkali, that is, a substance in all respects unlike 

 an acid ; and he had found that unlike substances nearly 

 always attract each other most strongly. So he made 

 some * fixed air ' and tested it with blue litmus, and, as the 

 litmus turned red directly, he knew that he was right in 

 supposing it to be an acid, and he called it * aerial acid,' or 

 acid air. He then weighed it and proved that it was heavier 

 than common air, and bypassing it through water he showed 

 that a large quantity of it would dissolve. 



Thus these two men. Black and Bergmann, had arrived 

 at a pretty good knowledge of this gas. They had proved 

 that it is an invisible heavy kind of air ; that it dissolves in 

 water ; that it is acid and joins itself to lime, forming lime- 

 stone or chalk ; that it destroys life when breathed, and 

 puts out a flame ; that it is given out by fermenting liquids, 

 and from mineral springs, and is contained in our breath. 

 One thing they had not found out, namely, that it is made 

 up of two elements ; this, as you will see (p. 238), was dis- 

 covered by Lavoisier in 1779, when he gave it the name of 

 * carbonic acid.' 



Discovery of Hydrogen by Cavendish, 1766. — The next 

 gas discovered was hydrogen, and its discoverer was Henry 

 Cavendish, grandson of the Duke of Devonshire, who was 

 bom in 1731. Cavendish was a very shy and reserved man, 

 who lived much alone and found his greatest pleasure in 

 studying science for its own sake. It is even related of him 

 that he taught all his servants to understand by signs what 



