540 SPECIAL METHODS OF DISCOVERY. 



Hales also, about the same time, made the same kind of 

 experiments, and obtained similar results. Dr. Black, in 

 the year 1755, made, by means of experiments and ob- 

 servations, one of the first steps towards the discovery of 

 the properties of individual gases. He found that lime- 

 stone consisted of a gas united to caustic lime, and that 

 by the union or fixation of this gas (or ' fixed air,' as it 

 was then called) to magnesia, lime, soda, or potash, these 

 bodies lost their causticity, and became ' mild ' alkalies. 

 He, by experiment, discovered the same gas in our breath, 

 and in fermenting beer. He further found out some of its 

 chief properties, that it would precipitate lime-water, &c. ; 

 but it was Bergmann who, by additional experiments, dis- 

 covered it to be an acid. 



Cavendish, in the year 1766, discovered hydrogen, by 

 adding zinc or iron to dilute sulphuric acid, and collecting 

 the gas, and testing its properties in various ways. He 

 found that it was combustible, burning with a blue flame, 

 that it would not support animal life, nor the combustion of 

 a candle flame, that it was very light, and that when mixed 

 with air the mixture exploded, on the application of a flame. 

 Bergmann, about the year 1770, was one of the first to 

 employ chemical tests, and by means of them to analyse sub- 

 stances, mineral waters, &c. He suspected the ( fixed air ' 

 of Dr. Black to be an acid, because it united itself to lime, 

 and he proved it by means of experiments, for he knew 

 that dissimilar substances united chemically together. He 

 discovered the specific gravity of that gas, and its solu- 

 bility in water, and called it ' aerial acid,' or acid air. It 

 was by means of well-conceived experiments upon oxide of 

 mercury that Priestley, in 1774, discovered oxygen ; and 

 Scheele, by using black oxide of manganese, also dis- 

 covered it during the same year. Wenzel made and pub- 

 lished, in 1777, many accurate experiments of chemical 



