502 SPECIAL METHODS OF DISCOVERY. 



chlorine in a somewhat similar manner, viz., by heating 

 black oxide of manganese with hydrochloric acid. 



The inflammable gas (now called hydrogen) evolved 

 from metals immersed in acids was also a neglected common 

 substance, and had long been known ; but, in the year 

 1781, Cavendish and Watt showed that this gas, by unit- 

 ing with oxygen in burning, produced water ; and after- 

 wards Lavoisier decomposed water into its elements ; 

 and subsequently also Humboldt and Gray Lussac dis- 

 covered that one volume of oxygen unites with exactly 

 two volumes of hydrogen to form water. Other investi- 

 gators had previously found different proportions. 



The heavy uninflammable gas (carbonic acid) produced 

 from limestone and in fermentation, was well known to 

 Paracelsus and Van Helmont, and was subsequently ex- 

 amined by Hales, Black, Priestley, and Bergmann ; but 

 Lavoisier showed it to be composed of carbon and oxygen, 

 and also was the first to prove carbon to be an element. 1 

 Scheele and Priestley, by investigating common air, dis- 

 covered that the atmosphere consisted of two kinds of gas, 

 one only of which supported life. Dalton's chemical theory 

 of the rule of multiple combining proportions of bodies, 

 in accordance with his theory of atoms, was suggested by 

 his experimental investigation of olefiant gas and carbu- 

 retted hydrogen. 



Anhydrous hydrofluoric acid was also long a neglected 

 substance. Several chemists of the greatest eminence made 

 limited investigations of it, but soon discarded the subject, 

 apparently on account of the extremely dangerous nature 

 of the substance ; the author then investigated the dis- 

 carded body during a period of about nine years, and was 

 enabled, by the combined use of vessels of platinum and 



1 Gmelin, Handbook of Chemistry, vol. ii. p. 82. 



