General Principles. 41 



Oxygen gas forms a considerable part of the 

 air of our atmosphere, (about 23 parts, in weight, 

 in 100,) the rest is nitrogen, or azotic gas, which 

 is not respirable, and will not support flame. 

 Oxygen is necessary also to animal life. Being 

 respired it unites with and removes the carbon 

 of the venous blood ; that which returns from 

 the lungs to the heart, thereby having acquired 

 a brighter colour. It has been supposed that 

 during respiration, oxygen becoming condensed, 

 parts with its caloric, and furnishes us with ani- 

 mal heat ; and that this is proved by every per- 

 son who uses laborious exercise breathing quick, 

 and becoming proportionably heated. But the 

 real origin of animal heat is not at present as- 

 certained. 



It supports combustion. An iron wire, after 

 being a little heated, and plunged in a vessel 

 containing oxygen gas, will burn and flame like 

 a candle. The oxygen, in this case, unites with 

 the metal, and converts it into an oxide, or calx 

 as it was formerly called, from its being produced 

 by burning, and resembling a calx or cinder. 



Oxygen has indeed a strong attraction for all 

 the simple acidifiable and combustible substances, 

 hydrogen, nitrogen, sulphur, phosphorus, car- 

 bon, and boron, forming different compounds. 

 With hydrogen it forms water; with nitrogen 

 atmospheric air ; with sulphur, sulphuric acid ; 

 with phosphorus, phosphoric acid ; with carbon, 

 carbonic acid gas, formerly called fixed air, and 

 with boron, boracic acid. 



