250 The Rise of the Modern [lect. 



preferred to call it, being unlike some other makers of new 

 terms chary of using his new word, oxygine) which disappeared 

 when an animal was made to breathe a measured atmosphere of 

 it for a given time. He also estimated the quantity of carbonic 

 acid gas given out during the same time ; and knowing by this 

 time the exact composition of carbonic acid, knowing how much 

 oxygen was present in a given quantity of carbonic acid, he found 

 that all the oxygen which disappeared did not reappear in the 

 carbonic acid respired. Some of the oxygen was used for 

 something else than the combustion of carbon and the pro- 

 duction of carbonic acid. 



Now in expired air there was nothing present in measurable 

 quantity except carbonic acid and the substance known as water. 



Here I must go back a little. 



Van Helmont found that his gas, of which, as seen in gas 

 sylvestre, the conspicuous feature was that it would not support 

 burning, though sometimes uninflammable, sometimes caught 

 fire and burnt. Boyle in 1672 recognized that the air or gas given 

 off when metals were dissolved in acids was inflammable. And 

 during the eighteenth century mention is from time to time made 

 of factitious air, and of this air being often inflammable. Hales 

 refers to it; and Haller speaks of factitious air, such as is produced 

 by the action of acids on metals, and is frequently inflammable, 

 as being unfit for respiration, although it is elastic. As we have 

 seen, Haller attributed much importance in respiration to the 

 fact that air in being breathed lost its elastic power, and 

 thought the possession of elastic power a feature of respirable 

 air; hence he found a difficulty in elastic factitious air not 

 being respirable. 



It is with Cavendish however, and his experiments 

 on factitious air in 1766, that our real, exact knowledge of 

 inflammable factitious air begins; and when in 1781 he dis- 

 covered the composition of water, this mysterious gas became 

 henceforth known as hydrogen. It was the last of the four 

 chief physiological gases to be ruQ to earth. As we have seen, 

 carbonic acid gas first laid hold of van Helmont in 1640 

 or thereabouts, was more firmly grasped by Black in 1757. 

 Nitrogen was first observed by Rutherford in 1772. Oxygen, 

 prepared by Priestley in 1774, was recognized by Lavoisier in 



