CHEMICAL HISTORY OF RESPIRATION. 93 



ing life, either when inhaled in a pure state, or when mixed, 

 in considerable quantity, with atmospheric air. Its occurrence 

 in mines and wells, as Choke-damp, was also noticed, and the 

 means of removing it from wells and similar places, by means 

 of lime-water, described, with some anecdotes in illustration of 

 its fatal effects in such situations, and of the ease with which 

 those effects might have been avoided*. 



With this previous information, the alteration induced in 

 the air by breathing was considered. The subject was opened 

 by some particulars of the circulation of the vital fluid ; and 

 it was then explained in the following order: the purple colour 

 of blood drawn from the veins is changed to the florid red of 

 arterial blood, on its exposure to the air, and this change 

 is more rapidly effected by its exposure to pure oxygen: 

 the same obvious change takes place with the venous blood 

 exposed, in the lungs, to the action of the air, in breath- 

 ing : Dr. Black (the discoverer of the distinct existence of 

 carbonic acid gas) h'rst noticed that the air exhaled from the 

 lungs contains a considerable quantity of carbonic acid: Dr. 

 Priestley, some years afterwards, observed that air is rendered 

 unfit for supporting flame or animal life, by the process of re- 

 spiration, from which fact it was probable that oxygen is con- 

 sumed during that process : the French chemist Lavoisier sub- 

 sequently established the facts, that during respiration oxygen 

 gas disappears, and that carbonic acid is evolved in its place ; 

 and it has since been ascertained, by our own countrymen 

 Messrs. Allen and Pepys, that, in the respiration of man, the 

 quantities of oxygen consumed, and of carbonic acid evolved, 

 are equal ; so that the oxygen of the air is employed in with- 

 drawing a portion of carbon from the venous blood. Each of 

 these stages of discovery (excepting the last) was illustrated by 

 the appropriate experiments, of detecting the presence of car- 

 bonic acid gas in respired air by its precipitating lime-water, 

 ascertaining that respired air extinguished a taper, and ob- 

 serving the diminution in bulk of the respired air on removing 

 the carbonic acid by lime-water. 



After a brief general view of the Simple Non-metallic Com- 

 bustible Bodies, the specific consideration of the more import- 

 ant of them (regarded with respect to the objects of the course 

 of instruction here detailed) was commenced with that of 

 HYDROGEN. Hydrogen gas being prepared on the lecture- 

 table, in the usual manner, by the decomposition of water 

 effected by the affinities which come into play when iron or 



* See the remarks offered in p. 36, on the use of Chemical knowledge, 

 in reference to this subject. 



