368 PRINCIPLES OF CHEMISTRY 



parts of carbon to thirty-two of oxygen. The composition by volume 

 of this gas is determined from the fact that during the combustion of 

 charcoal in oxygen, the volume remains unchanged ; that is to say, 

 carbonic anhydride occupies the same volume as the oxygen ivhich it 

 contains that is, the atoms of the carbon are, so to speak, squeezed in 

 between the atoms of the oxygen. O 2 occupies two volumes and is a 

 molecule of ordinary oxygen ; CO 2 likewise occupies two volumes, and 

 expresses the composition and molecular weight of the gas. Carbonic 

 anhydride exists in nature, both in a free state and in the most varied 

 compounds. In a free state it is always contained (Chap V.) in the 

 air, and in solution is in all kinds of water. It is evolved from vol- 

 canoes, from mountain fissures, and in some caves. The well known 

 Dog grotto, near Agnano on the bay of Baioe, near Naples, is the 

 best known example of such an evolution. Similar sources of carbonic 

 anhydride are also found in other places. In France, for instance, 

 there is a well-known poisonous fountain in Auvergne. It is a round 

 hole, surrounded with luxurious vegetation and constantly evolving 

 carbonic anhydride. In the woods surrounding the Lacher See, near 

 the Rhine, in the neighbourhood of extinct volcanoes, there is a de- 

 pression constantly filled with this same gas. The insects which fly to 

 this place perish, animals being unable to breathe this gas. The birds 

 chasing the insects also die, and this is turned to profit by the local 

 peasantry. Many mineral springs carry into the air enormous quan- 

 tities of this gas. Yichy in France, Spriidel in Germany, and Narzan 

 in Russia (in Kislovodsk near Piatigorsk) are known for their car- 

 bonated gaseous waters. Much of this gas is also evolved in mines, 

 cellars, diggings, and wells. For this reason sometimes people descend- 

 ing into such places are suffocated and die. The evolution of carbonic- 

 anhydride in the earth is accounted for by the slow oxidation of 

 organic matter underground. The combustion, putrefaction, and fer- 

 mentation of organic substances give rise to the formation of carbonic 

 anhydride. It is also introduced into thk atmosphere during the respi- 

 ration of animals at all times and during the respiration of plants 

 in darkness and also during their growth. Very simple experiments 

 prove the formation of carbonic anhydride under these circumstances ; 

 thus, for example, if the air expelled from the lungs be passed 

 through a glass tube into a transparent solution of lime (or baryta) 1 

 in water a white precipitate will soon be formed which contains an 

 insoluble compound of lime and carbonic anhydride. If a funnel 

 be held over a substance in a state of combustion for instance, 

 over a lighted candle or brazier and the air be collected from it by 

 means of an aspirator, the presence of a large amount of carbonic- 



