880. PRINCIPLES OF CHEMISTRY 



position, showing that it contains twelve 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 which 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 (Chapter V.) in the air, and in solution is in all kinds of 

 water. It is evolved from volcanoes, from mountain fissures, and in 

 some caves. The well-known Dog grotto, near Agnano on the bay of 

 Baise, near Naples, furnishes the best known example of such an evolu- 

 tion. 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 nea'r the Rhine, in .the neighbourhood of extinct vol- 

 canoes, there is a depression 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 quantities of this gas. Vichy in France, Spriidel in Germany, 

 and Narzan in Russia (in Kislovodsk near Piatigorsk) are known for 

 their carbonated gaseous waters. Much of this gas is also evolved in 

 mines, cellars, diggings, and wells. People descending into such places 

 are suffocated. The combustion, putrefaction, and fermentation of 

 organic substances give rise to the formation of carbonic anhydride. 

 It is also introduced into the atmosphere during the respiration 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) in water a white 

 precipitate will soon be formed consisting of an insoluble compound of 

 lime and carbonic anhydride. By allowing the seeds of plants to grow 

 under a bell jar, or in a closed vessel, the formation of carbonic anhy- 

 dride may be similarly confirmed. By confining an animal, a mouse, for 

 instance, under a bell jar, the quantity of carbonic acid which it evolves 

 may be exactly determined, and it will be found to be many grams per 

 day fqr a mouse. Such experiments on the respiration of animals have 



