II. THE ATMOSPHERE 23 



Theoretically, assuming that the composition of the atmosphere is 

 not modified by any disturbing influences, the amount of carbon di- 

 oxide should diminish with the height above the sea-level. Experi- 

 mental evidence on this point is conflicting. Truchot found distinctly 

 less at heights of 1440 ('020 per cent) and 1880 metres (-0172 per 

 cent) than nearer the sea-level (-0313), while recently (1899) Thierry 1 

 found on Mont Blanc -0262 per cent at 1080 metres and -0269 per 

 cent at 3050 metres. Miintz and Aubin 2 found in 1882, as a mean 

 of many analyses, -0286 volumes on the Pic du Midi (2877 metres), 

 practically the same as in the plains. The earlier observers, too, 

 found more on the tops of mountains than at sea-level. . It is obvious 

 that vegetation must have a great local influence on the amount of 

 carbon dioxide and may, in some cases, cause misleading indications. 



Among the many causes tending to increase the amount of carbon 

 dioxide in the air, the following are the most important : 



1. Emission from volcanoes, deep springs, and other subterranean 

 sources ; immense quantities are thus sent into the atmosphere. 



2. Oxidation of carbonaceous matter ; this occurs in the processes 

 of respiration of animals and plants, the decay and fermentation of 

 animal and vegetable products, and the combustion of most fuels. 



3. The dissociation of carbonates by heat ; this is seen in such 

 processes as lime-burning. 



4. The decomposition of calcium bicarbonate by shell-fish, the cal- 

 cium carbonate being retained in building up the shell, and the carbon 

 dioxide evolved. 



The main cause tending to diminish its quantity is the decomposition 

 effected by the green portions of plants under the influence of light. 

 The rapidity with which this absorption of carbon dioxide occurs is 

 astonishing (vide Chap. XI). Other causes which remove it are the 

 weathering of rocks, e.g., the conversion of felspar into kaolin 



AL,0 3 K 2 0.6SiO,+ CO., + 10H 2 O = Al 2 O 3 .2SiO 2 .2H 2 + K 9 C0 3 



+ 4H 4 Si0 4 . 



and the conversion of normal into acid carbonates. Indeed, to this 

 last- mentioned action and its reverse, in the case of calcium carbonate 

 in sea-water, Schloesing attributes the maintenance of the constancy 

 of composition of the atmosphere above the ocean. If, from any cause, 

 the quantity of carbon dioxide in the air above the ocean increases, an 

 increased amount goes into solution as calcium bicarbonate, whereas, if 

 the quantity in the air diminishes, a portion of the dissolved bicarbonate 

 dissociates, thus liberating some of the gas. 



Ammonia is a small but important constituent. It exists, prob- 

 ably, as carbonate, nitrite and nitrate in the air. Its amount is 

 very variable and is always greater in town than in country air. 



Truchot in 1874 found 0'93 to 2'79 milligrams of ammonia in 1 

 cubic metre of air collected in Auvergne at a height of 395 metres 

 above the sea, on the Puy de Dome (1446 metres) he found 3-18 milli- 

 grams, and on the Pic du Saucy (1884 metres) 5 -5 milligrams per 

 cubic metre. He concludes that the amount of ammonia increases with 



iCompt. Rend., 129, 315. , 2 76td., 93, 797. 



