AMMONIA. 29 



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 calcium 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 absorp- 

 tion of carbon dioxide occurs is astonishing (v. Chap. IX.). 

 Other causes which remove it are the weathering of rocks, 

 e.g., the conversion of felspar into kaolin 



+ 4H 4 SiO 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 atmo-' 

 sphere above the ocean. If, from any cause, the quantity of 

 carbon dioxide in the air above the ocean increases, an in- 

 creased 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, 

 probably, 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 '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 milligrams, and on the Pic du Saucy (1884 

 metres) 5*5 milligrams per cubic metre. He concludes that 

 the amount of ammonia increases with the elevation and is 

 greater in cloudy than in clear weather.* These amounts are 

 much greater than have been found by other observers, and 

 his conclusions as to distribution of the ammonia are not gene- 

 rally accepted. Levy at Montsourisf found as a mean 1*68 



* J.C.S. 1874, abstracts, 223. t J.C.S. 1877, 509 ; 1878, 243, and 1880, 848, 



