280 NITROGEN. 



Besides these constituents, the atmosphere always contains 

 a variable quantity of watery vapour and carbonic acid gas. The 

 presence of the latter is observed by exposing to the air a bason 

 of lime-water, which is soon covered by a pellicle of carbonate 

 of lime. Its proportion is generally ascertained by adding ba- 

 rytes-water of a known strength, from a graduated pipette, to 

 a large bottle of the air to be examined 5 agitating after each 

 addition, till a slip of yellow turmeric paper is made permanently 

 brown by the barytes-water after agitation, which proves that 

 more of the latter has been added than is neutralized by the 

 carbonic acid of the air. The carbonic acid is in the equivalent 

 proportion (by weight) of the quantity of barytes which has 

 been neutralized. Like every subject connected with the atmo- 

 sphere, the proportion of carbonic acid which it contains has 

 been ably investigated by the Saussures. The elder philosopher 

 of that name detected the presence of this gas in the atmosphere 

 resting upon the perpetual snows of the summit of Mont Blanc, 

 so that there can be no doubt that carbonic acid is diffused 

 through the whole mass of the atmosphere. The younger Saus- 

 sure has ascertained, by a series of several hundred analyses of 

 air that the mean proportion of carbonic acid is 4.9 volumes in 

 10,000 volumes of air, or almost exactly 1 in 2000 volumes 5 

 but it varies from 6.2 as a maximum to S.J, as a minimum in 

 10,000 volumes. Its proportion near the surface of the earth is 

 greater in summer than in winter, and during night than during 

 day upon an average of many observations. It is also rather 

 more abundant in elevated situations, as on the summits of 

 high mountains, than in the plains ; a distribution of this gas 

 which proves that the action of vegetation at the surface of the 

 earth is sufficient to keep down the proportion of it in the at- 

 mosphere, within a certain limit.* An enormous quantity of 

 carbonic acid is discharged from the elevated cones of the active 

 volcanoes of America, according to the observations of Bous- 

 singault, which may partly account for the high proportion of 

 that gas in the upper regions of the atmosphere. The gas 

 emitted from the volcanoes of the old world, according to Davy 

 and others, is principally nitrogen. 



Carbonic acid is a constituent of the atmosphere which is 

 essential to vegetable life, plants absorbing that gas, and all 



* Saussure, An. de Ch. et de Ph. t. 38, p. 411, and t.44, p. 5. 



