Action of' the Atmosphere upon neKhj- deepened Soil. 425 



The atmosphere also seems always to contain a variable Ijut 

 small proportion of carbonic acid. The proportions of this in- 

 gredient in 10,000 volumes of air have been found to be, by 



Vulumes. 



Dalton G-S 



Saussure, maximum .. .. 5'74 1 -ni -mt i i- j. ^ 



' , T r- I i I'om lOi ODseiTatious takou near 



,, mean 4-15 > ,-, 



" ■ • o T o I (jreneva. 



Thenard 3-91 



Boussiugault and Levy .. 3'253 At Paris in October. 



,, .. 2-980 At Amdilly, 16 miles from Paris, ■ 



Saussure's investigations were continued for two years, and he 

 states that the proportion of carbonic acid varies at the same 

 place within short intervals of time, is greater in summer than 

 in Avinter, and more abundant in the night than in the day. 

 Saussure also found that a continuance of heavy rains dhninished. 

 the quantity of carbonic acid in the air by dissolving or carrying 

 down part of it to the ground. When the soil is soaked with 

 water it has the power of imbibing this gas, and its greater 

 abundance in the air in frosty weather has been thought owing 

 to the effect of the frost in keeping the air and soil dry. It has 

 been found more plentiful in elevated situations and on moun- 

 tain tops than in tlie lower grounds ; wind having a tendency to 

 augment it in those localities. 



Another substance apparently always present, but in still 

 smaller proportion, is ammonia. The quantity is so minute that 

 its presence cannot be recognised in any given quantity of air, 

 but Liebig showed that it could be detected l)y looking for it in 

 rain-water, which carries down the ammonia in its passage 

 tlirough the air, and according to his own experiments invariahlij 

 contains it, the form being that of a carbonate. The quantity is 

 greatest in the first shower after a continuance of dry weather, and 

 least after a succession of rains. Its amount prol)ably varies in 

 difftn'ent places and at different times, and from the researt lies of 

 Barral and Bousslngault it would appear to occur more abund- 

 antly in the atmospliere surrounding cities. 



Another atmospheric ingredient is nitric acid. Cavendish had 

 observed that by passing electric sparks through moist air its 

 voluitu! was diminished, and an acid soluble in water was at tlie 

 same time produced. He also established the fact that the o.wgen 

 and nitrl)g('n of the air unite to form nitric acid when acted upon by 

 electricity. Henry also noticed that ammonia mixed with oxygen 

 is by the same agency converted into nitric acid. 



Recent observations tend to prove that the quantity of nitrogen 

 in the nitric acid of the air exceeds that contained in the form 

 of aunuonia — it would seem then that a direct oxidation oi" the 



