ITS SOURCE THE ATMOSPHERE. 47 



The experiments of De Saussure have proved, 

 that the upper strata of the air contain more car- 

 bonic acid than the low^er, which are in contact with 

 plants ; and that the quantity is greater by night 

 than by day, when it undergoes decomposition. 



Plants thus improve the air, by the removal of 

 carbonic acid, and by the renewal of oxygen, which 

 is immediately applied to the use of man and animals. 

 The horizontal currents of the atmosphere bring 

 with them as much as they carry away, and the in- 

 terchange of air between the upper and lower strata, 

 which their difference of temperature causes, is 

 extremely trifling when compared with the horizon- 

 tal movements of the winds. Thus vegetable culture 

 heightens the healthy state of a country, and a 

 previously healthy country would be rendered quite 

 uninhabitable by the cessation of all cultivation. 



The various layers of wood and mineral^coal, as 

 well as peat, form the remains of a primeval vegeta- 

 tion. The carbon which they contain must have 

 been originally in the atmosphere as carbonic acid, 

 in which form it was assimilated by the plants which 

 constitute these formations. It follows from this, 

 that the atmosphere must be richer in oxygen at the 

 present time than in former periods of the earth's 

 history. The increase must be exactly proportional 

 to the quantity of carbon and hydrogen contained 

 in these carboniferous deposits. Thus, during the 

 formation of 353 cubic feet of Newcastle splint-coal, 

 the atmosphere must have received 643 cubic feet 

 of oxygen produced from the carbonic acid assim- 

 ilated, and also 158 cubic feet of the same gas 

 resulting from the decomposition of water. In 

 former ages, therefore, the atmosphere must have 

 contained less oxygen, but a much larger proportion 

 of carbonic acid, than it does at the present time, 

 a circumstance which accounts for the richness and 

 luxuriance of the earlier vegetation. 



But a certain period must have arrived in which 

 the quantity of carbonic acid contained in the air 



