ITS SOURCE THE ATMOSPHERE. 19 



our winter, when artificial warmth must replace deficient heat 

 of the sun, carbonic acid is produced in superabundance, and is 

 expended in the nourishment of tropical plants. The great 

 stream of air, which is occasioned by the heating of the equator- 

 ial regions and by the revolution of the earth, carries with it in 

 its passage to the equator the carbonic acid generated during our 

 winters ; and, in its return to the polar regions, brings with it the 

 oxygen produced by the tropical vegetation. 



The experiments of De Saussure have proved, that the uppei 

 strata of the air contain more carbonic acid than the lower, 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 interchange of air between the upper and lower strata, caused 

 by their difference of temperature, is extremely trifling when 

 compared with the horizontal movements of the winds. Thus 

 vegetable culture heightens the healthy state of a country, so 

 that a previously healthy country would be rendered quite unin- 

 habitable by the cessation of all cultivation. 



The various layers of wood and mineral coal, as well as peat, 

 form the remains of a primeval vegetation. The carbon con- 

 tained in them 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 equal in volume to the carbonic acid abstracted in the 

 nourishment of a former vegetation, and must, therefore, corres- 

 pond to the quantity of carbon and hydrogen contained in the 

 carboniferous deposit. Thus, by the deposition of ten cubic feet 

 Hessian (5-51 cubic feet English) of Newcastle splint coal (of 

 the formula C 2 4 H 1 3 0, and specific gravity 1228), the atmosphere 

 must have been deprived of above eighteen thousand cubic feet 

 Hessian (9918 cubic feet English) of carbonic acid, and must 

 'nave been enriched with the same quantity of oxygen. A further 



