[367] SCIENTIFIC MANUAL. 81 



While the composition of the atmosphere cannot be 

 changed by man, it is supposed that the application of 

 sulphate of lime (gypsum) to the surface of plants facili- 

 tates the absorption of a portion of its nitrogen as am- 

 monia. 



Rain, also, especially when accompanied by electricity, 

 carries down to the soil, for the use of plants, ammonia 

 from the atmosphere. 



In periods of drouth, the evaporation of moisture from 

 the surface of the leaves of plants is often in excess of its 

 absorption by their roots, and wilting of the leaves and 

 general contraction of the plant results. The plants re- 

 sume their normal condition during the night, by the res- 

 toration of the equilibrium between the evaporation from 

 the leaves, and the absorption of moisture by the roots. 



Unlike the soil, the atmosphere cannot be exhausted 

 either of its constituents necessary in plant nutrition, or of 

 the oxygen essential to animal respiration. 



When we consider the vast amount of carbon in the ve- 

 getation which covers our globe, and that the whole of it 

 has been derived from the atmosphere, the question natur- 

 ally arises, " How is this carbon restored to the air ?" 

 The carbon in all vegetation is derived from the atmos- 

 phere in the form of carbonic acid. 



This is a product of the decay and combustion of vege- 

 tation and animal matters, the combustion of coal and 

 oils, and of the respiration of animals. 



Each decaying leaf, each flickering taper, each respira- 

 tion of an animal, however small, yields up its contribu- 

 tion of carbonic acid to the atmosphere. 



Plants absorb carbonic acid during the day, through 

 their leaves and other green parts, assimilate the carbon, 

 and give off the oxygen to the air, while a small quantity 

 of carbonic acid is given off during the night. Animals 

 retain the oxygen, and exhale carbonic acid at all times, 



