INFLUENCE OF THE ATMOSPHERE. 73 



pounds, such as hydrogen, nitric acids, sulphureted and car- 

 bureted hydrogen. It is also probable that odoriferous, sa- 

 line, and metallic particles float in it ; all of which, save the 

 first, are found in exceedingly small quantities. The agency 

 of the atmosphere may be studied under the following heads. 

 Influence of its oxygen, of its nitrogen, of its ammonia, of its 

 nitric acid, of its sulphureted hydrogen, of its carbonic acid, 

 and its mechanical agency. 



I. Injluence of the oxygen of the air in vegetation. We 

 have seen, that oxygen is a most important agent in the pro- 

 cess of germination, combining with the hydrogen of the de- 

 composed water, and with the carbon of the seed, and that 

 carbonic acid is almost the only gaseous product evolved. 



Oxygen is no less necessary to the growth of plants. 

 This is proved by the fact, that when all other conditions are 

 supplied, if the plant is deprived of oxygen it will wither and 

 die ; hence, when the roots of trees are surrounded with 

 stagnant water, no oxygen being supplied to them, the leaves 

 turn yellow and fall, but when fresh water is added, yielding 

 the requisite quantity of oxygen, the tree will revive. 



Oxygen acts principally upon the roots and leaves oi plants- 

 The mode of its action in the roots has been differently rep- 

 resented by different chemists. There are four theories. 1. 

 The absorbed oxygen combines with the carbon of the plant. 



2. It combines with the hydrogen of the decomposed water. 



3. It is assimilated. 4. It combines with substances in the 

 soil by which food is prepared. 



First theory. The roots absorb oxygen and convert it by 

 means of their carbon into carbonic acid. The truth of this 

 theory is supposed to be proved by placing fresh roots depri- 

 ved of their stems under a bell-glass receiver. They will di- 

 minish the quantity of air, by abstracting its oxygen, and 

 forming carbonic acid. The volume of oxygen consumed is 

 never greater than the bulk of their roots. 



Place the roots thus saturated with oxygen in a receiver 



