44 OF THE ASSIMILATION OF CARBON. 



sun's light, the carbonic acid is, after a time, found 

 to have disappeared entirely from the water. If the 

 experiment is conducted under a glass receiver filled 

 with water, the oxygen emitted from the plant may 

 be collected and examined. When no more oxygen 

 gas is evolved, it is a sign that all the dissolved car- 

 bonic acid is decomposed ; but the operation recom- 

 mences if a new portion of it is added. 



Plants do not emit gas when placed in water which 

 either is free from carbonic acid, or contains an al- 

 kali that protects it from assimilation. 



These observations were first made by Priestley 

 and Sennebier. The excellent experiments of De 

 Saussure have further shown, that plants increase in 

 weight during the decomposition of carbonic acid 

 and separation of oxygen. This increase in weight 

 is greater than can be accounted for by the quantity 

 of carbon assimilated ; a fact which confirms the 

 view, that the elements of water are assimilated at 

 the same time. 



The life of plants is closely connected with that 

 of animals, in a most simple manner, and for a wise 

 and sublime purpose. 



The presence of a rich and luxuriant vegetation 

 may be conceived without the concurrence of animal 

 life, but the existence of animals is undoubtedly de- 

 pendent upon the life and development of plants. 



Plants not only afford the means of nutrition for 

 the growth and continuance of animal organization, 

 but they likewise furnish that which is essential for 

 the support of the important vital process of respira- 

 tion ; for besides separating all noxious matters from 

 the atmosphere, they are an inexhaustible source of 

 pure oxygen, which supplies the loss which the air 

 is constantly sustaining. Animals on the other hand 

 expire carbon, which plants inspire ; and thus the 

 composition of the medium in which both exist, name- 

 ly, the atmosphere, is maintained constantly un- 

 changed. 



It may be asked, — Is the quantity of carbonic acid 



