20 OF THE ASSIMILATION OF CARBON 



quantity of oxygen amounting to 4480 cubic feet Hessian (2468 

 English) must have been furnished to the air by the decomposi- 

 tion of water, for 10 cubic feet Hessian of coal contains hydro- 

 gen corresponding to this amount. 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 ; q 

 circumstance which accounts for the richness and luxuriance of 

 the earlier vegetation. When this became entombed, the condi- 

 tions were established, under which higher forms of animal life 

 were capable of existing. (Brogniart.) 



But a certain period must have arrived in which the quantity 

 of carbonic acid contained in the air experienced neither increase 

 nor diminution in any appreciable quantity. For if it received 

 an additional quantity to its usual proportion, an increased vege- 

 tation would be the natural consequence, and the excess would 

 thus be speedily removed. And, on the other hand, if the gas was 

 less than the normal quantity, the progress of vegetation would 

 be retarded, and the proportion would soon attain its proper stand- 

 ard. When man appeared on the earth, the air was rendered 

 constant in its composition. 



The most important function in the life of plants, or, in other 

 words, in their assimilation of carbon, is the separation, we 

 might almost say the generation, of oxygen. No matter can be 

 considered as nutritious, or as necessary to the growth of plants, 

 which possesses a composition either similar to or identical with 

 theirs ; because the assimilation of such a substance could be 

 effected without the exercise of this function. The reverse is 

 the case in the nutrition of animals. Hence such substances, 

 as sugar, starch, and gum, themselves the products of planL-, 

 cannot be adapted for assimilation. And this is rendered certain 

 by the experiments of vegetable physiologists, who have shown 

 that aqueous solutions of these bodies are imbibed by the roots of 

 plants, and carried to all parts of their structure, but are not 

 assimilated ; they cannot, therefore, be employed in their nutrition. 



In the second part of the work we shall adduce satisfactory 

 proofs that decayed woody fibre (humus) contains carbon and the 

 elements of water, without an excess of oxygen ; its composition 



