26 OF THE ASSIMILATION OF CARBON. 



dus operandi of humus have their origin in the 

 false notions entertained respecting the most im- 

 portant vital functions of plants ; analogy, that 

 fertile source of error, having unfortunately led to 

 the very unapt comparison of the vital functions 

 of plants with those of animals. 



Substances, such as sugar, starch, &c. which con- 

 tain carbon and the elements of water, are products 

 of the life of plants, which live only whilst they 

 generate them. The same may be said of humus, for 

 it can be formed in plants, like the former sub- 

 stances. Smitkson, Jameson, and Thomson, found, 

 that the black excretions of unhealthy elms, oaks, 

 and horse-chesnuts, consisted of humic acid in com- 

 bination with alkalies. Berzelius detected similar 

 products in the bark of most trees. Now, can it 

 be supposed, that the diseased organs of a plant 

 possess the power of generating the matter, to which 

 its sustenance and vigour are ascribed ? 



How does it happen, it may be asked, that the 

 absorption of carbon from the atmosphere by 

 plants is doubted by all botanists and vegetable 

 physiologists, and that by the greater number the 

 purification of the air by means of them is wholly 

 denied ? 



These doubts have arisen from the action of plants 

 on the air in the absence of light, that is, during the 

 night. 



The experiments of Ingenhouss were in a great 

 measure the cause of this uncertainty of opinion, 



