I 



SEPARATION OF OXYGEN. 49 



mus has only to enter into combination with water, 

 in order to effect the formation of woody fibre, and 

 other substances similarly composed, such as sugar, 

 starch, and gum. But they forget, that their own 

 experiments have sufficiently demonstrated the inapt- 

 itude of these substances for assimilation. 



All the erroneous opinions concerning the modus 

 operandi of humus have their origin in the false 

 notions entertained respecting the most important 

 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. Smithson, 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 vigor 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 physiolo- 

 gists, and that by the greater number the purification 

 of the air by means of them is wholly denied ? 



The action of plants on the air in the absence of 

 light, that is during night, has been much miscon- 

 ceived by botanists, and from this we may trace 

 most of the errors which abound in the greater part 

 of their writings. The experiments of Ingenhouss 

 were in a great degree the cause of this uncertainty 

 of opinion regarding the influence of plants in puri- 

 fying the air. His observation, that green plants 

 emit carbonic acid in the dark, led De Saussure and 



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