18 



AGRICULTURAL CHEMISTRY. 



mals. Hence such substances as sugar, 

 starch, and gum, which are themselves pro- 

 ducts of plants, cannot be adopted for as- 

 similation. 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 struc- 

 ture, but are not assimilated, they cannot, 

 therefore, be employed in their nutrition. 

 We could scarcely conceive a form more 

 convenient for assimilation than that of 

 gum, starch, and sugar, for they ah contain 

 the elements of woody fibre, and nearly in 

 the same proportions. 



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 j its composition differing from that 

 of woody fibre in its being richer in carbon. 



Misled by this simplicity in its constitu- 

 tion, physiologists found no difficulty in dis- 

 covering the mode of the formation of 

 Woody fibre ; for they say,* humus has only 

 to enter into combination with water, in 

 order to effect the formation of woody fibre, 

 and other substances similaily composed, 

 such as sugar, starch, and gum. But they 

 forget that their own experiments have suf- 

 ficiently demonstrated the inaptitude of these 

 substances for assimilation. 



All the erroneous opinions concerning the 

 modus operandi of humus have their origin 

 m 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 com- 

 parison of the vital functions of plants with 

 those of animals. 



Substances, such as sugar, starch, &c., 

 which contain carbon and the elements of 

 water, are products of the life of plants 

 which live only while they generate them. 

 The same mav 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 combination 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 substance and vigour are ascribed? 



How does it happen, it may be asked, that 

 the absorption of carbon from the atmos- 

 phere 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 ? 



The action of plants on the air in the 

 absence of light, that is during night, has 

 been much misconceived 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 



Meyeu, Ffanzenphysiologie, II., S. 141. 



were in a great degree the cause of tms un- 

 certainty of opinion regarding the influence 

 of plants in purifying the air. His obser- 

 vation that green plants emit carbonic acid 

 in the dark, led t)e Saussure and Grischow 

 to new investigations, by which they ascer- 

 tained that under such conditions plants do 

 really absorb oxygen and emit carbonic acid ; 

 but that the whole volume of air undergoes 

 diminution at the same time. From" the 

 latter fact it follows, that the quantity of 

 oxygen gas absorbed is greater than the 

 volume of carbonic acid separated ; for, if 

 this were not the case, no diminution could 

 occur. These facts cannot be doubted, but 

 the views based on them have been so false, 

 that nothing, except the total want of obser- 

 vation and the utmost ignorance of the 

 chemical relations of plants to the atmos- 

 phere, can account for their adoption. 



It is known that nitrogen, hydrogen, and 

 a number of other gases, exercise a pecu- 

 liar, and in general, an injurious influence 

 upon living plants. Is it, then, probable, that 

 oxygen, one of the most energetic agents in 

 nature, should remain without influence on 

 plants when one of their peculiar processes 

 of assimilation has ceased ? 



It is true that the decomposition of car- 

 bonic acid is arrested by absence of light. 

 But then, namely, at night, a true chemical 

 process commences, in consequence of the 

 action of the oxygen in the aii, upon the 

 organic substances composing the leaves, 

 blossoms, and fruit. This process is not at 

 all connected with the life of the vegetable 

 organism, because it goes on in a dead plant 

 exactly as in a living one. 



The substances composing the leaves of 

 different plants being known, it is a matter 

 of the greatest ease and certainty to calcu- 

 late which of them, during life, should ab- 

 sorb most oxygen by chemical action when 

 the influence of light is withdrawn. 



The leaves and green parts of all plants 

 containing volatile oils or volatile constitu- 

 ents in general, which change into resin by 

 the absorption of oxygen, should absorb 

 more than other parts which are free from 

 such substaruces. Those leaves, also, which 

 contain either the constituents of nutgails, 

 or compounds in which nitrogen is present, 

 ought to absorb more oxygen than those 

 which do not contain such matters. The 

 correctness of these inferences has been dis- 

 tinctly proved by the observations of De 

 Saussure ; for, while the tasteless leaves of 

 the Jlgave americana absorb only 0-3 of 

 their volume of oxygen in the dark, during 

 24 hours, the leaves of the Pinus Jlbies, 

 which contain volatile and resinous oils, 

 absorb 10 times, those of the Quercm Robur 

 containing tannic acid 14 times, and the 

 balmy leaves of the Popiihts alba 21 times 

 that quantity. This chemical action is 

 shown very plainly also, in the leaves of 

 the Cotyledon calycinum, the Cacaliaficoidej, 

 and others; for they are sour like sorrel in 

 the morning, tasteless at noon, and bitter in 



