connected mth the Theory of Agriculture, 387 



of this faculty, and how far it may be dependent upon the 

 stimulus of light. 



Thus distinct experiments are requisite to determine, 

 whether plants can decompose sulphate of ammonia, sulphate 

 of lime, sulphate of magnesia, sulphate of soda, &c., as well as 

 the chlorides of sodium, of magnesium, and of calcium; 

 whether nitrates are converted into carbonates under the in- 

 fluence of vegetable processes, &c. 



The power of deriving carbon from certain kinds of vege- 

 table matter, as for example from humus^ is attributed to 

 plants by some, but denied to them by others ; for although 

 the latter thrive more in rich vegetable mould than they would 

 do in any mere combination of earthy and alkaline ingre- 

 dients, yet it is still disputed whether the mould does not as- 

 sist vegetation, merely in proportion as it itself undergoes de- 

 composition, and evolves in consequence carbonic acid. 



Nor is this inquiry merely a speculative one, since if humus, 

 as such., is capable of nourishing plants, everything which can 

 contribute to render it soluble will augment its nutritive 

 quality ; whilst if it only acts by virtue of becoming decom- 

 posed, then our endeavours ought rather to be directed towards 

 the acceleration of that decomposition, than towards the pro- 

 motion of its solubility. 



Another point upon which vegetable physiologists are di- 

 vided is, whether plants are capable under any circumstances 

 of absorbing nitrogen from the atmosphere ; and although my 

 own opinion inclines strongly to the negative side of this ques- 

 tion, yet I am quite alive to the importance of having it set at 

 rest by a more precise series of experiments than has ever 

 yet been instituted. It would be needless indeed to remark, 

 how much our practice would be modified by ascertaining 

 that a plant is capable of furnishing itself with the nitrogen it 

 requires. The whole theory of chemical manures would be 

 changed by such a supposition, since, instead of setting them 

 down, in so far at least as the volatile ingredients of plants are 

 concerned, as the food, we should rather regard them in the 

 light of stimulants to the vital forces of those organic bodies 

 to which they are applied. 



Another question, of much practical as well as theoretical 

 moment, relates to the degree in which the adhesive or co- 

 hesive affinity, which subsists between the parts or particles 

 of bodies, may act as a counteracting power to that of assimi- 

 lation. How far, for instance, will the secretion of lime by a 

 plant be favoured by the earth having already past through 

 the system of an animal, as in the case of an oyster-shell, shelly 

 marl, &c. ? What difference will there be in the fertilizing 



2 C 2 



