208 PROCEEDINGS OF THE CHEMICAL DEPARTMENT. 



substances which, dissolve most readily should be in the most 

 favourable position, but it is by no means clear that this is 

 invariably true. If so, it would necessarily follow that those 

 substances which dissolve readily in water should be most 

 useful. But experience by no means confirms this opinion, for 

 it is not found that plants grow readily in solutions of their 

 essential constituents, although it is possible to make them do so 

 with proper care and precaution. Moreover, soils bring the 

 most essential elements of plants into insoluble or nearly in- 

 soluble forms of combination, in which, being attached to the 

 clay and organic matter present in them by a feeble bond, they 

 are kept there ready to supply the wants of the plant. It is far 

 from improbable that the only constituents of the soil capable of 

 passing into the vegetable organism are those which exist in the 

 particular state of combination in question, and which have 

 either been introduced from without, or having been liberated by 

 the action of external agents, such as air and moisture, on the 

 minerals forming the basis of all soils, have been retained in that 

 looser state of combination ready to supply the wants of vegeta- 

 tion. 



It is impossible, however, to decide this question in a 

 thoroughly satisfactory manner, because no means are known of 

 separating from one another those portions of the soil constituents 

 which exist in these different states. We can only seek to arrive 

 at some approximation to the solution of the problem, by ac- 

 cumulating knowledge as to the different degrees of solubility 

 of these substances under different conditions, and comparing the 

 results with those obtained in the field, which is necessarily a 

 very slow process, and requires much patience to amass the 

 requisite number of facts, and much self-denial to prevent our 

 attempting to draw conclusions from an insufficient number of 

 data. The determination of the matters dissolved by acetic 

 acid is a step in this direction, which will probably pave the way 

 for a still more minute investigation. 



It has been stated at the outset that acetic acid, which is, in 

 fact, more familiar to us as pure vinegar, was selected as the 

 solvent to be applied to the soil, on account of its very feeble 

 affinities, which seemed likely to be more comparable with the 

 action the roots are supposed to exercise on the soils than the more 

 powerful acids, and I propose to direct attention to the quantities 

 of the different substances which it has extracted from the soil. 

 It is necessary to remark, that if a soil of the most fertile kind is 

 boiled two or three times with successive quantities of water, 

 the quantity of matters extracted from it varies from one-tenth 

 to one-third of a per cent., rarely exceeding the latter number — 

 of which one-half at least consists of humus and other organic 

 matters, which are generally understood to be incapable of serving 



