INTRODUCTION.

XVII

passed the subject, there is still a very great deal whichrequires investigation ; many important points are asyet imperfectly, or even not at all, explained, andmany questions must be satisfactorily settled beforea complete system of Agricultural Chemistry can beestablished. Till these difficulties are removed, it ispremature to expect that Chemistry can be of more thanpartial assistance to Agriculture, for whilst many of thefundamental laws of Agricultural Chemistry are stillscarcely understood, all attempts to apply them topractice must be incomplete, and liable to error.

The composition of the principal varieties of organicmatter is well known ; the substances which, by com-bining together, form the various constituents of plants,have been ascertained. The food of plants, the greatsources whence they derive it, and the manner in whichthey absorb it, are known. The various changes whichorganic matter undergoes, the conversion of one sub-stance into another, and the influence which thesechanges have on the growth of plants, is likewise easilyunderstood ; nearly all the purely chemical operationswhich are concerned in their nutrition can be explained,by reference to simple chemical laws ; but there aremany most important phenomena which are as yetwholly in the dark. Thus, for example, the manner inb