INTRODUCTION. XXV 



points. are as yet imperfectly, or even not at all, 

 explained, and many questions must be satisfactorily 

 settled before a complete system of Agricultural 

 Chemistry can be established. Till these difficulties 

 are removed, it is premature to expect that Chemistry 

 can be of more than partial assistance to Agricul- 

 ture; for, whilst many of the fundamental laws of 

 Agricultural Chemistry are still scarcely understood, 

 all attempts to apply them to practice must be in- 

 complete, and liable to error. 



GQhe composition of the principal varieties of 

 organic matter is well known ; the substances which, 

 by combining together, form the various constituents 

 of plants, have been ascertained. The food of 

 plants, the great , sources whence they derive it, and 

 the manner in which they absorb it, are known. 

 The various changes which organic matter undergoes, 

 the conversion of one substance into another, and 

 the influence which these changes have on the growth 

 of plants, is likewise easily understood ; nearly all 

 the purely chemical operations which are concerned 

 in their nutrition, can be explained by reference to 

 simple chemical laws; but there are many most im- 

 portant phenomena which are as yet wholly in the 

 dark. Thus, for example, the manner in which wood 

 is formed ; and, indeed, all those natural operations 



