VI INTRODUCTION. 



Surrey. The consequence of this has been the publication 

 from time to time, and especially in the Transactions of 

 the Highland and Agricultural Society of Scotland, of the 

 results of numerous experiments with saline and other 

 substances applied to different crops, in soils of nearly all 

 varieties, and upon many geological formations. 



These experiments, however, are often contradictory in 

 their results, often incorrectly or carelessly made, and 

 occasionally exhibit, when criticised, evidences of total un- 

 trustworthiness. But they have never been criticised as 

 a whole the good separated from the bad, and the value 

 of the experimental data they afford us candidly weighed. 



This I propose to do in the following pages. And 

 though my examination of what has hitherto been done in 

 the way of field experiment, has led me to the conviction, 

 that scarcely any of the results we have as yet obtained 

 are to be relied upon as secure grounds for scientific 

 opinions ; yet they may be considered to have cleared the 

 path to surer results, by pointing out sources of error pre- 

 viously unknown, and thus indicating the precautions 

 which must be adopted in future trials. 



In my previously published works, I have embodied, so 

 far as I could, all that was known, with any degree of 

 certainty, in regard to the applications, especially of 

 chemical science, to practical agriculture. But there is a 

 vast deal yet unknown altogether scarcely half under- 

 stood or in regard to which we possess only the assertions 

 or opinions of individual men. To these obscure parts of 

 the subject, in so far as they appear capable of being im- 



