2 INTRODUCTION. I. 



In the application of chemistry to agriculture cases often 

 arise in which the truth of the old adage "A little knowledge is 

 a dangerous thing" becomes strikingly apparent and the con- 

 clusions arrived at from the consideration of a particular 

 problem from the standpoint of ordinary elementary chemistry 

 are quite opposite to the results of actual practice. Such 

 contradictions arise, not from any inaccuracies in general 

 principles, but through leaving out of consideration the effects 

 produced by some apparently insignificant circumstances or 

 conditions. 



It is evident, therefore, that although there is no distinct 

 agricultural chemistry, yet the problems which arise in agricul- 

 ture demand a knowledge of chemistry in which due attention 

 is given to the peculiar circumstances under which the re- 

 actions take place. 



In this work it will be assumed that the reader possesses an 

 acquaintance with general elementary chemistry and is familiar 

 with the properties of the more commonly occurring elements 

 and their chief compounds. 



The student of agricultural chemistry soon finds that of the 

 seventy odd elements which are known, only a comparatively 

 small number, some 12 or 14, are concerned in most of the 

 changes which are brought before his notice. It may perhaps 

 be advisable to very briefly remind the reader of the properties 

 of these important elements, the mode of their occurrence, and 

 the characteristics of some of their compounds. The elements 

 which are most important to living organisms are hydrogen, 

 oxygen, nitrogen, carbon, sulphur, phosphorus, potassium, 

 sodium, calcium, magnesium, iron, silicon, aluminium, chlorine, 

 and fluorine. 



Hydrogen. This substance, as its name implies, is a con- 

 stituent of water. Its most important chemical properties are 

 its strong tendency to combine with oxygen, the act of union 

 being accompanied by the evolution of a large amount of heat, 

 and its power of uniting in a vast number of different propor- 

 tions with carbon to form that very numerous and important 

 group of bodies known as the hydrocarbons. It also enters 

 into the composition of almost all compounds existing in the 



