A MANUAL OF 



AGRICULTURAL CHEMISTRY. 



CHAPTER I. 

 INTRODUCTION. 



CHEMISTRY has an intimate connection with the processes of life, 

 both animal and vegetable. The processes involved in vital phen- 

 omena, those occurring in the soil and indeed, in most natural opera- 

 tions, are attended by slow chemical changes, often so complex in 

 character that they are difficult to unravel. The ordinary chemical 

 student acquires in his training a familiarity with reactions, which, as 

 a rule, proceed quickly to a definite and well-marked termination. 

 This, indeed, is the case with the vast majority of chemical changes 

 occurring in the industries, and, for the most part, such reactions are 

 thoroughly understood and can be satisfactorily explained. 



But with the changes taking place in animals and plants or even 

 in the soil, the reactions are much more involved, partly because of 

 the complexity of the substances concerned, partly on account of the 

 conditions under which they occur, and partly because of the numerous 

 changes which may proceed simultaneously. Chemical reactions 

 occur between substances in solution, and the final condition of equili- 

 brium between two reacting substances is determined by the relative 

 concentration of the solvent in the various dissolved substances. In 

 nature, the solutions are usually very dilute, and the effect of the relative 

 masses of the reacting substances influences the final result to a far 

 greater extent than is the case in ordinary laboratory reactions. 



Agricultural chemistry has often to deal with changes of this com- 

 plex character, and in all attempts to explain such changes it is 

 necessary to take into careful consideration the conditions under 

 which they occur. 



The border line between chemistry and physics, as, indeed, be- 

 tween any two conventional divisions of natural science, is not clearly 

 defined, and, among the factors affecting chemical changes of the kind 

 under discussion, physical conditions, e.g., temperature and pressure, 

 are often of much importance. Other purely physical phenomena, 

 among which may be mentioned diffusion, osmosis and surface pressure, 

 play an important part in vital processes and are often closely inter- 

 woven with chemical changes. 



In the present volume reference will have to be made to many 

 phenomena which are not purely chemical, and it will be necessary to 

 give some account of subjects which, perhaps, may be regarded as 

 belonging to physics, geology, or biology rather than to chemistry. 



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