EXPERIMENTS WITH SINGLE SALINE SUBSTANCES. 87 



2. Practical and theoretical purposes for which experiments 

 with single substances are made. 



Experiments with single saline or mineral substances are 

 made with the view of ascertaining 



1. If they are required by, or can be profitably applied to a 

 particular crop, on a given soil, when treated in a given way. 

 For example 



a When, during the making of the manure on a farm, the 

 ammoniacal matter is allowed to run to waste, either in the 

 form of liquid manure or of gaseous ammonia will land, dressed 

 with such manure, be grateful for, and give a profit upon, the 

 application of salts of ammonia, or other salts containing nitro- 

 gen, the most valuable constituent of ammonia ? 



b When, by the growth and sale of corn crops, by the export 

 of cheese, or the rearing of young stock, the land has been, or 

 is presumed to be, exhausted of phosphates will it be specially 

 advantageous to apply preparations containing phosphoric acid? 

 and so on. 



2. The abstract general effect of a given elementary or other 

 substance upon vegetable growth. 



3. The abstract special effect, if any, which it exercises upon 

 this or that particular crop. 



4. The influence of known circumstances in modifying this 

 general or special effect. 



5. How far the state of chemical combination, in which a 

 substance is applied, interferes with or changes its general or 

 special effects. Whether these several compounds perform the 

 part of distinct agents with peculiar actions upon plants, dis- 

 tinct from those of their constituents or whether their action is 

 compounded of the joint influences of the substances of which 

 they consist? 



The first of the objects above stated is altogether practical ; 

 the others are theoretical or scientific in the first instance. All 

 such inquiries, however, are sure to lead, either directly or 

 indirectly, to important practical and economical deductions. 



How the several classes of experiments are to be made will 

 be explained hereafter. 



