98 EQUIVALENT WEIGHT OF 



Such general trials as these are likely, when made with 

 various substances in varied circumstances, to lead to useful and 

 instructive results. 



8. Equivalent quantities or weights of different saline sub- 

 stances used in comparative field experiments. 



In making field experiments, we wish to ascertain, among 

 other points, what are the comparative effects of different sub- 

 stances applied to the same soil and crop under similar circum- 

 stances. But in making such comparisons, we are not at 

 liberty to select at random the quantity of a substance A, which 

 we are to apply in comparison with a given weight of a sub- 

 stance B. We must not even apply them in equal weights if 

 we wish to obtain results which can be correctly compared with 

 each other. This is a point which has not hitherto been brought 

 under the notice of practical experimenters, with whom the 

 comparative money value has generally been the standard by 

 which the quantity applied was measured, rather than the 

 relative chemical energies of the substances employed. 



It will be granted, I believe, that the action upon the soil 

 and plant, of the various saline and other substances we employ, 

 is a chemical action though the precise nature of that action 

 may as yet in many cases have escaped us. But if this be so, 

 they must act in definite quantities : for example- 

 Suppose I wish to try the relative effects of gypsum and 

 sulphuric acid upon a soil abounding in lime, then I must not 

 take equal weights of the two and sprinkle them over the land. 

 The moment the sulphuric acid enters the soil, it combines with 

 lime and forms gypsum, every 100 pounds of the strongest acid 

 of the shops forming about 170 pounds of gypsum. If there- 

 fore we add only 100 pounds of gypsum in comparison with 

 1 00 of the strong acid, we give the latter an unfair advantage. 

 To be strictly comparative, 170 pounds of gypsum must be 

 tried against 100 of the acid. 



Again suppose a soil very poor in lime, upon which I wish 

 to try whether this substance will produce the best effect, in 

 the state of quicklime, of slaked lime, or of gypsum, it will be 

 understood at once that, to make the experiments comparative, 



