OF ARTIFICIAL MANURES. 163 



ciprocal comparison, a counterbalancing, experiment- 

 ing, and practical knowledge, than upon fixed princi- 

 ples, and must therefore be contingent upon circum- 

 stances. If the prices specified are shown on fur- 

 ther examination to be untenable, they must be 

 altered ; they cannot lay claim to permanent weight, 

 inasmuch as they are subject to the same fluctuations 

 as those of other articles of trade. If from this kind 

 of computation, in general, a beneficial result for 

 practical guidance merely is to be expected, it must 

 then be treated in the same way as the merchant 

 deals with his price-current; that is, the prices must 

 be revised from time to time, and, when necessary, 

 altered. 



The individual substances of manures^ for which 

 specific valuations were calculated, are the follow- 

 ing : — 



1. Putrid nitrogen {ammonia): — l\h.= ls. ^d. 

 This price may, perhaps, appear somewhat high, but 

 it is not so in truth. I believe that it would rather 

 bear a further addition than a reduction, since in the 

 cheapest ammoniacal salt (crude sulphate of ammo- 

 nia) it costs from 2s. to 2s. 2d., and in the cheapest 

 nitric acid salt (crude Chili saltpetre), from Is. 6d. to 

 Is. 9d. 



2. Nitrogen {not putrid): — llb.= llc?. The 

 lower price of nitrogen, when not putrid, is justified 

 by its slower operation. In rape-meal it is computed, 

 according to the present market value, at from 9d. to 

 lid. ; in woollen rags, at from 5d. to 7d. only. The 



