CONSTITUENTS TO BE DETERMINED 49 



in citric acid solutions is likely to lead to erroneous conclusions. 

 Moreover, the fertilizing value of any material of this kind is 

 not a constant quantity, but varies always with the character of 

 the soil to which it is applied and of the seasonal environment to 

 which it is subjected. The chemist has done all that could be 

 reasonably expected of him when he has determined the fineness 

 of the subdivisions of the material and its relative solubility in 

 certain reagents which indicate the relative amount of acid pres- 

 ent which readily passes into solution under the ordinary condi- 

 tions to which it is likely to be subjected. It must be remem- 

 bered, however, that in the field the processes of solution go on 

 constantly for a period of three or four months; in fact, as 

 long as the plant continues feeding. A very feeble solubility in 

 the soil, therefore, would render the phosphoric acid constantly 

 available in the proportion in which it is used. If a reagent of 

 the same feeble power was used in the laboratory it would neces- 

 sarily have to be kept in activity over a long period of time to 

 yield results which are comparable to those found in the soil. 

 Therefore, it seems only reasonable to use a stronger reagent 

 for a limited period of time such as can be used practicably by 

 the analyst for his determinations. 



While the use of the various reagents which have been pro- 

 posed for the valuation of these materials may not lead to results 

 directly comparable to those that take, place in the growing sea- 

 son, they do, undoubtedly, give an idea of the availability, which 

 is of great practical importance. 



ANALYTICAL PROCESSES 



55. Constituents to be Determined. The most important point 

 in the analysis of mineral phosphates is to determine their con- 

 tent of phosphoric acid. Of equal scientific interest, however, 

 and often of great commercial importance is the determination 

 of the percentage of other acids and bases present. The analyst 

 is often called on, in the examination of these bodies, to make 

 known the content of water both free and combined, of organic 

 and volatile matter, of carbon dioxid, sulfur, chlorin, fluorin, 

 silica, iron, alumina, calcium, manganese, magnesia, and the al- 

 kalies. The estimation of some of these bodies presents problems 



