212 AGRICULTURAL ANALYSIS 



all cases, however, the method which he has proposed must be 

 carefully followed out, that is, all the manipulations must follow 

 each other directly without delay, a condition which is easily se- 

 cured. If, on the contrary, the citric -acid extract, or the pre- 

 cipitations with citrate solution and magnesia mixture, or the 

 citrate-holding magnesia mixture, are allowed to stand for several 

 hours, which, in spite of the precise directions given, still some- 

 .times happens, it can readily occur that large quantities of silicic 

 acid come down with the phosphoric acid precipitate and the re- 

 sults are, in consequence, too high. In such cases it is easy to ex- 

 plain why the precipitated phosphoric acids filter badly. In 

 carrying out of the method in cases of bad filtration, it has been 

 observed by Bottcher, in the conduct of over 800 determinations, 

 that if a solution filters badly it is a proof that in some way or 

 other silicic acid has been precipitated, and naturally in such a 

 case, the silicic acid must be removed by evaporation with hydro- 

 chloric acid in order that correct results be obtained. 



Many comparisons are given by the author of the data obtained 

 by direct precipitation and by precipitation after the separation 

 of the silicic acid. The differences are in all cases negligible 

 between the two methods. 



Following are the data from two samples in which the magnesia 

 pyrophosphate was obtained by four different methods, namely : 



(1) Direct precipitation. 



(2) Direct precipitation after separation of silicic acid. 



(3) Direct precipitation by molybdate method. 



(4) Direct precipitation by molybdate method after separation 

 with silicic acid. 



The data obtained are as follows : 



Weight of Sample I. Weight of Sample II. 



Grams. Grams. 



Method (i) 0.1472 0.1435 



(2) 0.1466 0.1410 



(3) 0.1475 0.1420 



(4) 0.1476 0.1436 



These analytical data show that basic slags which are not 

 capable of being correctly analyzed by the direct citrate precipita- 

 tion method are of very seldom occurrence. Nevertheless a pre- 



