134 AGRICULTURAL ANALYSIS 



contain decomposable silicates giving gelatinous silica with hydro- 

 chloric acid. 



There are two methods [see (6) and (7)] of securing a solu- 

 tion of the sample which varies from one to five, and even to 10 

 grams, according to the apparent quantity of phosphoric acid in 

 the material to be analyzed. 



(6) Solution by Filtration and Washing. The ordinary method 

 can be employed consisting in decomposing the substance by 

 an acid, filtering, washing the residue upon the filter and com- 

 bining all the wash-waters to make a determinate volume. After- 

 wards an aliquot fraction of the whole is used for the precipita- 

 tion. This method is long, and presents some chances of error 

 when the insoluble residue is voluminous and contains silica 

 which obstructs the pores of the paper and renders the filtration 

 difficult. 



(7) Volumetric Solution. It is advisable to substitute volu- 

 metric solution for solution by filtration and washing, which is 

 accomplished by decomposing the substance in a graduated flask, 

 the volume being afterwards made up to the mark with distilled 

 water after cooling. The solution is filtered without washing, 

 and by means of a pipette an aliquot part of the original volume 

 is removed for analysis. Thus all retardations in the process 

 are avoided, and likewise the chances of error from washing on 

 the filter. It is true that this method may lead to a certain error 

 due to the volume of the insoluble matter which is left undecom- 

 posed, but since this insoluble matter is usually small in quantity, 

 and since it is always possible to diminish the error therefrom by 

 correspondingly increasing the volume of the solution, this cause 

 of error is much less to be feared than those due to the difficulties 

 which may occur in the other method. Let us suppose, in order to 

 illustrate the above, that we are dealing with a phosphate contain- 

 ing 50 per cent, of insoluble sand which may be considered as an 

 extreme limit. In working on four grams of the material in a 

 flask of 100 cubic centimeters capacity, there will be an insoluble 

 residue of two grams occupying a volume of less than one cubic 

 centimeter, the density of the sand being generally above two. The 

 100 cubic centimeter flask will then contain more than 99 cubic 



