2O2 AGRICULTURAL ANALYSIS 



tiire, after which the filter is broken and the precipitate washed 

 into a beaker with two per cent, ammonia and the filter washed 

 therewith until about 100 cubic centimeters have been used. If 

 the solution is turbid from the presence of silicic acid it should be 

 precipitated a second time by addition of molybdic solution. The 



Fig. ii. Wagner's Digestion Apparatus for Slags. 



ammoniacal solution of the yellow precipitate is treated, drop by 

 drop, with constant stirring, with 15 cubic centimeters of mag- 

 nesia mixture, and set aside for two hours. The precipitate is 

 collected, washed, ignited and weighed in the usual manner. The 

 direct precipitation of the phosphoric acid by the magnesia solu- 

 tion in presence of citrate is not advisable because of the almost 

 general presence of silicic acid, which would cause the results to- 

 be too high. 



The chief objection to this method of Wagner lies in the fail- 

 ure to control the temperature at which the digestion with citrate 

 solution is made. Huston has shown, as will be described further 

 on, that the temperature exercises a great influence in digestion 

 with citrate. Since the laboratory temperature, especially in this 

 country, may vary between 10 and 40, it is evident that on the 

 same sample the Wagner method would give very discordant re- 

 sults at different seasons of the year unless the digestions were 

 m^de at one temperature. In order to control the temperatures 



