84 AGRICULTURAL ANALYSIS 



dish. If no chlorid of lime is contained in the original soil be- 

 fore the evaporation takes place a half gram of calcium carbonate 

 is added to the solution. The residue from drying is heated care 

 fully and in order to avoid too rapid evaporation a platinum or 

 nickel plate is placed between the source of the heat and the 

 dish. After complete dryness has been secured, the flame is so 

 regulated that the bottom of the dish containing the residue is 

 heated almost to a low red heat, but even in this case the volatili- 

 zation of the aluminium chlorid is not to be feared. After the 

 vigorous evaporation of the vapors of hydrochloric acid has 

 diminished somewhat the platinum or nickel plate is laid upon 

 the top of the dish. The mass within the dish soon reaches a 

 condition in which it can be rubbed into a fine powder by a glass 

 rod, after which it is heated a while longer and stirred contin- 

 ually until all parts of the material are reduced to a fine pow- 

 der. After about an hour of treatment of this kind the organic 

 substances which the .solution may contain are completely de- 

 stroyed and the residue is ready for further investigation. 



The contents of the dish are washed in a 125 cubic centimeter 

 flask with a sufficient quantity of water to fill the flask half full 

 and the contents of the flask are boiled over a low flame for about 

 half an hour; after cooling, the flask is filled to the mark, thor- 

 oughly mixed and filtered through a very small dry filter into a 

 dry vessel. The filtrate must be completely colorless and at most 

 only slightly alkaline, and if the operation above indicated has 

 been properly carried out, it is entirely free from iron, phosphoric 

 acid and silicic acid. One hundred cubic centimeters of the fil- 

 trate, corresponding to 20 grams of the original soil, is the quan- 

 tity used, if the volume of insoluble material is not taken into 

 consideration. * 



If a correction is desired for this, the ignited residue is weighed 

 and it's volume approximately calculated by taking into consid- 

 eration its specific gravity and weight. 



In a porcelain dish 100 cubic centimeters of filtrate are evap- 

 orated to dryness, a few drops of hydrochloric acid added and a 

 sufficient quantity of platinum chlorid for the determination of 

 potassium salts in the usual manner. Inasmuch as other salts 



