270 AGRICULTURAL ANALYSIS 



coming from the covered dish are introduced into a flask grad- 

 uated to 125 cubic centimeters, which should not be completely 

 filled. Some solid carbonate of ammonia is added and the mix- 

 ture heated for half an hour to about 50, adding from time to 

 time a little of the carbonate. By this process the silica is, as a 

 rule, completely precipitated. Nevertheless, as it is important 

 that not the least trace of silica be in solution, it is recommended 

 to finish the separation with oxid of zinc. For this purpose the 

 volume of the mixture is completed to the mark and the contents 

 of the flask filtered into a dried beaker. One hundred cubic cen- 

 timeters of the filtrate are placed in a porcelain dish and 10 cubic 

 centimeters of solution of oxid of zinc in ammonia, in all about 

 0.3 gram of oxid of zinc, are added. The solution is evaporated 

 almost to dryness and some water added, and the evaporation 

 repeated in this way two or three times. The precipitate of car- 

 bonate of zinc formed carries down the last traces of silica. The 

 whole precipitate is finally collected upon a filter and washed. 

 Since the quantity of carbonate of soda present is not exactly 

 known, there are added to the solution a few drops of tropeolin, 

 and carefully, afterwards, diluted hydrochloric acid, until a rose 

 tint is produced, and without waiting 10 cubic centimeters of a 

 solution of carbonate of soda containing 300 grams of the crys- 

 tallized salt per liter. This quantity of carbonate of soda should 

 be prepared in advance, so that it can be added instantly, as it is 

 not safe to leave the solution acid, because it will attack the glass 

 and bring a small quantity of silica into solution. After the addi- 

 tion of the carbonate of soda solution the mixture is boiled for 15 

 minutes. To the nearly boiling solution there is added a slight 

 excess of calcium chlorid, say about 10 cubic centimeters of a 

 solution containing 300 grams of the crystallized calcium chlorid 

 per liter. Stir thoroughly and allow to settle for a short time 

 and filter while still hot. Wash the precipitate, which is com- 

 posed of calcium carbonate and calcium fluorid. The precipitate 

 is dried and burned, the temperature being raised with great cau- 

 tion, so as to avoid a partial fusion, a phenomenon which is pro- 

 bably due to the existence of a fluocarbonate which has not yet 

 been isolated. The ignition is conducted in a large platinum cruci- 



