36 



Ten grams of the air-dried soil are treated with 200 c. c. of hydro- 

 chloric acid, specific gravity 1.115, for five days, on a steain-bath, at 

 100 C., in a large porcelain dish, covered with a clock-glass, keeping 

 up the volume by the addition of distilled water when necessary. At 

 the end of the five days the solution being, from time to time, stirred 

 with a glass rod during this treatment, is allowed to cool. and settle 

 completely. Then filter off the solution and wash the residue of in- 

 soluble matter well with boiling water, allowing it to settle before de- 

 canting through the same filter. .Repeat this washing by decantation 

 two or three times, not using more than 50 c. c. of boiling water at a time. 

 Finally transfer the insoluble matter to the filter, washing it thoroughly 

 out of the dish with hot water. It often happens that some portions 

 will remain sticking to the sides of the dish. These are removed by rub- 

 bing them with a rubber-covered glass rod, and washing them onto 

 the filter. A few drops of the wash-water are tested with argentic 

 nitrate, on a watch-glass, to see that all the soluble chlorides have been 

 washed out. This testing is not done until about 200 c. c. of water has 

 passed through the funnel. If any chlorides be present in the wash- 

 water a cloudiness is produced op the addition of the argentic nitrate 

 solution, in which case the washing of the insoluble residue is con- 

 tinued until, on testing, all the chlorides are removed. (Fres.,140, II, a.) 

 The filtrate A. and washings should not exceed 5001). c. 



DETERMINATION 6F THE INSOLUBLE RESIDUE. . 



Dry the filter containing the insoluble residue A in the air-bath at 

 120 0. When dry transfer it to a previously weighed platinum cru- 

 cible and ignite ifc carefully, at first, until the filter paper is consumed 

 and then raise the heat until the crucible and contents are at a bright 

 red and continue it until all the organic matter is consumed. The in- 

 soluble residue should become white, or nearly so, by this treatment. 

 Gool and weigh. The increase in weight gives that of the insoluble 

 and hydrated silica, plus the filter-ash, which must be deducted in all 

 cases. 



' DETERMINATION OF THE HYDRATED SILICA. 



To ascertain how much of the silica found exists in combination with 

 the bases of the clay, how much as hydrated acid, and how much as 

 quartz sand, or as a silicate present in the form of sand, proceed as fol- 

 lows: The insoluble residue A, after it has been ignited and weighe'd, 

 is transferred, iii small portions at a time, to a boiling, rather concen- 

 trated, solution of sodium carbonate, contained in a large platinum 

 dish holding about 200 c. c. ; boil for some time, aud filter off each 

 time, still very hot. When all is transferred to the dish, boil repeatedly 

 with the strong solution of sodium carbonate until, a few drops of the 

 fluid remain clear on warming with ammonium chloride. . 



