§ 58. SILICIC ACID 75 



the addition of fresh quantities of acid if necessary, until no 

 gritty particles can be felt under the end of the stirring 

 rod. Heat the residue to a temperature somewhat above 

 100°, in an air-bath, made by suspending the dish on Avires 

 inside of an iron dish, so that there shall be a space of 

 about 12 mm. between the two at all points; when the 

 whole is completely dry and no more acid fumes escape, 

 moisten the residue with concentrated hydrochloric acid, 

 let it stand half an hour, add water, and digest the mix- 

 ture awhile, wash the insoluble residue two or three times 

 by decantation, wash well on the filter, dry, and ignite. 

 The residue is generally pure silicic acid. 



All the bases with w^hich the silica was combined can 

 be determined in the filtrate from it. 



2. Sometimes this residue is mixed with sand which it 

 may be desired to estimate. 



In this case collect the mixture on a dried and weighed 

 filter, dry it at 100° C, and weigh it ; then separate it 

 from the filter as completely as possible without tearing 

 the latter, and boil it with several portions of a concen- 

 trated solution of sodic carbonate, or with sodic carbonate 

 to which about ^ |^ of sodic hydrate has been added, or 

 w^ith sodic hydrate alone ; dilute each portion of the 

 liquid if it contained much free alkali, let it cool, and 

 throw it on the same filter from which the mixture of 

 silica and sand was taken ; finally, transfer the insoluble 

 sand to the same filter, wash it well, dry, and ignite. If 

 the extraction of the silica was performed in a silver dish, 

 the amount taken into solution by the alkaline liquids 

 may be estimated also ; for this purpose, evaporate all the 

 filtrates and washings to dryness, after having added an 

 excess of hydrochloric acid, and determine the silica as 

 in a. 



3. Sometimes, in agricultural analyses, this residue con- 

 tains, besides silica and sand, free carbon, or coal. In 

 this case, diT the whole at 100° and weigh it, separate it 



