VOLATILE MATERIALS IN SILICATES. 308 



acid until wholly dissolved. This washing is done on 

 a filter, so that the sulphate may be acted upon more 

 readily by the hot water. The residue on the filter is 

 ignited, and should consist of only a very small quan- 

 tity of silica. 



Fluoride of silicium and fluorine may be in excess, 

 so that it may be possible to suppose an expulsion of 

 free fluorine and fluoride of silicium. But this is not 

 probable, because generally the silica is in excess of 

 the fluorine, and cannot be set free without combining 

 with the silica ; the topaz containing a large quantity 

 of fluorine, and but little silica, only sets free fluoride 

 of silicium. Admitting, however, that an excess of 

 fluorine may exist, it is at the same time supposed that 

 it contains no water. 



A sufficient quantity of silica is added so that only 

 fluoride of silicium will be set free, and two estimates 

 are made ; in the first the material alone is determined, 

 and in the second the loss of the substance to which 

 the silica has been added. The difference gives the 

 amount of silicium necessary to neutralize the fluorine, 

 and the quantity of free fluorine existing in the mate- 

 rials is deducted from it. 



