Chemical Examination of American Minerals. 13 



of the cheeks in a small stream, upon the upper margin of 

 the filter ail round. And this process is continued till the 

 filter is quite clear, and till the water, which passes through 

 when evaporated from a silver spoon over a spirit lamp, 

 leaves no sensible residue. 



The filter and silica thus washed clear, is left on the glass 

 funnel till it can bear handling without risk of being torn. It 

 is then taken oft', put between two folds of brown paper, and 

 placed on the sand bath, in a place cold enough not to char 

 the paper, nor injure the filter, but hot enough to dissipate 

 the moisture and dry the silica. As soon as it has become 

 quite dry, and has fallen into the -stale of a fine powder, its 

 weight is determined by placing the two filters in the scales 

 in the same manner as at first, and then finding what weight 

 is requisite to equipoise the silica. Let this weight be a. It 

 gives us the weight of hydrous silica extracted from our mine- 

 ral. The weight of this silica in an anhydrous state, is de- 

 termined in the following way. 



A small platinum crucible about an inch in height, is equi- 

 poised in the scales, and as much of the silica as can be con- 

 veniently taken oft' the filter, is put into it, and its weight de- 

 termined. Expose the crucible to the flame of a spirit lamp 

 till it becomes red hot, and keep it in that state for about ten 

 minutes. As soon as it is cold, replace it in the scales, and 

 determine the loss which it has sustained. From this it is 

 easy to determine the loss of weight which the whole silica 

 would have sustained, had it been subjected to ignition. 

 Thus the weight of the whole anhydrous silica becomes 

 known. 



6. The liquid which has passed through the filter may con- 

 tain alumina, lime, oxide of iron, and a fixed alkali. Let it 

 be reduced by evaporation to so small a quantity that it can 

 be conveniently operated upon ; then add caustic ammonia in 

 slight excess. The alumina and iron will be thrown down, 

 but the lime and the fixed alkali will remain in solution. 



