292 SILICATES. 



and washings, as much in the capsule as upon the fil- 

 ter, which is used for decantations. All these decanta- 

 tibns should be made upon the same filter. 



The filter is again placed over the silica in the cap- 

 sule, the whole gently dried upon the sand-bath, then 

 moderately calcined, when the silica should become 

 white. 



The crucible and its cover are placed upon the 

 balance, and quickly weighed. Inasmuch as the 

 crucible cools, the weight that it is necessary to place 

 on the side of the silica to obtain an equilibrium di- 

 minishes more and more, by reason of the cooling of the 

 surrounding air; on the other hand, as this cooling 

 takes place, the silica absorbs the moisture to such an 

 extent that its weight is changed and augmented even 

 so as to be seen. 



The crucible is then placed again warm upon the 

 balance, the weights taken away from the side where 

 the silica is, until the increase of weight of the silica 

 ceases to be rapid. At the moment the balance is at 

 rest, the weight is noted, which gives the weight of 

 the silica. 



At this point in the analysis the weight has been 

 found; first of the silica, secondly of the mixture of 

 alumina and iron containing a little manganese. 



To be sure that the silica is pure, it is dissolved in 

 very dilute hydrofluoric acid ; if quite pure it will 

 leave no perceptible residue, except the ash of the 

 filter. It is evaporated with a little sulphuric acid, and 

 should leave no residue. 



After having weighed the crucible which contains 

 the material after the ignition of the alumina and 

 iron, the mixture is carefully removed and placed in 

 a small platinum boat, previously weighed in a small 

 corked tube; the boat is heated to redness, and again 

 weighed with its case or tube. The boat is then intro- 

 duced into a platinum or porcelain tube, by aid of a 



