302 VOLATILE MATERIALS IN SILICATES. 



silicium is completely expelled. "When it is certain 

 that the loss of the topaz is terminated, the crucible is 

 taken from the fire and weighed ; there should be no 

 loss except of the carbonic acid of the lime and the 

 water. The quantity of the carbonate of lime being 

 known, and therefore that of carbonic acid, it follows 

 that the loss of weight, that of the carbonic acid being 

 deducted, depends entirely upon the quantity of water 

 which may have passed through the lime without 

 being absorbed. 



This done, the crucible is slightly inclined, the lime 

 taken out with the greatest care, generally rendered 

 compact and adhering to the crucible by the presence 

 of the fluoride of calcium and the silicate of lime. 

 When a sufficient quantity of lime has been taken out 

 to free the medium crucible, this is withdrawn, and 

 then the interior crucible is free ; it is weighed, and 

 the loss of weight gives the fluoride of silicium which 

 is evolved. 



It is necessary to prove that this is fluoride of sili- 

 cium. The composition of fluoride of silicium is SiF 3 ; 

 if it .is passed through the lime, it forms SiO 2 ,CaO-h 

 3CaF, a mixture of fluoride of calcium and silicate 

 of lime, which gives an excess of lime again. It is ne- 

 cessary to take all the substance around the medium 

 crucible into the large crucible and boil it with nitrate 

 of ammonia, which does not decompose the fluoride 

 of calcium and silicate of lime. The quicklime is thus 

 disposed of, and fluoride of calcium and silicate of lime 

 remain in the proportions indicated above Si0 2 CaO-f 

 CaF. 



The substance treated with sulphuric acid should be 

 completely converted into sulphate of lime and fluoride 

 of silicium. Therefore, after having treated it with 

 nitrate of ammonia and washed it, sulphuric acid is 

 added until no vapors are given off; the sulphate of 

 lime thus formed is treated with boiling water and 



