CRYOLITE. 155 



of finely -powdered silicic acid (that prepared from tetra- 

 ^fluoride of silicon is the best), the mixture introduced 

 into an apparatus similar to that employed in alka- 

 limetry, and the sulphuric acid, which must for this 

 purpose be very concentrated, allowed to flow upon it. 

 With the aid of a gentle heat, tetrafluoride of silicon is 

 formed, which is allowed to escape in the gaseous state 

 through a tube filled with chloride of calcium; the 

 last portions are withdrawn from the apparatus by 

 sucking air through it, for which purpose, there is 

 attached to the chloride-of-calcium-tube a small tube 

 filled with fragments of moist hydrate of potassa, 

 through which the air is drawn. The loss of weight 

 expresses the amount of tetrafluoride of silicon which 

 has been evolved. 



85. CRYOLITE.* 



The analysis may be made by means of concentrated 

 sulphuric acid as in the case of fluorite. The fluorine 

 is determined by the loss. For the direct determina- 

 tion, the mineral is decomposed in a platinum retort, 

 'and the fluorine contained as fluoride of calcium, as 

 with fluor-spar and topaz. The excess of sulphuric 



* This remarkable mineral is found in an immense deposit 80 

 feet thick and 300 feet long in Greenland at the head of Arksut Bay, 

 jiear Cape Farewell. It is often associated with crystals of galena, 

 spathic iron, copper and iron pyrites, etc. 



The Pennsylvania Salt Company introduced to our country this 

 valuable material, and now prepare from it caustic soda, carbonates 

 and other salts of soda, sulphate of alumina, etc. 



One hundred pounds of cryolite yield 44 pounds dry caustic 

 'soda ; or 75 pounds dry carbonate of soda, 203 pounds crystallized 

 carbonate soda ; or 119 pounds bicarb, soda, and 24 pounds 

 of. alumina. 



