WRITINGS OF JAMES SMITIISON. 



97 



fused on the platina plate at the mouth of the tube, made 

 an abundant deposit of silica over its interior surface ; and 

 the bit of logwood paper at the end of it had its blue colour 

 altered to yellow. 



In the trial in this way of substances of difficult fusion, 

 an apparatus of the following construction is more favoura- 

 ble than the one above described. 



d 



a. A bottle cork.. 



b. A slice of the same fixed with three pins. 



c. A wire. 



d. A cylinder of platiua foil introduced into the mouth of 

 the glass tube, to prevent its being softened and closed by 

 the flame. 



6. A platina wire, at the end of which is cemented with 

 clay the subject of trial. 



I formerly suggested that topaz might be a compound of 

 silicate of alumina, and of fluate of alumina.* I am now 

 convinced that no oxygen exists in it ; but that it is a com- 

 bination of the fluorides of silicium and aluminum. 



This system produces a considerable alteration in the pro- 

 portions of its elements. 



* Philosophical Transactions for 1811. 



