COMPOSITION OF N.S.W. LABRADORITE AND TOPAZES. 195 



ates. To separate the silica from silicates combined with fluorine 

 he employed a solution of zinc carbonate in ammonia; zinc silicate 

 is formed and the solution is freed from the rest of the zinc oxide 

 by evaporation. 



H. Rose in 1849 separated the fluorine as calcium fluoride, a 

 more convenient form than the sodium fluoride employed by 

 Berzelius. He precipitated the calcium fluoride together with 

 calcium carbonate to help in its filtration. He pointed out that 

 some fluorides which were not associated with silica were not 

 completely decomposed by fusion with alkaline carbonates. A 

 combination of the methods of Berzelius and Rose is still the best 

 method known for the estimation of fluorine. Wohler in analys- 

 ing topaz fused with sodium carbonate alone. 



(b) Fluorides decomposable by sulphuric acid. — For the analysis 

 •of these a large number of methods have been proposed; Wohler 

 added silica and sulphuric acid weighed and determined the 

 fluorine by loss due to escape of silicon tetra-fluoride. 



Liversidge (Chem. News, 1871) decomposed with silica and 

 sulphuric acid, and collected the silicon tetra-fluoride in ammonia 

 solution. By partial evaporation the gelatinous silica was dis- 

 solved. He then added potassium chloride and weighed the 

 fluorine direct as potassium silicon fluoride. 



1872, Fresenius collected the silicon tetra-fluoride in tubes 

 containing moistened pumice. 



Penfield (Journ. Chem. Soc. 1879) passed the silicon tetra- 

 fluoride into a solution of potassium chloride and alcohol, and 

 titrated the hydrochloric acid set free by the reaction. 



Garnot (Comptes Rendus, 1893) collected the silicon tetra- 

 fluoride in a ten per cent, solution of potassium fluoride and 

 weighed as potassium silicon fluoride. 



Three of the above methods were tried for the estimation of 

 fluorine in topaz : — 



First method, fusion with alkaline carbonates. — The topaz first 

 analysed was from Mudgee, N.S.W. It was in the form of 



