Pen field and Minor — Composition of Topaz, etc. 387 



Locality and 

 Description. 



I. 

 San Luis, Mexico. 

 Small crystals ; 1, 

 2, white ; 3, brown. 



II. 

 Ilrnen Mts. Large 

 white crystals. 



III. 



Scbneckenstein Sax- 

 ony. Small gray- 

 white crystals. 



IV. 

 Brazil. Large 

 brown crystal. 



Substance 

 taken. 



0-5128 gr, 



0-5234 



0-4890 



0-5422 

 0-5554 



0-5743 

 0-5564 



0-4954 

 0-6308 

 0'5263 



Water 

 found. 



0-0040 gr. 



0-0046 



0-0036 



0-0057 

 0-0055 



0-0073 

 0-0071 



00140 

 0-0164 

 0-0140 



Per cent. Average. 



0-79 



0-88 } = 0-81 ^ 



0-74 



1-05 ) 

 0-99 f 



1-27 

 1-28 



2-82 ) 

 2-60 V = 2 

 2-66 ) 



= 1-02$ 



1-28^ 



To assure ourselves that the presence of the water was not 

 due to organic substances or other impurities in the lead oxide 

 we made two additional determinations ; one, of the water 

 given off by the lead oxide alone, the other conditions of the 

 experiment being observed ; the second, of the water in the 

 specimen from the Ilmen Mountains by means of lead oxide 

 which had been previously fused. In the first determination 

 the calcium chloride tube increased in weight by an amount 

 corresponding to less than - l per cent of the quantity of min- 

 eral usually taken for analysis, and the second experiment 

 yielded results agreeing with those previously obtained. 



Whether the presence of water will necessitate any radical 

 change in the formula of topaz we are not yet in position to 

 state. We are at present engaged, however, in making seve- 

 ral complete analyses of different varieties of the mineral, and 

 hope soon to be able to decide this question. 



Heidelberg, Feb. 17, 1894. 



Art. XLYI. — On the Chemical Composition and Related 

 Physical Properties of Topaz; by S. L. Pen field and 

 J. C. Minor, Jr. 



The chemical composition of topaz has never been satis- 

 factorily settled. The results of the analyses thus far published 

 show clearly that silicon and aluminium are present in the 

 proportion of 1:2, but the percentage of fluorine as given in 

 them varies from 16-12-18-83. The formula that is usually 



Am. Jour. Sci— Third Series, Vol. XLVII, No. 281.— May, 1894. 

 26 



