464 G. J. Brush — Chemical Composition of Durangit 



Finding, therefore, no red substance capable as such of i 

 iingthes * " - • - ■ ,. . . , 



of AgBr to green light, and, 

 other hand, many colorless substances which have that effect, 

 I am entirely confirmed in the opinion originally expressed in 

 the pages of this Journal, that there exists no relation between 

 the color of a substance and that of the rays to which it increases 

 the sensitiveness of silver bromide. 

 Philadelphia, March 13, 1876. 



Art. LX.— Contributions from the Sheffield Laboratory of Yale 

 College. No. XXXIX.— Ow the Chemical Composition of 

 Durangite ; by George J. Brush. 



In an article^ on this rare mineral, published in 1869, I 

 expressed the hope to make further examination of its chemical 

 composition whenever sufficient material could be obtained for 

 this purpose. Several years elapsed before any new discoveries 

 of the mineral in Durango were made. I am again indebted to 

 Mr. Henry Gr. Hanks of San Francisco for a new supply of the 

 crystals obtained in recent explorations. These crystals are 

 much smaller than those previously examined, being from one 

 to three millimeters in diameter, and they are of a darker shade 

 of color. The former were loose detached crystals, while these 

 are associated with, and in some cases attached to, rolled frag- 

 ments of crystallized hematite and cassiterite. The density of 

 the small dark colored crystals is 4'07, while that of the purest 

 of the bright colored crystals before described is 3-937. In all 

 other physical characters there is a perfect correspondence 

 between the two varieties. 



The chemical examination of the dark colored small crystals 

 has been undertaken, at my request, by mv assistant Mr. 

 George W. Hawes, first to estimate the amount of fluorine m 

 the mineral, which in two determinations he found to be 7-67 

 and 7-49 per cent, and Mr. Hawes has also placed at my dis- 

 posal for this article a complete analysis of this variety of the 

 mineral. The fluorine was determined directly by Wohlers 

 method as modified by Fresenius.f To determine the arsenic 

 acid, and the bases, the mineral was decomposed by sulphunc 

 acid, and the arsenic weighed as sulphide ; the alumina, iron 

 and manganese obtained in the analysis v^ere carefully ex- 

 amined to ascertain their purity. The soda and lithia were 



* This Journal, II, xlviii, 119. 



t Fresenius' Quantitative Analysis (Johnson's edition), p. 406. 



