432 
POPULAR SCIENCE EEYIEW. 
shaped groups. They show an eminent basal cleavage. Soft. The specific 
gravity of the purest scales (showing less than one per cent, of impurities) 
was found to be 2-938 ; another specimen of less purity gave 2-921. Lustre 
pearly, inclining to submetallic. Colour dark clove-brown to greenish- 
brown, sometimes dark brownish-green. Before the blow-pipe it fuses 
easily to a black glass, colouring the flame slightly pink. With salt of 
phosphorus gives a skeleton of silicic acid, a dark yellow bead in the oxidis- 
ing flame, and an emerald-green bead in the reducing flame. Only slightly 
acted upon by acids, even by boiling concentrated sulphuric acid: but 
readily decomposed by dilute sulphuric acid, when heated in a sealed tube 
at a temperature of about 180° C., leaving the silicic acid in the form of 
white pearly scales, and yielding a deep bluish-green solution. With sodic 
carbonate it fuses to a white mass. 
The Chemical Composition of Durangite. — Mr. G. Brush, whom 
mineralogical readers will remember described this mineral as long ago as 
1869, says that he is again indebted to Mr. Henry G. Hanks, of San Fran- 
cisco, 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 colour. The former were loose detached crystals, while these are asso- 
ciated with, and in some cases attached to, rolled fragments of crystallised 
hematite and cassiterite. The density of the small dark-coloured crystals 
is 4-07, while that of the purest of the bright-coloured crystals before 
described is 3-937. In all other physical characters there is a perfect cor- 
respondence between the two varieties. The chemical examination of the 
dark-coloured small crystals has been undertaken, at my request, by my 
assistant, Mr. George W. Hawes, first to estimate the amount of fluorine in 
the mineral, which in two determinations he found to be 7'67 and 7'49 per 
cent., and Mr. Hawes as also placed at my disposal for this article a com- 
plete analysis of this variety of the mineral. The fluorine was determined 
directly by Wohler’s method as modified by Fresenius. To determine the 
arsenic acid and the bases, the mineral was decomposed by sulphuric acid, 
and the arsenic weighed as sulphide; the alumina, iron, and manganese 
obtained in the analysis were carefully examined to ascertain their purity. 
The soda and lithia were weighed as sulphates and then converted into 
chlorides and separated by ether and alcohol. The results of the analysis 
are as follows : — 
i. n. 
Arsenic acid 
53-11 
— 
Alumina . 
17-19 
— 
Ferric oxide 
9-23 
— 
Manganic oxide. 
2-08 
— 
Soda . 
. . 13 06 
— 
Lithia 
. . 0-65 
— 
Fluorine . 
7-67 
7-49 
102-99 
Silliman's American Journal, June 1876. 
