of Arkansite. ; 43 
to permit the use of the reflecting goniometer. M on M is 
given by Prof. Shepard as 101°. The specimens which ] have 
are nearly of the form figured by him in his “ farther account 
of the Arkansite.”  (Silliman's Journal, Vol. IV. new series, 
p. 229.) Like the crystals described by him, they are im- 
planted on quartz crystals, and about one fifth of an inch in 
diameter. Hardness 6; Specific gravity 4.085; the other 
external characters are nearly as given by Prof. Shepard. 
The chemical composition of the Arkansite seems to have 
been a matter of some doubt with the Professor, for, from his 
first examination, he concludes the mineral to be a titanate of : 
yttria, and possibly zirconia and thorina; on a second and 
more deliberate investigation, he decides that it is a “ niobate 
of yttria and thorina.” J need hardly say that the detail of 
the investigations does not justify either of these conclusions. 
On examining the Arkansite before the blow-pipe, titanic 
acid was readily detected in it, it being found to have exactly 
the reactions of this substance as it occurs in nature in the 
form of rutile, Brookite or anatase. The specific gravity of 
the mineral is given by Prof. S. as 3.854 ; this seemed to me 
hardly probable, as, were the mineral essentially titanic acid, 
as I at once supposed it to be, having the crystalline form of 
Brookite, (M on M, as given by Prof. S. on crystals too dull 
to allow of accurate measurement, only differing 1? from 
Brookite,) it could not be expected to have a specific gravity 
as low as that of anatase. I therefore carefully determined 
it on about three grammes of the small crystals, and found it 
to be 4.085, which is very near that of Brookite, a very little 
less, some of the crystals probably being slightly intermixed 
with the quartz, from which it was difficult to free them 
entirely. This agreement in the specific gravity was a con- 
firmation of my views with regard to the composition of the - 
mineral ; I therefore examined it qualitatively, and have fully 
satisfied. myself that the Arkansite is titanic acid, with a trace 
of iron, having the crystalline form and specific gravity of 
rookite. Its insolubility in acids is already strong presump- 
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