374 Reéramination of American Minerals. 
pared and examined, the first two being of the greenish variety 
and the third of the yellow variety, which color is doubtless due 
to a peroxydation of a minute quantity of the protoxyd of iron 
entering into the constitution of the mineral. Mr. L. Saemann 
in a communication made to the American Association some 
time since, attributed this change to magnetic iron undergoing 
decomposition ; but this, however, does not appear to me to be 
the case, for the reasons that crystallized magnetic iron is a min- 
eral difficult of decomposition, and the color is not in fissures as 
would be the case if the peroxyd arose from a substance foreign 
to the composition of the mineral, but enters into its most inti- 
mate structure. 
The hardness of Boltonite is found to be, as already stated, be- 
tween 5and 6. The specific gravity was taken on three speci- 
mens; Nos. 1 and 2, on a gramme each of fine particles; No. 3 
on a piece of :150 gramme, all possible precautions being used to 
arrive at correct results : 
No. 1, 3270 No.2, 3-208 No, 3, 3'328 
No. 3 is to be regarded as by far the most reliable, as in taking 
the sp. gr. of fine grains it is almost impossible to detach the 
last particles of air, and consequently the sp. gr. they indicate 1s 
below the true number. 
The analyses of three portions gave— 

J 0.1 No. 
ili > 42°56 41:95 42-41 
Magnesia, : - 51°77 51°64 50:06 
Protox. iron, . : i Oo 3°20 3°59 
Alumina, ; ; : 0:10 0°2 f 
Loss by heat, . E é3:1988 1-58 not estimated. 
99°00 98°62 
Nos. 1 and 2 were the greenish variety, No. 3 the yellowish. 
The oxygen ratio of the silica and protoxyds are— 
= No. 1. No. 2. No. 3. 
Silica, ; ‘ ; 2241 21°77 2a 08 
Magnesia, . a ‘ 20°35 20°30 196 
Protoxyd of iron, ‘ i “52 it “15 
This being as one to one within a small fraction, the formula 
therefore is (Mge)* Si, or of the general form ki, which of 
course proves it to be chrysolite, a fact sustained in every respect 
by its physical characters. 
38. Iodid of Silver. 
In this reéxamination of American minerals it was not origil- 
_ally designed to include those of South America: but my recent 
examination of the minerals obtained by Lieut. Gilliss of the U.S. 
Chili Expedition, has afforded an opportunity of analyzing a 
. . 
tain minerals that it was well to investigate, and among 
