RE-EXAMINATION OP AMERICAN MINERALS. 143 



yellow variety, which color is doubtless due to a peroxidation 

 of a minute quantity of the protoxide of iron entering into the 

 constitution of the mineral. Mr. L. Saemann, in a communi- 

 cation made to the American Association some time since, 

 attributed this change to magnetic iron undergoing decom- 

 position ; 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 peroxide arose from a substance 

 foreign to the composition of the mineral, but enters into its 

 most intimate structure. 



The hardness of boltonite is found to be, as already stated, 

 between 5 and 6. The specific gravity was taken on three 

 specimens ; Nos. 1 and 2 on a gramme each of fine particles ; 

 No. 3 on a piece of 0.150 gramme, all possible precautions being 

 used to arrive at correct results. 



No. 1 3.270 No. 2 3.208 No. 3 3.328 



No. 3 is to be regarded as by far the most reliable, as in 



taking the specific gravity of fine grains it is almost impossible 



to detach the last particles of air, and consequently the specific 



gravity they indicate is below the true number. The analyses 



of three portions gave 123 



Silica 42.56 41.95 42.41 



Magnesia 51.77 51.64 50.06 



Protoxide of iron 2.35 3.20) Q r Q 



Alumina 10 .25/ rf,oy 



Lossbyheat 2.22 1.58 not estimated. 



99.00 98.62 



Nos. 1 and 2 were the greenish variety, No. 3 the yellowish. 

 The oxygen ratios of the silica and protoxides are 



12 3 



Silica 22.11 21.77 22.03 



Magnesia 20.35 20.30 19.67 



Protoxide of iron .52 .71 .75 



This being as 1 to 1 within a small fraction, the formula 



therefore is (MgFe) 3 Si, or of the general form ft 3 Si, which 



of course proves it to be chrysolite, a fact sustained in every 



respect by its physical characters. 



38. Iodide of Silver. 

 In this re-examination of American minerals it was not origin- 

 ally designed to include those of South America, but my recent 



