and Equivalent Volume of some Mineral Species. 215 
so that the formula becomes sissOs«(M2:H:)O2s=82RO, 
while some amorphous tales give sis sOs«(M:i9H1; )Oss=88RO, 
or the formula of hornblende, plus 4MO and 10MO. 
gives according to Scheerer’s view, the relation of augite, or 2:1; 
but if the water be excluded, we obtain the relation of hornblende 
or 9: 4, and with the water, the formula is nearly sis1Oss(Maa 
Hs)O32=86RO; a similar relation appears in the nephrites 
which Scheerer has referred to the augite type. In certain other 
hydrous magnesian minerals, affording from five to ten per cent. 
of alumina, he has endeavored to show that 3Al2QOs: replace 2Si 
Os, and thus deduces for the various species, between the oxygen 
of the silica, with that of two-thirds of the alumina, and the 
oxygens of the protoxyds with that of one-third of the water, a 
ratio of 2: 1. But it will be found that his analyses admit of a 
simpler interpretation ; if we conceive al O to replace si O, as in 
hudsonite aud carinthite, we obtain, for all the aluminous species 
which he has referred to the augite type, (with but one exception, ) 
a ratio between the sum of the oxygen of the silica and alumina 
on the one hand, and the oxygen of the protoxyd bases on the 
other, (that of the water being excluded,) of 9 : 4, while the oxy- 
gen of the water is equal either to 1 or 2, so that the formulas 
may be written (si al)s«O51, M2sOa« plus 6HO, and 12HO, = 
S4RO and90RO. <A diopside from Reichenstein affording 1:10 
of alumina and 1-9 of water, is the exception referred to, and is 
an aluminous augite, plus water. 
Without having considered the densities of these minerals, we 
give the above formulas as representing the general type, although 
perhaps not the real equivalents of the species. 
We now proceed to examine a number of isomorphous species, 
some of which will serve to illustrate the question of hydrous 
Silicates. The arragonite group and the species which Rose has 
compared with it, crystallize in right rhombric prisms, in which 
the angle M: M varies from 112° 6’ in bournonite to 119° in 
nitre. Mr. Dana has pointed out another isomorphous trimetric 
group, having for the same prism 119°-120°. They are epsom- 
salt, chrysolite, picrosmine, serpentine, villarsite, and chrysoberyl, 
to which last he has compared the crystallization of topaz. We 
add iolite and aspasiolite, without knowing any determination of 
the vertical axes of these species. ; 
Epsom-salt is generally written SOs, MgO, 7HO, and with a 
density of 1-689 (P. and Joule) gives 73°3 for its equivalent vol- 
ume ; the formula S:Mg:O2s, 49HO, will then correspond to a 
volume of 513, while the higher density 1-75 which has been, 
assigned to it gives 492. The formula of chrysolite (Mg Fe)s Bi, 
may be multiplied by four, and the analyses of many specimens 
then lead to the formula sissOus(Mgs aFes Oss, which gives 
