68 T. STERRY HUNT : SUPPLEMENT TO 
waters—the protoxyd bases are liberated from combination with the silica, which itself is, 
in great part, like the protoxyds, dissolved. In the case of the aluminous double silicates 
thus decomposed (chiefly feldspars and scapolites) while the protoxyd bases, with more 
or less silica, are in like manner dissolved, the alumina remains behind as a definite 
hydrous silicate. The zirconia of the few zirconie protopersilicates known, would, it is 
conjectured, behave like alumina; and the existence in nature of such compounds as wol- 
chonskoite and chloropal shows in chromic and ferric oxyds (which are not dissolved by 
carbonated waters) a similar affinity for silica. The changes in oxydation of iron and man- 
ganese from atmospheric oxygen, on the one hand, and from certain organic matters on 
the other, modify, in ways which are well known, the relations of these metals in decay- 
ing silicates. The titanium which may be present in silicates thus decomposing probably 
separates as titanic dioxyd, since in the younger crystalline schists it generally appears in 
this state, crystallized as rutile. 
§ 8. In the essay on a Natural System in Mineralogy to which this is a supplement, 
attention was called ($$ 29, 31, 65) to the unlike effects of agents which, like chlorhydric 
and nitric acids, combine with their bases, upon silicates of similar centesimal composition. 
As examples of this were cited the cases of meionite as compared with zoisite, and of wollas- 
tonite with amphibole and pyroxene. Illustrations, moreover, are not wanting in the 
similarly related species of other orders, as in the behavior of the different carbon-spars, 
such as calcite, dolomite and magnesite, with the same acids, or of tridymite and quartz with 
- 
Tage V.— OpxiToips. 




SPECIES. FORMULA. ip D Vv 
I 
Serpentine. - - | (mg,siyo; t2uq | 15-33 . 
Retinalite. - - | (mg,si,)o,; + 23aq | 15-00 
Deweylite. -. - | (mg,si,)o; +3aq_ | 14-00 
Genthite. - - - | (ni,si,)o, + 3aq 18-25 
Aphrodite. - - | (mg,si,)o; + faq | 15:13 
Cerolite. - - - | (mg,si,)o, + 13aq | 14-11 
Chrysocolla. - - |(eu,si,)o, + 2aq | 17°53 
Spadaite. - - - | mg;si,:)0,, + 4aq | 15.04 
Rensselaerite. - | (mg,Sij))oy + 1aq | 15-93 
Sepiolite. - - - | (mgisi,)o, + lag | 14°80 
Glauconite. = -| - = - - - - 




a solution of sodium carbonate. The conclusion was then announced (§ 35) that for species so 
related “the hardness and chemical indifference are inversely as the value of V, or in other words, 
they increase with the condensation, the relative amount of which in the species compared 
is shown by the diminution of V ;’ which is the quotient got by dividing the combining 
weight of the chemical unit, or unit-weight, P by the density or specific gravity D 
(water = 1.000). We were thus led to insist upon the fundamental importance of specific 
gravity in solid species, considered not in itself, but in relation to the mean unit-weight. 
This, which was assumed for Silicates and for Oxydates—as that of the compound of eight 
Ww 
