ON THE FORMATION OF MINERALS. 205 
Siliéa S>ica Pap Shrek. 59-004 
FeO? 22 /0RROe 13-836 
MoO SPRL See Mi 10: 8-351 
AA OU: be nuaioiie: E 8-103 
BOO"! sostiel Ie oie. | 11-800 
101-094 
The slow metamorphosis which some slates appear to have undergone, and 
their insensible transition from slate to chloritic slate, show, as I think, the 
latent part that alkalies have had in that transformation, by their reaction on 
alumina, protoxide of iron, and magnesia, and also by their faculty of partially 
separating from the combination once formed. Chlorite always contains more 
or less alkalies ; and even Andalusite found in these rocks often retains traces 
of alkalies as the last evidence of its mode of formation. 
The colloid condition assumed by these aluminous silicates, obtained at a 
moderate temperature, may lead to the conclusion that the foliated structure 
assumed by chloritic schists is more or less connected with phenomena of this 
Jind. 
Silicate of protoxide of iron dissolved in caustic potash is not precipitated by 
the alkaline sulphides, and the solution acquires the well-known green tint 
which the slight traces of sulphide of iron remaining in solution give it when 
we precipitate a salt of iron by an alkaline sulphide. 
Some drops of acid added to the solution of the silicate in the alkaline sulphide 
give an emerald-green precipitate, which is decomposed with evolution of 
sulphide of hydrogen on the addition of an excess of acid. The green sub- 
stance loses its colour as soon as it ceases to be under the influence of the 
sulphides. A porous body saturated with this solution loses its green tint by 
desiccation ; the colour reappears with a bluish tinge if the substance be 
exposed to the vapours of sulphide of ammonium. The colour may be thus 
revived for a certain number of times, after which the phenomenon no longer 
takes place. 
This phenomenon has relation to the natural formation of ultramarine, a 
substance which is always accompanied by pyrites of iron. The silicate of 
iron dissolved in the sulphides of potassium leaves upon the side of the glass 
an ultramarine blue tint; but other circumstances may lead us rather to 
suspect that this blue colour is due to a molecular condition of the sulphur 
itself, since a sulphide left to the air in a vase exhibits on the sides of the 
glass a fugitive blue tint. 
Action of the Alkalies on Silicate of Lime. 
The direct action of the alkalies, when carbonic acid is not present, on the 
hydrated silicate of lime is very simple, and may be briefly stated thus :—If 
the hydrated silicate of lime, 2 S10, CaO, 2 HO, be treated with caustic potash, 
it loses an equivalent of silica, and becomes transformed into 810, CaO, HO; 
this silicate loses its equivalent of water at a dull red heat, and is then found 
to have the composition of tabular spar, CaO, Si0,,. 
IL. Propucrion or SunpHuret or Zinc, Brenve, SELENITE, AND CaLAMINE, 
UNDER THE INFLUENCE OF PUTREFACTION oF ANIMAL MArrer. 
The reaction of sulphate of zinc on carbonate of lime or magnesia easily 
explains the production of Smithsonite, or carbonate of zinc; but when we 
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