204 , REPORT—1868. 
This substance, when dried, had a semivitreous transparent aspect. Heated 
to a dull red, it becomes insoluble in acids. 
2nd Experiment.—A saturated solution of the hydrated silicate of mag- 
nesia (2Mg0,3 SiO, + 4HO) in caustic potash, treated by a dilute solution of 
bicarbonate of magnesia, gives a gelatinous precipitate, which, after having 
been washed till the action of acids no longer disengages carbonic acid, had 
the following composition :— 
DURCH eta ne nals ae 40°285 
Magnesia ........ 38°250 
Waters isc os 5% ws 19-428 
Carbonic acid ...... 1-450 
99-413 
The substance thus obtained would represent a serpentine with three equiva- 
lents of water; it has the composition of the Deweylite of Thompson, which 
is, in fact, a variety of serpentine. 
As the bicarbonate of magnesia which remains in solution with the silicate 
of potash has a tendency to form with the latter double salts but slightly soluble, 
it is well to employ only dilute liquors. This tendency of magnesia to replace 
the alkalies in silicates is exemplified in a great number of hydrated magnesian 
compounds. On the other hand, the zeolites are in general remarkable by 
the absence of magnesia; and in one or two exceptional cases, such as the 
Picrothomsonite for example, in which magnesia enters into the constitution 
of the mineral, the augmentation of magnesia is attended by a corresponding 
diminution of the alkalies. Thus serpentine should have been the result of 
the action of water containing alkalies on magnesian rocks. The same phe- 
nomenon is shown on a small scale in certain basaltic tufas, in which we meet 
with a deposit of magnesian silicate, accompanied often by arragonite and 
calcareous spar, containing more or less magnesia, while the alkalies of the 
basalt have completely disappeared *. 
Solubility of Silicate of Hydrated Protowxide of Iron. 
The silicate of the hydrated protoxide of iron is soluble in the cold in 
alkalies, if we take care to exclude air; this solution is more readily effected 
in presence of magnesia, which appears to protect the liquor from further 
oxidation. We can obtain compounds in which magnesia and protoxide of 
iron exist in various proportions. 
The solutions in potash of the silicates of protoxide of iron, magnesia, and 
alumina, left exposed to the air on a plate, dry and acquire a gelatinous state 
without undergoing alteration, the iron remaining in the state of protoxide ; 
the potash separates itself partially from the compound; the substance, then 
washed and dried, has the form of green scales. Weak hydrochloric acid re- 
moves the bases, leaving the siliceous skeleton unaltered, in the shape of soft 
and nacreous scales like certain chlorites. 
The following analysis will give an idea of the solubility of the silicate of 
protoxide of iron, and of the number of bases which can thus be dissolved and 
precipitated by evaporation, or by the action of Co, :— 
* Besides the artificial Deweylite, of which I have just given the analysis, I have 
obtained a great number of other precipitates of variable composition, which strikingly 
represent the composition of many serpentine-rocks, to which I shall return on another 
occasion. 
