Art. XXX.—On the absorptive properties of Silica; and its direct hydration by contact with water.
[Read before the Wellington Philosophical Society, July 17, 1869.]
In No. 157 of the “London Chemical News,” I communicated the fact that silica is hydrated and dissolved by aqueous solution of ammonia. Evidence in favour of this being given in a recent number of the same Journal, together with particulars as to the amount of this solubility, I thought it desirable to ascertain whether ammonia is absolutely necessary to ensure this, the first of these reactions, the hydration of the silica, it occurred to me that water might effect it of itself;—the action of ammonia, in this instance, being confined to bringing the silica, thus hydrated, into solution.
The following experiments tend to show this assumption to be correct.
Rock-crystal, finely pulverized in an agate mortar, then agitated with water, did not completely subside, even after the lapse of some days; the water remained turbid like clay-water, and like it, is soon clarified by the addition of an acid of a neutral salt.
The effects of such additions would, I conceive, rather retard the precipitation of the silica, by increasing the gravity of the fluid, were it not that combination between the silica and the water had commenced—were it not also for an affinity of this substance for water under these conditions,—feeble, no doubt, as to intensity, but insatiable as to quantity.
There appears to be one weak point in the evidence here tendered, namely, that agate (the substance of the mortar used), is not pure silica; still, it is so nearly pure, that upon the whole it is, I think, quite safe to leave this matter out of further consideration.
In reference to other absorptive properties of silica, I find that massive quartz, rock-crystal, and silica, prepared for estimation in the usual way, take sesqui-oxide of iron from solution of its acetate, but not from the chloride.
Prepared silica, especially, manifests this property, if ignited at a low temperature; and, besides, takes oxides of chromium and copper from their acetates, and removes certain organic matters from their aqueous solutions. These reactions are more apparent in this case, because the silica is in a finely divided state, chemically pulverized in fact.
These reactions show silica to be a feeble mordant, and I think they have an intimate relation to what is termed the physico-mechanical absorption of soils, etc., since we thus see that one of the main constituents of rocks and soils, supposed to be at once the most insert and the most insoluble in an ordinary way, are capable of chemically absorbing certain substances to an extent proportionate to that of the surfaces exposed; such surfaces, even those of rock-crystal itself, are certain to be in a hydrous, in fact in a pulpy state, whenever water has had prolonged contact with them. It follows, therefore, if a substance, which has hitherto been held to be so inert and so unassailable, in these respects, as quartz, in thus actually affected in this manner, we may be certain that the great bulk of our soils, and our more porous rocks, have been affected by water and saline substances in a similar manner;—we may be quite certain, that the surfaces of every siliceous stone, and of every grain of siliceous sand in our soils, is hydrated, and, by so far, advanced to the possession of what is termed the physico-mechanical absorptive power for plant-food.
It only remains for me to state that the reactions here described tend to resolve the so called “physico-mechanical absorption of soils for plant-food,” into a simply chemical one, or, at least, as much a chemical one as are any of those undisputably recognized as such.