The Edinburgh Philosophical Journal

forces or influences which determine mineral bodies to assume a crystalline form, or concerning the causes which produce such a diversity of forms among crystals. Haüy states, that “when the molecules of a body are suspended in a fluid, which afterwards, either by evaporation, or through some other cause, abandons them to their reciprocal affinities, if no disturbing force should interfere, the molecules would unite by those planes the most disposed to such union, and would, by their combination, produce the regular solids which we term Crystals."

But this explanation of the manner in which a change of state from solution to solidity may take place, does not assist our inquiry into the nature of the causes which predispose the molecules to form solids of particular shapes, or which determine particular planes of those molecules constantly to unite with each other.

We may, for the purpose of adapting a theory to the facts we observe, suppose the molecules of bodies influenced by different forces; one of which acts only within very small distances, and be termed the cohesive force; the others, acting at greater VOL. XI. NO. 23. JAN. 1825.

may

distances, introducing attraction or repulsion among the particles subject to their influence, and possibly bearing some close analogy to electricity. We may suppose these latter forces acting upon molecules held in solution at considerable distances, and predisposing them to approach each other, and to arrange themselves at the same time in some regular order; and we may imagine the same forces continuing to exert themselves, until the particles are brought within that distance from each other at which the cohesive force begins to operate. We may then imagine this cohesive force to fix and retain the molecules in those positions in which, when their numbers are sufficient, they will constitute, by their aggregation, visible and regular solids.

It is evident that we may frame other theories to account for the formation of visible solids; but there does not appear to be any hypothesis capable of accounting for the variety of forms under which crystals present themselves.

I shall, however, proceed to consider shortly the general phenomena of the production of crystals, and the circumstances under which the crystallisation of minerals probably takes place.

The different hypotheses which assign an aqueous or an igneous origin, to what has been termed the crust of our earth, a small portion only of the depth or thickness of which has been yet penetrated by the industry of man, suppose the minerals which have been discovered upon or beneath its surface to have been produced by corresponding causes.

It is, however, certain, that many of the natural crystals with which we are acquainted, were not formed contemporaneously with the bed in which they have been discovered, but that they have been produced at different periods, and possibly under very different circumstances. Sometimes their origin may be ascribed to the action of heat, sometimes to the solvent power of some fluid, and in other instances to the united influence of both these causes.

The observations of Sir Humphrey Davy " upon the state of water and aëriform matter in cavities found in certain crystals," printed in the Phil. Trans. for 1822, render it not improbable that natural crystals are formed under very different states both of pressure and temperature.

There are several appearances not uncommon among crystals,

from which we may infer that they have been sometimes slowly formed, and that accompanying crystals of different minerals have been deposited at very different periods.

The crystals of carbonate of lime which are found at Ecton in Staffordshire, frequently contain numerous minute crystals of copper-pyrites, apparently first deposited on small crystals of the carbonate of lime. This substance has then formed over the pyrites, and produced a larger crystal of the carbonate of lime, upon which a second deposit of copper-pyrites has taken place. This has been again enclosed within a still larger crystal of the calcareous matter, upon which other crystals of pyrites have been again deposited. And we may discover, when the crystals are large, several alternations of these two minerals successively covering each other.

The crystals which are termed pseudomorphous, afford very distinct evidence of successive formation, as the period at which these were produced must have been posterior to that of the crystal whose form they imitate. I have observed in one instance a mould in preparation, if I may so term it, for a pseudomorphous crystal, from which a part only of the model, a crystal of fluate of lime, had been removed.

The mould itself was crystallised quartz, which had coated the crystal of fluor. The size of this crystal was originally more than a cubic inch; but it had been subsequently reduced to a rounded mass, loose within the mould, of about half that bulk, with an irregular and smooth surface, like that of partially dissolved salts.

Hollows of various forms, contained within crystallised quartz, are not uncommon; but I do not recollect any other instance of the crystal, whose removal had produced the vacuity, being only in part destroyed, as if its dissolution had been recently going on. Numerous other examples might be cited as evidences of the gradual and the successive formation of crystals, by processes which are probably still in operation, although we are uninformed with respect to their nature.

There are circumstances which render it almost certain that some crystals have been produced from solution in a fluid. The water found in the cavities of certain crystals of quartz, would seem to refer their origin to solution in that fluid; and we

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The Edinburgh Philosophical Journal, 12권