Electricity, and light, and heat, exert remarkable powers, and accelerate or retard crystallisation according to the conditions under which these forces are brought to bear on the crystallising mass. We have recently obtained evidence which appears to prove that some form of magnetism has an active influence in determining the natural forms of crystals, and we discover that magnetism exerts a peculiar influence in relation to the optic axes of crystals, which is not exerted in lines at right angles to these. Electricity appears to quicken the process of crystalline aggregation--to collect more readily together those atoms which seek to combine--to bring them all within the limits of that influence by which their symmetrical forms are determined; and strong evidence is now afforded, in support of the theory of magnetic polarity, by the refined investigations of Faraday and Plücker, which prove that magnetism has a _directing_ influence upon crystalline bodies.[40] It has been found that crystals of sulphate of iron, slowly forming from a solution which has been placed within the range of sufficiently powerful magnetic force, dispose themselves along certain magnetic curves, such as are formed around a magnet by steel filings; whereas the crystals of the Arbor Dianæ, or silver tree, forming under the same circumstances, take a position nearly at right angles to these curves. Certain groups of crystals have been found in nature, which appear to show, by their positions, that terrestrial magnetism has been active in producing the phenomena they exhibit; indeed, nearly all our mineral formations indicate the influences of this, or some similarly acting power.[41] During rapid crystallisation, some salts--as the sulphate of soda, boracic acid, and arsenious acid crystallising in muriatic acid--exhibit decided indications of electrical excitement; light is given out in flashes. We have evidence that crystals exhibit a tendency to move towards the light, and that crystallisation takes place more readily, and progresses with greater activity in the sunshine than in the shade. Professor Plücker has recently ascertained that certain crystals--in particular the cyanite--“point very well to the north, by the magnetic power of the earth only. It is a true compass needle; and, more than that, you may obtain its declination.” We must remember that this crystal, the cyanite, is a compound of silica and alumina only. This is the amount of experimental evidence which science has afforded in explanation of the conditions under which nature pursues her wondrous work of crystal formation. We see just sufficient of the operation to be convinced that the luminous star which shines in the brightness of Heaven, and the cavern-secreted gem, are equally the result of forces which are known to us in only a few of their modifications.