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any required direction, or to discover such polarization when it exists.
Moreover, M. Biot has discovered that certain solid bodies, and even certain fluids, possess the faculty of changing progressively polarization previously impressed on rays passing through them; and by an analysis of the phænomena produced by those substances he has shown that the same faculty resides in their smallest molecules, so that they preserve it in all states solid, liquid, and acriform, and even in all combinations into which they may happen to enter. M. Fresnel has found certain analogies between these phænomena and those of double refraction, which seem to connect the two together most intimately through the intermediation of total reflexion.
Since reflexion and refraction, even of the ordinary kind, modify the polarization of light, we may expect to find this effect produced when rays of light are made to pass through media of regularly varying density. It is accordingly found that all transparent bodies which are sufficiently elastic to admit of different positions of their particles round a given state of equilibrium, as glass, crystals, animal jellies, horn, &c. produce phænomena of polarization when they are compressed or expanded, or made unequally dense by being considerably heated and then cooled suddenly and unequally. These phænomena, discovered originally by M. Seebeck, have been since studied and considerably extended by Dr. Brewster, who has moreover remarked, that successive reflexions of light on metallic plates produced phænomena of colours in which both M. Biot and he have recognized all the characters of alternate polarization.
Knowing, by what has preceded, the experimental laws, according to which light is decomposed in crystals endued with double refraction, we may consider these effects as proofs proper to characterise the mode of intimate aggregation of the particles of such bodies, and to give some insight into the nature of their crystalline structure. Light becomes thus, as it were, a delicate sounding instrument with which we probe the substance of matter, and which, insinuating itself between their minutest parts, permits us to study their arrangement at which Mineralogists previously guessed only by inspection of their external forms. M. Biot has shown the use of this method, applying it to a numerous class of minerals desig-
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