The poetry which surrounds these, in every way, mysterious conditions of the solar beam, is such, that it is with difficulty that imagination is restrained by the stern features of truth. The uses of this peculiar property in great natural phenomena are not yet made known to us; but, since we find on every side of us the natural conditions for thus separating the beam of light, and effecting its polarization, there must certainly be some most important end for which it is designed by Him who said, “Let there be Light.” It must not be forgotten that we have at command the means of showing that the chromatic phenomena of polarized light are due to atomic arrangement. By altering the molecular arrangement of transparent bodies, either by heat or by mere mechanical pressure, the unequal tension or strain of the particles is at once indicated by means of the polarized ray of light and its rings of colour. Differences in the chemical constitution of bodies, too slight to be discovered by any other mode of analysis, can be most readily and certainly detected by this luminous investigator of the molecular forces.[101] Although we cannot enter into an examination of all the conditions involved in the polarization of, and the action of matter on, ordinary light, it will be readily conceived, from what has been already stated, that some most important properties are indicated, beyond those which science has made known. Almost every substance in nature, in some definite position, appears to have the power of producing this change upon the solar ray, as may be satisfactorily shown by examining them with a polarizing apparatus.[102] The sky at all times furnishes polarized light, which is most intense where it is blue and unclouded, and the point of maximum polarization is varied according to the relative position of the sun and the observer. A knowledge of this fact has led to the construction of a “Solar Clock,”[103] with which the hour can be readily determined by examining the polarized condition of the sky. It has been stated, that chemical change on the Daguerreotype plates and on photographic papers is more readily produced by the polarized than by the ordinary sunbeam.[104] If this fact be established by future investigations, we advance a step towards the discovery so much desiderated of the part it plays in natural operations.