390         ON THE  CHANGE  OF  REFIUNOIBIUTY   OF  LIGHT.
well as in the crystals themselves, which are doubly refracting. In all probability therefore the molecular vibrations by which the dispersed light is produced are not vibrations in which the molecules move among one another, but vibrations among the constituent parts of the molecules themselves, performed by virtue of the internal forces which hold the parts of the molecules together. It is worthy of remark that it is chiefly among organic compounds, the ultimate molecules of which we are taught by chemistry to regard as having a complicated structure, that internal dispersion is found. It is true that peroxide of uranium furnishes many examples of internal dispersion; but then the anhydrous peroxide is itself insensible, it is only some of the compounds into which it enters that are so remarkably sensitive ; and the chemical formulae of these compounds, so far as they are known, are not by any means extremely simple, although it is true that they may not be more complicated than formula) relating to other oxides. Why this particular oxide should be disposed to enter into tottering combinations I do not pretend even to conjecture; but it seems not a little remarkable that peroxide? of uranium, which is so peculiar with respect to its optical properties, should also present some singularities in its mode of chemical combination, which led M. Peligot to regard it as the protoxide of a compound radical.
229. We are, I conceive, at present, far from an explanation of the phenomena of internal dispersion in all their details. They appear to be associated with the inmost structure, of chemical molecules, to such a degree as to throw even the phenomena of polarization into the shade. In this respect, indeed, absorption seems superior to polarization, since most, of t.he phenomena of polarization refer rather to the state of crystalline aggregation of the molecules than to their constitution ; but the phenomena of internal dispersion appear to be much more searching than those, of absorption. There is one law however relating to internal dispersion so striking and so simple, that it seems not unreasonable to look for an explanation of it; I allude to that according to which the refrangibility of light is always lowered in the process of dispersion. I have not hitherto been able altogether to satisfy myself respecting a dynamical explanation of this law, but the following conjectures will not perhaps be deemed altogether unworthy of being mentioned.