550 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
sense that the molecules retain the same mean state ; and that the disturbance which 
the molecules in turn communicate to the ether must be such as cannot be expressed 
by circular functions of a given period, namely, that of the incident vibrations. 
228. It is very remarkable with what pertinacity a particular mode of internal 
dispersion attaches itself to a particular chemical substance. Thus the singular 
dispersion of a red light exhibited by the green colouring matter of leaves is found 
in a green leaf, or in a solution of the green colouring matter in alcohol, ether, sul- 
phuret of carbon, or muriatic acid. The dispersion exhibited by nitrate of uranium 
is found in a solution of the salt in water, as 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 com- 
pounds, the ultimate molecules of which we are taught hy 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 formulm of these com- 
pounds, 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 formulae relating to other 
oxides. Why this particular oxide should be disposed to enter into tottering com- 
binations 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 the 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. 
230. Reasons have already been brought forward for regarding the molecular 
vibrations as performed under the influence of forces not proportional to the dis- 
