Memoirs presented to the Cambridge philosophical society on the occasion of the jubilee of Sir George Gabriel Stokes, bart., Hon. LL. D., Hon. SC. D., Lucasian professor.

ABSORPTION SPECTRA OF SOLUTIONS OF DIDYMIUM AND ERBIUM SALTS. 315
short, but probably shorter when the complementary part is more massive, as in the
case of a nitrate, than it is in the case of a chloride. But between complete freedom
and complete incorporation in a chemical compound there is a considerable gradation,
and the capacity of the part to vibrate at particular rates will have a corresponding
gradation, and the part may moreover be frequently under the influence of molecules,
or parts of molecules, with which it does not combine. This influence will probably be
greater as the molecule exerting the influence is greater whether more massive, or, as
in the case of such colloids as alcohol, more voluminous. These considerations reconcile
all the facts as to the spectra I have observed with the hypothesis I have made.
There are, however, other facts to be reconciled with that hypothesis. I mean
the facts of ionization, of osmotic pressure and the correlative facts of the rise of boiling
point, and fall of crystallizing point, of solutions. In regard to all these effects the
freedom of the parts is the primary postulate, far more definitely so than in the case
of vibrations such as my observations relate to. The laws I have tried to investigate
appear to hold good up to the point of saturation of the solutions, which is not the case
with the laws of osmotic pressure and of change of boiling and freezing points, which
have been established for dilute solutions. Further, ionization implies a certain distribution of energy in the field, the ions are charged with electricity. That is not necessary for the absorption of light, which will depend, primarily at least, on the form of
the internal energy of the vibrating mass, that is on its structure. That a redistribution of energy occurs at every rupture of a molecule seems certain, solution is attended
with thermal effects and so is dilution, and it is only when equilibrium is reached,
and as much change takes place in one direction as in the opposite, that the manifestation of such redistribution ceases. How much of the intrinsic energy of the
molecules takes the form of heat and how much is retained in the field at the rupture
of the molecules we do not know. It is however quite conceivable that the circumstances under which the rupture takes place may determine whether any, or how much,
energy is retained by the field, that is whether any, or how many, of the ruptured
parts become ions.
The plates, which are all reproductions of photographs, will be found at the end of the
volume.
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