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. 309
There are here four facts to deal with:
1. The identity of the spectra of the different salts of the same metal in the
dilute condition.
2. The constancy of this spectrum in the case of chloride and sulphate in different
dilutions so long as the thickness of absorbent is proportional to the dilution, a constancy
holding good in the chlorides for a great range of concentration.
3. The modification, for I take it to be only a modification, of this spectrum in
the case of the nitrate, by some cause which has increasing effect with increasing concentration.
4. The absorptions at the most refrangible end of the spectrum, which are somewhat
different for different salts of the same metal, and diminish with increased dilution.
The first of these facts is certainly strongly suggestive of the interpretation put on
it by Ostwald, that the spectrum common. to all the salts of the same metal is due to
the metallic ions. Against this the second fact militates, for the ionization is supposed to
increase with dilution, and the absorptions by the ions should increase in intensity by
dilution when the total quantity of salt, dissociated and undissociated, through which the
light passes remains the same. The third fact points to some cause, affecting the diffuseness of the bands, which is more effective in concentrated solutions. This cause may be
encounters between the molecules of the salt, or of its products in solution, which would
be more frequent in more concentrated solutions.
Ionization should be increased by heating the solutions, and diminished by the addition
of acid. I proceed to describe what I have observed of the effects of heating and of
acidification on the absorption spectra.
EFFECTS OF TEMPERATURE ON THE SPECTRA.
The rise of temperature which could be employed was, as described above, only from
the temperature of the rooin, about 20~, to a few degrees below the boiling-point of the
water bath, or to about 97~. This rise of temperature produced the same kind of effect
on all those absorption bands which are common to all the salts of the same metal, whether
it be didymium or erbium, and that effect was to render them more diffuse, to spread
them out, make their limits less definite, and in the case of weak bands make them appear
weaker. The effect of heat was also the same in kind on dilute as on concentrated
solutions. Heat also caused the broad diffuse absorption at the most refrangible end to
extend itself downwards in a marked degree. Plates 13, 14 and 15 are reproductions of
photographs of the spectra of three salts, in various degrees of dilution, cold and hot.
It will be noticed that the absorption bands are not increased in intensity by heat, but
from the greater diffusion they seem weaker, except the very strong bands which are so
intense that they bear diffusion without letting enough light through to affect the plate.
The creeping down with the higher temperature of a diffuse absorption from the most