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.

314 PROF. LIVEING, EFFECTS OF DILUTION, TEMPERATURE, ETC. ON THE
On a review of the whole series of observations I conclude that the characteristic
absorptions of didymium compounds, namely those which are common to dilute aqueous
solutions, and are only modified by concentration, by heat, and by variations of the
solvent, are due to molecules which are identical in all cases, though their vibrations
are modified by their relations to other molecules surrounding them. The like conclusion
holds for erbium compounds. It appears to me quite incredible that the atoms of
didymium should retain in chemical combination so much individuality and freedom as
to take up their own peculiar vibrations unaffected by the rest of the matter combined
with them, as must be the case if we supposed the combined didymium in the nzolecules
to give the common spectrum of all the salts in dilute solution. When I speak of
atoms of didymium in the salts, I mean of course masses equal to the atoms of
didymium metal, but having different energy, which means different internal motions,
probably different structure, and different capabilities of vibration. No chemical compounds shew the absorptions which their separate elements exhibit. Sodium vapour,
though monatomic, has a very strong absorbent power which is quite lost when it has
parted with energy in combining with chlorine. Nevertheless the molecule of a chloride
breaks up, in general, into masses equal to those of the atoms of its elements more
easily than in any other way, and there is pretty good evidence that in encountering
a molecule of water this also is sometimes broken up, and ultimately, if not immediately,
new molecules of hydroxide and acid are formed, as well as, by a similar process, new
molecules of the salt. In the interval between the rupture of a molecule and the
recombination of its parts with each other, or with parts of other molecules, the parts
have a certain freedom, and capability of vibrating, which they do not possess in combination. Now if we suppose the number of such parts as have the capability of taking
up vibrations of frequency corresponding to the characteristic absorptions of didymium
to be directly proportional to the concentration of the didymium salt and to the time
of their freedom, the observed facts will be all in agreement with the hypothesis.
Increased concentration, and increased temperature, will mean more frequent encounters
amongst the molecules, and more frequent ruptures, but at the same time more frequent
encounters of the parts and consequent shortening of their times of freedom. These
effects will exactly compensate each other and leave the average number of absorbent
parts of molecules constant under changes either of concentration or of temperature.
The continuous absorption of the more rapid vibrations increasing with concentration
and rise of temperature points to an action depending only on the number of encounters
of the molecules of the salt with one another. It is not every encounter which is
attended with disruption, and the continuous absorption may be due to molecules in
encounter without rupture, but at all events it seems due to the condition of the
molecules during encounter, but not to occur at the encounters of a molecule of salt
with the very much less massive molecules of water. Encounters of a molecule of salt
with a molecule of acid will in all probability cause effects very similar to those of
encounters between two molecules of salt, and this supposition is quite in agreement
with the observed facts.
The time of complete freedom of a vibrating part of a molecule must be very