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.

310 PROF. LIVEING, EFFECTS OF DILUTION, TEMPERATURE, ETC. ON THE
refrangible end is seen in all, and with the nitrate and sulphate seems to be independent
of the concentration, while with the chloride it is barely noticeable with any but the most
concentrated solution. In the last exposure with the sulphate the light is a little weaker
throughout. The solution was the weakest and in the longest tube, and therefore most
likely to be troubled with bubbles on the inner faces of the terminal quartz plates which
could not be removed. I have no doubt this general weakening of the light was due to
this cause. A general weakening of the light has the effect of making the absorption
bands appear stronger. This appearance is deceptive; for the examination of a great many
photographs, as well as direct observations of the spectra by eye, have led me to the
conclusion that the effect of heat is to diffuse and not to strengthen the absorption bands
which are ascribed to the metals. On the other hand it looks as if the diffuse absorption
at the most refrangible end, which certainly creeps down lower with hot solutions, were
strengthened as well as diffused, for in the region above that included in the plates, the
limit of complete extinction of photographie effect is considerably lower with the hot than
with the cold solutions.
On the whole the effects of heat on the spectrum afford no confirmation to the supposition that the absorptions are due to an increase of the number of ions; but rather
suggest that they may be due to the increased energy of the motions of translation of the
molecules, causing more frequent encounters.
EFFECTS OF ACIDIFYING TIHE SOLUTIONS.
The solutions compared with a view to ascertain these effects had in every case equal
quantities of the metallic component per litre, but while one was neutral the other had
twice as much of its acid component as the first; and they were usually compared in
various degrees of dilution and in thicknesses proportional thereto. With didymium salts,
chloride and nitrate, the acid made very little difference in the bands, as will be seen
by examination of plate 18, which gives the spectra of four solutions of the chloride,
two neutral and two acid, The creeping down of the absorption at the most refrangible
end is, however, very evident in the most concentrated solution of acidified chloride;
and some diffusion of soine of the bands of the nitrate by the addition of the
acid is just traceable in photographs of some of the weaker bands of the more concentrated solution. The increased diffusion of the bands of the nitrate by the addition
of nitric acid can be easily seen directly by eye, using weak solutions in no great
thickness. The addition of acid also produces a slight shift of the places of greatest
absorption in the strong groups in the yellow and green. Whether this is due only to
the expansion, and consequent overlapping, of the several bands in these groups, or
whether there is a real shift, I have not been able to satisfy myself; but the general
appearance resembles the changes produced in those bands by the use of different solvents
which are described below, and it is very likely that similar causes are at work in the
two cases. Nothing of this kind can be seen on the addition of hydrochloric acid to
the chloride.