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.

306 PROF. LIVEING, EFFECTS OF DILUTION, TEMPERATURE, ETC. ON THE
chloride transmits a sensible amount of light as high as X315 (the highest part of the
spectrum included in my photographs) but with a gradually fading intensity frorn about
348 upwards. And this diffuse absorption creeps further down as the solution is stronger
until with a solution half the strongest, in the same thickness, it reaches X 360. Didymium bromide produces a similar diffuse absorption which extends lower than in the case
of the chloride; and didymium sulphate shews something of the same kind.
This diffuse absorption, which creeps far down the spectrum of the most concentrated solutions of the chlorides of both didymium and erbium, seems to belong
to a different category from that to which the other bands belong. For not only
is it diminished by dilution when the thickness of the stratum is proportioned to
the dilution, but it is diminished by diminishing the thickness of the strong solution,
without diluting it, at a greater rate than the other bands are diminished, for some
of the ultra-violet bands which are quite obscured by it when the liquid is 38 mm. thick
are visible in the photographs when the same liquid is only 6'7 mm. thick. The obvious
suggestion is that it is due in some way to the common element, the chlorine. Most
chlorides, however, produce no such absorption. I have tried solutions of calcium, zinc,
and aluminium chloride, respectively, and found them, in a thickness of 305 mm., very
nearly as transparent as water for the range of the spectrum included in my photographs,
namely below X 355. One chloride I have found, when in a concentrated solution, to
behave like the didymium and erbium chlorides, and that is hydrochloric acid, whether
it be dissolved in water or in alcohol. Plate 12 is a reproduction of a photograph of
the spectra of solutions in alcohol, and in water, of hydrochloric acid, in several thicknesses, and in proportional -degrees of dilution, along with one of distilled water for
comparison.
The increasing extent of the absorption with increasing concentration of the solution is manifest; and the most probable cause is some action between the molecules of
acid during their encounters, for it seems to depend on the number of molecules of
acid (or salt) and on their concentration, jointly. We cannot ascribe the absorption to
the chlorine ion, because the number of chlorine ions increases with dilution; but the
close correspondence of the effects strongly suggests a common cause in all the solutions
which give those effects. It should be observed that the percentage of chlorine in the
concentrated solution of the acid used in these experiments bore to that in the most
concentrated solution of didymium chloride the ratio of about 39 to 14'5. The extent,
down the spectrum, of the absorption now in question, is increased, as might be expected, by adding hydrochloric acid to the didymium solution, and also by raising the
temperature as described below. In connexion with this it may be remarked that concentrated neutral solutions of didymium, and erbium, chloride lose the clean pink tint,
by transmitted light, of their dilute solutions, and take up more of an orange hue,
due of course to the diminution of the rays at the blue end of the spectrum.
As above stated I have been unable to obtain a solution of didymrnium sulphate so
concentrated as my strongest solution of chloride; but using the solution containing