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.

308 PROF. LIVEING, EFFECTS OF DILUTION, TEMPERATURE, ETC. ON THE
nitrate as a single band narrower than the two given by the chloride, and the middle
band of the triplet in the blue is more diffuse with the nitrate.
The apparent shift above mentioned may be an effect of the overlapping of the
diffuse bands, and though a real shift does not seem to me improbable, it is not in
this case sufficiently decided to found an argument upon.
Plate 6 reproduces the spectra of 6'7 mm. of the strongest solution of didymium
nitrate and of 305 mm. of the same solution diluted to 45'5 times its bulk. The bands
of the strong solution are more diffuse and look somewhat washed out, notably the
narrow band about X 427, and the middle band of the triplet in the blue; and the
strong group in the yellow extends further towards the red and has the appearance of
being stronger with the strong solution than with the dilute.
Erbium nitrate behaves quite in the same way as didymium nitrate in regard to
the greater diffuseness of its bands with strong solutions, and their gradual contraction
and growing sharpness as the solution is diluted, until they come to be identical with
those of the chloride. This is better seen in the photographs of the erbium spectra
than in those of the didymium: see plate No. 5.
In plate 8 the spectrum of 6'7 mm. of solution containing 467 grams of erbium
nitrate to the litre is contrasted with that of 305 mm. of the same solution diluted
to 45'5 times its bulk. The greater diffuseness of the bands of the upper spectrum,
which is that of the strong solution, and apparently greater intensity of the ultra-violet
band on the left will be noticed. It may be compared with the corresponding plate No. 9
for the chloride, in which however the lower spectrum is that of the stronger solution.
Plate 10 contrasts the spectra of equivalent solutions of erbium chloride and nitrate, in
four degrees of dilution, the uppermost spectrum being that of the strongest chloride.
The greater diffuseness of the bands of the nitrate can be seen, and the gradual
approximation to identity in the spectra of the two solutions as they become more
dilute. It is the counterpart for erbium of plate 11.
The nitrates, as well as the chlorides of both metals, shew a general absorption
creeping down from the most refrangible end of the spectrum with increased concentration
of the solutions; but though similar in the two salts, that given by the nitrates is not
identical with that of the chlorides. Its edge is not so diffuse, but cuts off the spectrum
more sharply than that of the chloride; and in the strongest solutions it does not extend
so far down the spectrum as that of the chloride. On the other hand with the weak
solutions of didymium it extends lower than that of the chloride. With a solution
of didymium nitrate of -4 strength in thickness of 152 mm. all light above X333
seems to be absorbed, while with the chloride light gets through beyond X 315;
and the strongest solution of the nitrate in a thickness of 38 mm. does not entirely
cut off the light below 360, while the equivalent solution of chloride cuts it off much
lower.