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. 313
of the middle band of the triplet in the blue, which does not appear shifted, but of
this I am not sure for the photographs shew a trace of a washed-out band about midway between the two extreme bands of the triplet in addition to the stronger band which
is more refrangible. With glycerol the continuous diffuse absorption also creeps down the
spectrum as with alcohol.
In order to observe the effect of a crystallizable solvent other than water, some
didymium acetate was prepared and dissolved in glacial acetic acid, and for comparison
with it an aqueous solution of didymium nitrate was made of equal concentration.
Plate 20 shews the photographs of their spectra. Comparing the absorptions directly by
eye, the band in the red appeared stronger in the acetate and sensibly shifted to the
less refrangible side, the feeble band in the orange also was shifted in the same direction, the strong group in the yellow considerably extended towards the red but its more
refrangible edge not apparently shifted, doubtless because the widening of the bands
compensated the shift which was visible in all the other bands of the acetate though
they otherwise had the same general appearance as those of the nitrate. The shift
and change of character produced by acetic acid was less than was produced by alcohol.
Didymium tartrate is very insoluble in water, but the compound produced by
potassium hydrogen tartrate acting on didymium hydroxide dissolves in a solution of
ammonia. The spectrum given by this solution is contrasted with that of an aqueous
solution (not exactly of the same concentration), of didymium chloride in plate 23.
With the exception of the group in the yellow, the less refrangible of the groups in
the green, and the narrow band in the indigo, the bands seem all a good deal washed
out. All the bands are shifted towards the red, and the apparent shift increases as the
bands become more refrangible, but probably this appearance is the effect of the greater
dispersion of the more refrangible rays.
I had no crystals of didymium salts sufficiently large to enable me to see how
the diminished freedom of the molecules in the solid would modify the spectrum, but
had a rod of fused borax coloured with didymium. This was made by mixing weighed
quantities of didymium oxide and dried borax, fusing the mixture, and sucking the
fused mass into a hot platinum tube. After cooling the rough ends were cut off and
polished, and I was thus able to compare the spectrum given by a thickness of 25 mnm.
of this glass with that of an equivalent solution of didymium chloride. Photographs
of these spectra are shewn in plate 21. They are somewhat marred by dust on
the slit of the spectroscope, but this does not prevent a fair comparison. It will be
seen that the modifications produced by the glass are on the whole similar in character
to those produced by some of the liquid solvents. The strong group in the yellow
is much expanded and the components of the group unequally shifted towards the red,
the less refrangible of the groups in the green is shifted and its appearance modified
for the same reason. The more refrangible bands are much washed out and their shifts
appear very unequal. Nevertheless they appear to be still essentially the same bands
modified as to their rates of vibration by the diminished freedom of the molecules
producing them.
VOL. XVIII. 40