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.

302 PROF. LIVEING, EFFECTS OF DILUTION, TEMPERATURE, ETC. ON THE
per litre, and as the specific gravity of the solution of chloride is 8-295, it appears to
contain one molecule of the chloride to between 27 and 28 molecules of water.
Didymium sulphate is rather sparingly soluble in water, so that the most concentrated solution of it employed contained only 58'11 grams of it per litre. For comparison
with it, the strongest nitrate, or chloride, had to be diluted to 9'16 times its bulk.
Of erbium the most concentrated solutions used contained, respectively, of the nitrate
935 2 grams to the litre, of the chloride 726'6 grams. These each contain 2'67 grammolecules of the salt per litre. The solution of the nitrate was a saturated one at a
temperature of about 15~.
Less concentrated solutions were also prepared and used, containing, respectively,
566 grams of nitrate of erbium, and 440 grams of the chloride to the litre, or about
1'61 molecules in grams to the litre.
The more dilute solutions were obtained from these by taking measured quantities of
them and diluting up to the required volume. In fact the most concentrated of these
solutions were the stock solutions, and may conveniently be described as of strength
No. 1. Half strength will mean such a solution diluted until the bulk was doubled,
one-quarter strength will mean No. 1 diluted until its bulk was quadrupled, and
so on.
Other salts and solvents were employed, and will be described when the experiments
upon them are described. The solutions of nitrate and chloride above mentioned were, as
a rule, the standards of concentration.
THE ABSORPTION BANDS OBSERVED.
The didymium absorption bands of which I have taken notice in this investigation, are
as follows:
A band in the red at about 679.
A weak band at about X 623.
A rather weak band at about X 596.
The strong group extending from about X 590 to X 570, consisting of a number of
bands overlapping one another.
A rather weak band at about \ 531.
A strong group of about four, more or less overlapping, bands, extending from about
X528 to X 520.
A less strong group of two diffuse bands with the centre about \ 510.