260 Hartiey—The Action of Heat on the Absorption Spectra and 
Ceric Potassium Nitrate.—At \ 582 there appeared a faint sharp shade, and this 
doubtless proceeded from a trace of didymium, as it coincides with the centre 
of the strongest and blackest lines of absorption characteristic of didymium 
salts. Gladstone has shown that 0:05 per cent. of didymium, in a solution half 
an inch thick, may be detected by it.* 
Gold Chloride.—The darkening in this case is possibly in part due to concen- 
tration of the solution by evaporation. 
Potassium Pentasulphide—This was made by boiling potash with an excess of 
sulphur. The solution was kept in a stoppered bottle for two months unopened. 
It had the characteristic strong yellow colour. 
A mixture of Ferric Chloride and Potassium Sulphocyanate.—This solution was 
intensely dark ; it was therefore diluted until sufficiently transparent. 
None but the red rays were transmitted by the cold solution. The experi- 
ments of Schifff afford strong evidence that, on heating the solution of the mixed 
salts, the ferric sulphocyanate becomes converted into ferric chloride with the 
simultaneous formation of potassium sulphocyanate again. This is confirmed by 
an examination of its spectrum at 20° and 100° C. (Plate XIX.). — 
From an examination of these substances it appears evident— 
1st. That in compounds in which water forms no integral part of the molecule, 
the action of heat on the absorption spectrum causes little or no alteration. Any 
measurable change amounts to a slight darkening of the solution by increasing 
the absorption of the less or more refrangible rays (see Plate XIX.). 
2nd. Concentrated solutions of salts, in which water forms an integral part 
of the molecule, show a very marked change in their spectra when heated to 100’, 
inasmuch as there is a great increase in the absorption of light resembling that 
caused by dehydration of the solid substance at that temperature. This is more 
particularly to be remarked in the haloid compounds, and such substances as are 
more readily soluble than the sulphates and nitrates (Plates XVII. and XVIII.). 
It may be seen in the solutions of cupric chloride, in glycerine and hydro-chlori¢ 
acid, in alcohol, and when mixed with calcium chloride. The solutions were not 
so concentrated as the aqueous solution, and they do not, on that account, absorb 
quite so much of the red rays. The glycerine solution behaves, upon heating, in 
a similar manner, and shows the same change in its spectrum as a dilute blue 
solution of cupric chloride. A similar change may be noticed in nickel 
salts. 
8rd. The hydrated bromides of nickel and copper exhibit a remarkable 
change on heating to 100°, inasmuch as they become nearly opaque. 
4th. The absorption spectra of the most soluble salts undergo the greatest 
amount of change upon heating. 
* Jour. Chem. Soc., vol. x., 1857. + Ann. Ch. Phar. ex., p. 203. 
