310 HartieEy—The Action of Heat on the Absorption Spectra and 
place at all, or it takes place very slowly, occupying days or weeks instead of 
minutes, and is not a result therefore simply of a reduction of temperature. In 
this it differs from dissociation as generally understood. 
Taking any anhydrous series of salts of the same metal, the absorption bands 
shift towards the red the greater the molecular mass of the salt. This has been 
repeatedly shown in the case of organic substances,* and there was no reason to 
believe that compounds entirely composed of inorganic elements would behave 
differently ; but inasmuch as it might have been supposed that polymerisation 
had completely altered their properties, and therefore their spectra, it became 
necessary to ascertain whether this was really the case. It has been proved that 
these salts are not polymerised when they suffer dehydration, and therefore they 
do not deviate from the law discovered for organic compounds. It has also been 
shown in various hydrated salts, such as the sulphate, nitrate, and acetate of 
didymium, both in the solid and in the state of solution by Bunsen; in the 
chromium salts of organic acids by Lapraik;+ and as the foregoing descriptions 
show, it is applicable to copper, nickel, and cobalt salts in the state of solutions. 
Etard{ examined the spectra of chromium and cobalt solutions, and measured 
especially the bands in the red, which, as he observes, may be displaced or cease 
to exist with the same element, according to the nature of the molecules in 
solution or of the compound observed. In other words, the vibrations of the 
absorbed rays are of lesser frequency with the greater mass, the molecule being, 
so to speak, loaded, and its motion retarded. 
When the molecule of any haloid salt of copper, nickel, or cobalt is combined 
with water, we find that the greater the state of hydration the larger the amount 
of light transmitted, or conversely the less the absorption. As examples, we have 
the chloride and bromide of copper, also nickel and cobalt, chlorides, bromides, 
and iodides. 
This may be more generally stated in the followimg manner. In any series of 
hydrated compounds of the same salt, those with the largest amount of water in 
the molecule transmit the most light. The haloid nickel and cobalt salts are 
marked examples, also cupric bromide. 
There are one or two instances where this may appear to be not strictly true, 
* “Researches on the Relation between the Molecular Structure of Carbon Compounds and their 
Absorption Spectra,” Trans. Chem. Soc., vol. 89, p. 158, 1881; vol. 41, p. 45, 1882; vol. 47, p. 685, 
1885. Also British Assoc. Report, Section B, Dover. 1899, p. 19. 
+ Lapraik, Jour. fiir prakt. Chemie (2), 47, p. 805, 1883; also Chemical News, vol. 67, pp. 207-255, 
1893. 
if Etard, Comptes Rendus, vol. 120, p. 1057, 1895. It may be remarked that Lapraik attributed the 
shifting of the absorption towards the red to the greater complexity of the organic acid, and not directly 
to the increased molecular mass of the compound. 
