310 Haktley — 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 inoi-ganic 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 tlie case. It has been proved that 

 these salts are not pol3anerised when they sufEer 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 Iiapraik;t and as the foregoing descriptions 

 show, it is applicable to copper, nickel, and cobalt salts in the state of solutions. 

 EtardJ 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 

 witl) 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, clilorides, bromides, 

 and iodides. 



This may be more generally stated in the following 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, 



* " Eesearches on the Relation between the Molecular Structure of Carbon Compounds and their 

 Absorption Spectra," Trans. Chem. Soc, toI. 39, p. 153, 1881 ; vol. 41, p. 45, 1882 ; vol. 47, p. 685, 

 1885. Also British Assoc. Report, Section B, Dover. 1899, p. 15. 



t Lapraik, Jour, fiir prakt. Chemie (2), 47, p. 305, 1883; also Chemical News, vol. 67, pp. 207-255, 

 1893. 



X 6tard, 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 dii-ectly 

 to the increased molecular mass of the compound. 



