Chemical Constitution of Saline Solutions. 311 
as, for example, with magnesium platinocyanide,* the crimson colour of which 
becomes yellow and the crystals opaque by rise of temperature and loss of water 
(2H,O); but it must be remembered that the solution of the red salt is pale yellow 
or colourless. 
When crystalline hydrated salts dissolve in water without molecular change 
in composition, the absorption spectrum of the solution is that also of the salt, 
the composition and colour of which is known; therefore any alteration in the 
spectrum must be the result of a change in composition corresponding thereto. 
Hence hydrated salts are seen to undergo dissociation at elevated temperatures. 
Also when saturated solutions are submitted to dilution, the colours of the 
solution correspond with those of the different known hydrates; and when 
these changes are accompanied by an evolution of heat, the conclusion is 
inevitable that these hydrates are formed. 
Solutions of different salts of the same metal, the absorption spectra of the 
compounds of which are a marked characteristic, are not identical, but exhibit 
many variations. Here it is immaterial to what constituent or part of the molecule 
the characteristic of the spectrumis due. ‘The vibrations of the absorbed rays are 
of lesser frequency the greater the molecular mass. But in comparing molecules 
of cobalt and nickel salts, the atomic mass being very nearly the same in these 
elements, and also in praseodym and neodym, the difference in the molecular mass 
of their: salts is almost solely due to their non-metallic constituents ; and if we select 
different salts of the same metal, the increase in molecular mass is entirely caused 
by the sum of the masses of the non-metallic constituents, and therefore the 
molecules of these compounds vibrate as wholes or units, and not as acid and base, 
or as electro-positive and electro-negative ions. This has already been pointed out 
in the following words ;— 
** Molecules of compounds—that is to say, molecules composed of dissimilar atoms— 
vibrate as wholes or units, and the fundamental vibrations give rise to secondary vibrations 
which stand in no visible relation to the chemical constituents of the molecule, whether these 
be atoms or smaller molecules.” + 
Whatever their condition may be in enormously diluted solutions, in a state of 
moderate concentration the molecules of the salts remain absolutely unchanged in 
this respect at ordinary temperatures. Hence the molecule, as a distinct and 
* According to Buxhoeyden and Tammann (Zeitschrift fiir Anorganischen Chemie, vol. 15, p. 319, 
1897). ‘There is a series of red hydrates formed between 0° and 45°C. (with from 6°8 to 8 molecules 
of H,O); at 45° a bright yellow hydrate with 5H,0; at 60° one bright green, 4H,0; at 100° a white 
hydrate with 5H,0; and at 210° an orange red anhydrous salt. 
+ ‘‘ Researches on the Relation between the Molecular Structure of Carbon Compounds and their 
Absorption Spectra.” Part vm. Trans. Chem. Soc., vol. 41, p. 47, 1882; also Brit. Assoc. Report, 
Section B, Dover, 1899, p. 15. 
