Chemical Constitution of Saline Solutions. 311 



as, for example, with magnesium platinocyauide,* the crimson colour of which 

 becomes yellow and the crj-stals opaque by rise of temperature and loss of water 

 (2H2O) ; 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 sjjectra 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 spectrum is 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 ivholes or units, and the fundamental vibrations give rise to secoiidarij vibrations 

 tohich stand in no visible relation to the chemical constituents of the molecule, whether these 

 be atoms or smaller molecides."'\ 



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 BuxlioeTclen 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 H2O) ; at 45° a bright yellow hydrate with 5H2O ; at 60° one bright green, 4HjO ; at 100° a white 

 hydrate with 5H2O ; and at 210° an orange red anhydrous salt. 



f "Researches on the Relation between the Molecular Structure of Carbon Compounds and their 

 Absorption Spectra." Part vii. Trans. Chem. Soc, vol. 41, p. 47, 1882; also Brit. Assoc. Report, 

 Section B, Dover, 1899, p. 15. 



