Chemical Constitution of Saline Solutions. 295 



not only the widening of the bands by the action of heat, but also the way in 

 which those in the green become confused and unmeasurable. It was the 

 strongest of four solutions which were used. 



It is difficult to make accurate measurements of these bands on account of 

 their nebulous character. Light does not pass freely between them, but is 

 subject to more or less absorption in the intervening spaces. It was also 

 remarked that measuring four or five bands in permanganate occupied some 

 time, during which the solution was maintained at 100°, and at this temperature, 

 evaporation might occur; hence the figures obtained with the stronger solution 

 might be judged to be the more correct. Nevertheless the effect observed with 

 the weaker solutions, wliich admitted of the four principal absorption bands being 

 measured, was not of any consequence, being, at the most, when there was any 

 change at all, an increase in the width of a band by about a millionth of a 

 millimetre of wave-length. The majority of the saline solutions could undergo 

 no evaporation, because they were so concentrated that their boiling points were 

 far in excess of 100*^ C, and they were not maintained at this temperature for 

 any lengthened period. 



PART V. 



On the Molecular Constitution and Dissociation Phenomena of Saline 



Solutions. 



A general statement appeared in the preliminary notice,* and in the abstract of 

 the original Paper, f 



It has been already recognised that the absorption spectra of various salts of 

 the same metal bear some resemblance to each other, but it is a mistake to 

 suppose that they can all be described in general terms. For instance, all cupric 

 salts are not blue, nor nickel green, nor cobalt salts red. 



A careful comparison of the cupric salts or of the nickel and cobalt salts, with 

 both in the anhydrous state, as well defined crystallized hydrated salts, and in 

 solution, shows that each molecule has its own spectrum ; and the colour is that of 

 the molecule as a whole, and does not consist of the colom* of a base added to that 

 of an acid, or of the colour of one ion added to that of another, otherwise it 

 would be easy to predict what would be the colour of any compound. It is quite 

 otherwise when the salts are dissolved in water and the solutions greatly diluted. 

 In this case, whatever the colour of the solution originally may be, it must 

 approach more and more nearly to the absorption spectrum of water, according to 

 the increased proportion of water present. 



*Proc. Roy. Soc, vol. xxii., p. 241, 1874. ^jJ^mz. Eoy. See, vol. xxiii., p. 372, 1875. 



2S2 



