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, which 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. 
Pues \W 
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.t 
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 colour 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. xxil., p. 241, 1874. { Proc. Roy. Soc., vol. xxiil., p. 872, 1875. 
28 2 
