308 Harriry— The Action of Heat on the Absorption Spectra and 
is completely dissociated, should be made up additively from the absorption of rays 
by the positive and negative ingredients; and therefore that the colour of a dilute 
solution depends upon the colour of the free ions.” 
In short, with sufficient dilution, all copper salts, cobalt salts, and nickel salts 
should show the absorption spectra, not of the salts, but of the atoms of copper, 
cobalt, and nickel, respectively, when charged positively with electricity, and should 
therefore, when the rays are submitted to the action of an equal number of ions, be 
practically identical for each metallic series of compounds, no matter what the 
salts may be, provided the negative ions are colourless. Hwan’s experiments were 
ingeniously devised, and carried out with great care. More than a metre of liquid 
was used in each observation; and the coefficient of extinction was determined for 
each salt: nevertheless, his results were not in all cases concordant, which he 
believes was due to some of the salts, such as the acetates undergoing hydro- 
lysis. 
A discussion of this question lies outside the scope of the present communi- 
cation ; but it may be remarked that the facts in evidence of the formation of 
hydrated salts in solution, accompanied by a change of colour, and in a state of 
such concentration that electrolytic dissociation cannot have effected this change, 
have not been adequately taken into consideration. In this connexion, it has been 
stated by Ostwald* that ‘changes of the same nature in absorption spectra are 
evidence of the same kind of molecular change”: and when we are dealing with 
very dilute solutions of hydrated copper, nickel, and cobalt salts the change of 
colour caused by dilution is in each case characteristic of the formation of more 
complex molecules, and not of much simpler ones, such as would result from the 
dissociation of the metallic salts into ions. We know the colour and absorption 
spectra of the complex molecules, but we do not exactly know the colour and 
absorption spectra of the ions which are of simpler constitution. 
Summary and Conclusions. 
When one substance simply dissolves in another, the result is a homogeneous 
mixture of the two substances in the liquid state. 
When a salt dissolves at the ordinary temperature of the air, and without rise 
of temperature in a liquid which is without chemical action upon it, as a rule the 
salt is simply liquefied no matter what its constitution may be. Examples of this 
are afforded by the solution of uranyl nitrate in ether, of praseodym and neodym 
salts in alcohol or water, and of hexahydrated cobalt chloride and nickel chloride 
or copper sulphate in water. The spectrum of the solution is not the spectrum of 
the metal, but of the molecule, whatever it may be in solution. All copper salts 
* Zeitschr. phys. Chem., 9, p. 579, 1892. 
