182 DISCUSSION OF EVIDENCE. 



to disappear. If the absorption is sufficiently intense so that each 

 group of bands appears as a single band, these broad bands may widen 

 very unsymmetrically towards the red as the temperature is raised. 



In pure solvents the bands not only widen with rise in temperature, 

 but the edges become more diffuse. With mixtures of salts such as 

 calcium and neodymium chlorides, the bands become weaker with rise 

 in temperature. 



It is interesting to note that the absorption of a salt in mixtures of 

 two solvents often decreases in intensity with rise in temperature. 

 The effect of rise in temperature on the different "solvent bands" is 

 often quite different. Uranous bromide in 40 per cent water and 60 

 per cent alcohol showed, at ordinary temperatures, the "water" and 

 "alcohol " bands of equal intensity. When the temperature was raised 

 to 80 the "water" bands practically disappeared, while the "alcohol" 

 bands were scarcely widened at all. 



While the effect of rise in temperature is to produce a change in the 

 intensity of the "solvate" bands, it produces very little change in the 

 wave-lengths of the bands. In some cases, in mixtures of water and 

 alcohol, the alcohol bands increase in persistency as the temperature 

 is raised. This is important as showing that the hydrates and alcohol- 

 ates have different degrees of stability with respect to temperature. 

 The effect of rise in temperature is, in general, to increase the absorp- 

 tion in the short wave-lengths. 



An interesting question arose in connection with the effect of tem- 

 perature on the solvates. Does rise in temperature produce a perma- 

 nent change in the composition of the solvates? It would seem highly 

 probable that it would not. The composition of any given solvate is 

 determined by the amount of solvent relative to dissolved substance. 

 If the complex solvate is rendered simpler by rise in temperature, then, 

 when the solution cools down the solvate should have its original 

 complexity the original condition of equilibrium should be restored. 



The results from absorption spectra confirm the above conclusion. 

 When a salt is dissolved in a mixture of two solvents and the solution 

 heated, there is, as we have seen, a change in the spectra. When the 

 solution is cooled again the original spectrum is obtained. This shows 

 that the original solvates are, as we would expect, reformed. This 

 would seem to have some bearing on the nature of the solvates existing 

 in solution. The idea that was originally advanced as to the composi- 

 tion of the hydrates existing in aqueous solution was that a large 

 number of hydrates existed simultaneously in a given solution, the 

 composition for any given substance and any given solvent being 

 determined chiefly by the concentration of the solution, temperature 

 being constant. In a word, we had simply a condition of equilibrium 

 for any given substance between the combined and the free water. 

 This condition of equilibrium would be changed with rise in tempera- 

 ture, some of the combined water being set free, in accordance with 



