Chemical Constitution of Saline Solutions. 297 



hydrochloric acid. In that of calcium chloride we have a similar action, but 

 it is not so energetic. The action of alcohol has been clearly ascertained (see 

 p. 272) to be also that of a dehj'drating agent exerted upon the crystalline 

 hydrate in solution. The dehydrating action of glycerine is aided by a rise 

 of temperature. 



Let us now consider the action of a solvent which is not a dehydrating agent. 

 When six parts of ui-anyl nitrate U02(N03)2'6H20, or more precisely 6'1 parts, 

 are dissolved in 4-4 parts of washed ether, the crystals enter into a perfectly 

 definite and clear state of solution, from which they may be recovered unaltered by 

 spontaneous evaporation. The ether is saturated with water, and it cannot there- 

 fore dehydrate the salt ; but when -^V c.c. of water is mixed with 10 c.c. of this 

 ether, or in the proportion of 1 volume of water to 100 of ether, the water 

 separates as a distinct layer. But the solution of uranyl nitrate contains 1'307 

 parts of water in combination, or 21*43 per cent., and yet tliere is no separation 

 into two layers of liquid. If the specific gravity of the ether be taken as 0'75, the 

 proportion of water by volume to ether is as 1'307 to 3-3. Hence the water 

 molecules in this solution form an integral part of the molecule of the uranium 

 salt, and do not exhibit the physical properties peculiar to water. If, however, the 

 uranyl nitrate crystals are moist, the moistux'e separates from the ether as an 

 aqueous solution of the salt. 



I. Conclusion. — When a definite crystalline hydrate dissolves in a solvent which is 

 not ivater, and is not a dehydrating agent, the molecule of the salt remains intact. 



The Effect of Heat on Absorption Spectra. 



When saturated solutions of coloured salts are heated to 100° C, there are few 

 cases in which no change is noticed ; generally the amount of light transmitted 

 is diminished to a small extent by some of the more or less refrangible rays 

 being absorbed. There is frequently a complete difference in the nature of the 

 transmitted light. Anhydrous salts not decomposed, hydrated compounds not 

 dehj^drated at 100°, and salts which do not change colour on dehydration, give 

 little or no alteration in their spectra when heated. 



Solutions of hydrated salts, and most notably of haloid compounds, do change ; 

 and the alteration in the spectra, if not always absolutely identical with, is at least 

 very nearly the same, as that produced by dehydration and by the action of dehy- 

 drating liquids (such as alcohol, acids, and glycerine) on the salts in the crystalline 

 state or in solution. Allowance must be made for changes in the absorption spectra, 

 owing to differences in the solubility of the anhydrous and the hydrated comjjounds 

 at the particular temperature, and also in some cases for the different action of 

 solvents on the spectra. The reason of this may be indicated by stating that the 



