Nature of Solution. 95 



can be no doubt of the meaning of this; the specific gravity 

 of a solid is an index of the amount of attraction exercised by 

 its molecules on one another at the moment of solidification (if 

 the determination be made at once); and any further alteration 

 of the extent of the intermolecular spaces can go on but slowly, 

 owing to the exceedingly small internal motion of the mole- 

 cules of a solid. The separation of a salt from its solution is 

 really a slow process, however quickly (relatively speaking) 

 the solution may be cooled ; the molecules consequently are 

 able to arrange themselves in positions of stability for that 

 particular temperature ; and once this arrangement has taken 

 place, any further arrangement is, as has been said, extremely 

 slow. Kremers's experiments and mine show that the inter- 

 molecular attraction of salts is altered by the temperature, 

 and that this alteration is connected with the solubility : in 

 cases where the solubility increases, the attraction is weakened ; 

 and with sodium sulphate diminution in solubility is accom- 

 panied by increase of attraction. 



Further evidence on this point is furnished by Berthelot's 

 recent experiments on the heat of solution of various salts 

 before and after fusion*. When a salt dissolves in water the 

 thermal equilibrium is usually disturbed; in the case of most 

 salts, whether they contain water of crystallization or not, the 

 solution of the form which crystallizes at ordinary tempera- 

 tures is attended by absorption of heat, due to the heat ab- 

 sorbed by the liquefaction of the salt exceeding the heat evolved 

 by union with the water ; or, in other words, the work done 

 in separating the molecules of the salt requires more heat than 

 that given out by the union of water and salt. If the mutual 

 attraction of the salt-molecules be in any way weakened, then 

 less work has to be done to separate them, and consequently 

 the total absorption of heat is diminished. Berthelot finds 

 that in a number of cases the heat absorbed on the solution of 

 a salt in its normal state is greater than that absorbed when 

 the salt has been fused, allowed to cool, and at once dissolved ; 

 lastly, if a length of time sufficiently long be allowed to elapse 

 after fusion and before solution, the thermal effects of solution 

 are identical with those observed before fusion. Now a refer- 

 ence to Clarke's tables of specific gravityf shows that in nearly 

 every case where the specific gravity of a substance has been 

 taken before and after fusion, it has been found that the result 

 of fusion has been diminution of specific gravity : but the 

 data existing on this subject are very incomplete and in many 

 instances contradictory, as indeed is the case with the greater 



* Comptes JRendus, July 1882. 



t ' Constants of Nature/ Part I. Smithsonian Institution. 



