ON THE PHENOMENA AND THEOEIES OF SOLUTION. 449 



excursions from their normal centre, partly because they are subjected to 

 more violent encounter with the moving molecules of liquid' ('Phil. 

 Trans.' i. 1884, p. 30). Such a theory, however, serves to account only 

 for the initial stage in the process of solution, and does not explain the 

 selective power of solvents nor the limitation of solvent power of a given 

 liquid, &c. 



Thermal Phenomena. 



How far is it true that evolution of heat indicates chemical com- 

 hination ? Does the evolution of heat in dissolving a solid in water or 

 in adding more water to its solution indicate the formation of hydrates, 

 that is, of chemical compounds of the dissolved substance with water in 

 definite proportions ? Thomsen answers this question in the negative 

 (' Thermochem. Untersuchuugen,' iii. p. 20). 



Take the case of sulphuric anhydride, SO3. 



S03,H20 = + 21320 (Solid SO3 into liquid H.SO.) 



(S03H20),H,0 = 6379 (No change of state) 



(S032H20)H20 = 3039 



(S033H,,0)H.,0 = 1719 

 For next 2H2O average = 985-5 per H^O 



For next 4H2O average = 461 ,, ,, 



For next IOH2O average = 130-4 „ „ 



Up to 1599 HoO total = 17857 



The total heat of solution of SO3 in 1600 H,0 is therefore 21320 

 + 17857 = 39177. 



The following diagram (1) shows graphically the successive thermal 

 changes consequent upon adding this quantity of water gradually to 

 sulphuric anhydride. Although more than half the total heat evolution 

 occurs on addition of the first molecule of water, the succeeding mole- 

 cules give a quite appreciable amount ; the second gives, in fact, ^— "_ , 

 or nearly J of the whole. ' ^ ^ ' ' 



At what point in such a curve should we be justified in setting up a 

 distinction between the efiect due to chemical combination and that due 

 to other causes ? 



In the act of solution of solids, and especially of anhydrous salts in 

 water, the volume of solution is always less than the sum of the volumes 

 of the solid and its solvent, with the exception of some ammonium salts 

 in which expansion occurs. Similarly the addition of water to a solu- 

 tion is followed by contraction. This contraction may be due to mere 

 mechanical fitting of the molecules of the one liquid into the interspaces 

 between the molecules of the other, just as when one pint of small shot 

 is mixed with one pint of large shot the volume of the mixture is less 

 than two pints. This, I apprehend, would not by itself be attended by 

 loss of energy (See Mendelejeff", ' J. Russ. Chem. Soc' xvi. 643, 644. 

 Abs. in ' J. Chem. Soc' Feb. 1885, p. 114). Or it may arise from the 

 adjustment of the motion of the molecules of the constituents to the 

 conditions requisite for the formation of a uniform liquid (Thomsen 

 iii. p. 18). ' 



If we know the coefficient of expansion of the liquid and its specific 

 heat we can calculate the amount of heat that would be evolved for a 



1886. Q Q 



