Molecular 



Molecules 



Molecular 



Latent heat 



weight. 



diffused. 



conductivity. 



of solution. 



36-5 



989 



323 



17500 



81-0 



965 



311 



20000 



128-0 



994 



328 



19500 



63-0 



977 



334 



7500 



428 Dr. J. H. Long on the 



cules diffusod from n solutions in 24 hours), will be found 

 below. The heats of solution for one equivalent of each acid 

 are also given. 



Formula. 



HOI . 

 HBr . 

 HI . 

 HN0 3 



It is seen that the analogy is still preserved ; the four acids 

 are as closely related to each other in respect to rate of diffu- 

 sion as to electrical conductivity. The heats of solution of 

 the three similar acids are also nearly the same ; that of HN0 3 , 

 however, is less. 



Of all the above relations, that which seems the most natural 

 and most readily explained is the analogy between rates of 

 diffusion of similarly constituted salts and the amounts of heat 

 they absorb on going into solution. The rendering latent of 

 a large amount of heat indicates the performance of a corre- 

 spondingly large amount of work. The work in the present 

 case is probably the division of the molecules into smaller 

 aggregates ; and these, as is quite plain, are able to move 

 through the solution with greater facility than those which are 

 not so finely divided. Opposed to this, however, we should 

 not expect from a salt having a large molecular volume a 

 rapid rate of diffusion. Now it is possible that the large latent 

 heat corresponds to, and compensates for, the large volume, 

 and the rate of diffusion may not depend upon either. What 

 the true explanation of these facts is I cannot at present say, 

 but must content myself with the mere indication of the several 

 data and their analogies. 



On an entirely different footing stand the analogies between 

 rate of diffusion and electrical conducting-power. In both 

 cases a molecular motion is concerned ; and although at first 

 sight an explanation may seem as difficult as before, we have a 

 means of bridging the difficulty. In the first place, it will be 

 well to call to mind the theory of the constitution of solutions 

 as advanced by Clausius *, Williamson f, and others. Olausius 

 believes that in a solution of a salt, as NaCl for instance, the 

 two atoms cannot be assumed as remaining attached closely 

 together, or even as vibrating within limits near each other ; 

 but, on the contrary, they fly about in all directions, forming 

 or dissolving connexion with other atoms just according to the 



* Pogg. Ann. ci. p. 338. 



t See his "Theory of Etherifications," Chem. Soc. Quart. Journ. iv. 

 p. 229; also Ann, Chem. Pharm. lxxvii. p. 37. 



