478 Mr. W. Sutherland on the 



a true physical latent heat o£ evaporation, but includes the 

 heat of dissociation. We proceed to calculate the heat of 

 dissociation of dihydrol into hydrol and the true latent heat 

 of dihydrol. 



If we take 368 as the critical temperature of dihydrol, then 

 at | of the absolute critical temperature, that is at 155° C, 

 the gramme-molecular surface-tension of dihydrol must be 

 about 2-121(368-155) or 452. Now according to the fifth 

 method of finding the virial constant I for a substance (" Laws 

 of Molecular Force/' Phil. Mag. [5] xxxv. p. 258), 



where c=1209 when I is measured in terms of the megadyne 

 and a of the dyne ; for dihydrol 



I =1209 x 452 xu/M, 

 v=l/l-08942(l--0009xl55), M = 36; 

 .-. Z = 16192. 



The equation of the third method of finding I (ibid., p. 245) 

 is 



Z/u=66-5\-101T 6 /M, .... (29) 



where T$ is the boiling-point (absolute) , X the latent heat of 

 evaporation, and v the volume of a gramme of the liquid at 

 the boiling-point. Now \ does not vary rapidly with the 

 temperature, so we shall make but little error in taking 100° 

 as the boiling-point of dihydrol ; then 



T 6 = 373, M=36, w=l-009; 

 .-. X =257 calories (30) 



But the latent heat of water at 100° is 537, and is the heat 

 of evaporation of a solution of '217 gramme of trihydrol in 

 •783 of dihydrol, and of the dissociation of both into 

 hydrol. 



Now the removal from solution of the *217 gramme of 

 trihydrol would require by (28) 52*8 calories, and its dis- 

 sociation into dihydrol by (27) 38'4; so that to convert a 

 gramme of water at 100° into pure dihydrol requires alto- 

 gether 91*2 ; and therefore for the evaporation of a gramme 

 of dihydrol and its dissociation into hydrol 537 — 91 calories 

 are required ; and therefore the heat of dissociation of a 

 gramme of dihydrol into hydrol is 446 — 257 or 189 calories. 



We can partly check this result by the following rea- 

 soning. To dissociate a gramme of trihydrol into hydrol via 



