flic. Equilibrium Constant* of Chemical Reactions, etc. 477 



analogies existing between vapour pressures, solubilities, and equili- 

 brium constants of chemical reactions. 



III. Empirical Formula for Latent Heats of Vaporisation. During the 

 progress of the preceding work, I was led by a consideration of certain 

 empirical formulae put forward a number of years ago by W. C. Unwin,* 

 connecting the pressure, temperature, and volume of saturated vapours, 

 to a formula by means of which from the latent heats of vaporisation 

 of one substance the values of the latent heats of vaporisation of 

 another siibstance may be calculated. If LI is the known latent heat 

 at the absolute temperature T\ of the first substance, and L^> the latent 

 heat of the second substance at the temperature, To, at which the 

 vapour pressure of the second substance is equal to that of the first 

 substance at the temperature T T , then LI = L., T./, where a- is a 

 constant. A formula which is derived with greater strictness, but 

 which contains two constants, is L]/L. 2 = T^'/T/-. In some cases 

 the latter formula gives somewhat better values than the former, but 

 for most purposes the simpler formula may be employed. These 

 formulae appear to be suitable for calculating the latent heats of 

 vaporisation at not too high pressures, but break down in some cases 

 at pressures over 10,000 mm., although in other cases they hold even 

 at pressures of over 20,000 mm. 



In Tables VII and VIII are contained some of the results which 

 have been obtained using the simpler formula. 



Table VII. Benzene and Methyl Acetate. ./: = - 0-008847. 



As can be seen, the agreement between the determined and calculated 

 values of the latent heat of vaporisation, is very good. In some cases, 

 however, which I have investigated, e.g., in the case of benzene and 

 hexane, the agreement is not quite so good. 



The values of the latent heats of vaporisation of benzene, methyl 

 acetate, and ethyl alcohol, given in the fourth and fifth columns of t 

 * 'Phil. Mag.,' 1886, vol. 21, p. 299. 



