Page:Proceedings of the Royal Society of London Vol 69.djvu/497

Rh 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.

Heat of vaporisation

Pressure in mm.

Absol. temp, of benzene.

Absol. temp, of methyl acetate.

Heat of vaporisation of benzene.

of methyl acetate.

Deterd.

Calc.

cat.

cal.

eal.

1335

o

373

347 -3

92-06

96-95

4335 423

390-9

81-01

83-51

85-40

10R50 473

433-4

69-14

70-95

72 -95

22182 523

475 -0

49-71

1 52-28

52-49

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.