Surface Transition Layer of a Liquid Miature of Substances. 411 
part of the heat expended is small in comparison with the former 
‘part, and the internal heat of evaporation may therefore be taken 
_as approximately equal to the energy expended against molecular 
attraction. By definition U for a mixture is equal to the energy 
expended in overcoming the molecular attraction on separating 
‘the molecules of a gram of the mixture an infinite distance from 
one another, and hence approximately equal to the heat of 
‘mixture of the ingredients and their internal heats of evaporation 
in the pure state. If the ingredients of the mixture consist of 
the substances a and b in the proportion of n, and m by weight, . 
we have 
| ite Lgna + Longs 
ee mec, 
where L,, and L, denote the internal heats of evaporation of a gram 
of substance a and 6 respectively, and H,, the heat of mixture of 
the ingredients of a gram of the mixture. H,,, is usually small in 
comparison with both Z, and L;, and may therefore be neglected. 
| I have calculated the surface tension . of a few mixtures of 
substances of which Ramsay and Aston * have measured the ordinary 
surface tension X,. The constituent substances are not associated 
in the pure state and according to the experimenters mentioned 
do not undergo any radical chemical change on mixing, that 1s, the 
resultant substance is more or less a pure mixture. ‘The results 
T obtained are given in Table I. The values of U were calculated 
by means of equation (3) neglecting H,,, and using the internal 
heats of evaporation LZ, and ZL, given in the Table, which were 
interpolated from the values calculated by Millst. The absolute 
mass of the hydrogen atom is taken as in the previous paper equal 
to 161 x 10-* gram, the value obtained by Rutherford from ex- 
periments on the a particle. The difference d,—), is the external 
work done in the formation of the transition layer. 
It will be seen that the value of dA, for a mixture of C,H, and 
CCl, is practically independent of the relative concentrations of 
the ingredients. This is probably intimately connected with the 
fact that this holds also for the values of X,.. The results I obtained 
previously with pure substances are given in Table II for comparison, 
L, denoting the internal heat of evaporation. It will be seen that 
in the case of pure substances A, — A, Is practically independent of 
the temperature. But for mixtures this does not hold, its value 
increases with increase of temperature. At low temperatures 
A»— A, is smaller for a mixture than for each of the ingredients in 
the pure state. The difference in the behaviour of mixtures and 
pure substances is no doubt due to adsorption effects. 
* Zeit. f. Phys. Chemie, 15, 1894, p. 88. 
+ Journ. of Phys. Chem. vit. p. 405 (1904). 
