SURFACE- FILMS. 19 
contact—the liquid would spread over the whole solid. 
Whether such cases actually exist, except with soluble 
solids, is uncertain. 
It is clear that a continuation of the subject would 
necessarily involve a discussion of many aspects of the 
phenomena of solution and gel-formation, and this I must 
exclude, not only for lack of time, but also because, as 
soon as the masses of matter become very small, surface 
tension considerations no longer suffice, and it becomes 
necessary to deal directly with the inter-molecular forces 
as well and to have much greater knowledge concerning 
these than is available. 
The above conclusions concerning the part-tensions at 
solid-liquid interfaces must obviously be equally valid for 
all-liquid interfaces. but, owing to the mobility of both the 
apposed surface-strata, clear observational proof is more 
difficult to obtain. It would lead me too far to attempt 
any adequate consideration of emulsions and of the different 
phenomena observable when a little liquid is placed on the 
free surface of another liquid in which it is insoluble. 
The liquid-liquid interfaces have however one 
advantage over solid-liquid ones in that the total surface- 
tension can be measured. ‘T'aking this to be the algebraic 
sum of the two part-tensions, the hypothesis that the greater 
the ‘trans’ surface attractions on a surface stratum as 
compared with the ‘ cis’ surface attractions the smaller would 
be the part tension in that stratum, and that with sufficient 
preponderance of the former this tension would become 
actually negative, requires that the sum of the two part- 
tensions, 7.e., the total interface-tension, should always be 
less, and sometimes very considerably less, than the sum of 
the free surface tensions of the two liquids concerned. 
The available measurements show that this is the case. A 
few of Quincke’s values obtained at 20°C may be quoted in 
illustration. 
A = air, W = water, M = mercury, O = olive oil. 
Tw, 8] d ynes je 8] i ae 37 
Ty, 540 ,, To, 37 Ty, 540 
Tuw 418 _,, Tyo 33°5 Tyo 335 
