SURFACE-FILMS. 21 
in the part-tension T water waclin) accompanies a fall in the total 
tension, it is only necessary to assume that TJ jeoiin«water) 
has decreased more than sufficiently to compensate for the rise 
PT rater Ganiiny- 
It is desirable also to call attention to the probability 
that in some flocculations and precipitations, although they 
doubtless result from increased tension in the water stratum 
near the solid, this increase is not due to active adsorption at 
all, but to the precipitant (or one of its ions or hydrolytes) 
being ‘ crowded ’” into the liquid surface-stratum in spite of 
its raising the total tension of the interface, as well as the 
part-tension of the liquid. This is probably the most con- 
siderable factor in the ‘salting out’ of many colloids— 
since forcible intrusion of a tension-raising substance into 
the liquid near the solid would usually occur only when 
large quantities of the precipitant had been added. 
Discussion of the effects of adsorptions at the interfaces 
between the suspended liquid droplets of an emulsion and the 
surrounding liquid medium would take up too much time 
and must be kept for some other occasion. 
In connection with adsorption in general, there is, 
however, one point of much practical importance which can 
be put very briefly, namely, the effects of the presence of 
two adsorbable substances in the solution. It is clear on 
the principle of minimal surface energy that if these are 
physically and chemically indifferent to each other, the one 
which is capable of producing the greater lowering of 
surface-tension will tend to be adsorbed preferentially and 
to exclude the other. ‘This is readily demonstrable in the 
case of water containing both saponin and soap, the latter 
lowering the tension of a free water surface much more 
than does saponin—the solution gives typical soap-bubbles 
very different from saponin bubbles, it has the mobile air 
surface of soap solution instead of the rigid one of saponin, 
