682 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
When the film became a little thinner than the degree of tenuity represented by P 
a state of unstable equilibrium would be reached. The thicker parts of the film 
would tear the thinner parts asunder, but rupture would be prevented by the fact 
that when the point of minimum surface tension was passed the resistance would 
increase until, when the thickness corresponding to Q was attained, the tensions of the 
thin and thick parts of the film would be the same and equilibrium would again be 
possible. The equilibrium would be stable, because if the film became thinner its 
surface tension would increase, and it would tend to contract and thus to thicken. 
Such a theory would explain the approximately constant thickness of the black, on 
the ground that it would be impossible for it to exist except at a thickness such that 
its surface tension was nearly equal to that of the liquid in mass. When the film is 
very thin its tension would probably alter rapidly with the thickness, and thus the 
latter would be confined within narrow limits. 
These considerations would not apply to the liquid on the thicker side of the 
boundary. Its surface tension being independent of the thickness, the magnitude of 
the latter would be indifferent. 
If, as is possible by means of the electric current, the discontinuity was filled up, by 
liquid being forced into the black space, the equilibrium would be unstable. As soon 
as the external force was removed the “ grey,” intermediate in thickness to the black 
and coloured films, would be absorbed or stretched until it became black. 
(2.) If the simpler supposition is made that there is only one alternation of repul¬ 
sion and attraction to be considered, the law of the change of the surface tension 
might be represented as in fig. 2. 
In this case there would be a preliminary difficulty in explaining the formation of 
the black, inasmuch as when the point P' was reached any further decrease in thick¬ 
ness would be resisted by an increase in tension. As a matter of fact, however, the 
black is generally first formed in small specks, and if, owing to a sudden disturbance, 
the film were thinned to below the maximum of surface tension, equilibrium would be 
possible if the thickness were that corresponding to Q'. The parts of the film of 
thickness intermediate to P' and Q' would shrink, and an apparent discontinuity 
would be established. 
