SURFACE-FILMS i 
That the remarkable property which some solutions 
possess of yielding more or less durable bubbles and froths 
when shaken was in some way connected with the presence 
of such particles was an obvious possibility. On testing the 
various solutions which had yielded mechanical surface 
aggregates, I found that by far the greater number of them 
were conspicuously capable of yielding durable thin films 
m air. Conversely, on testing every solution I could come 
across which had exceptional frothing power, it was found 
that all yielded solid mechanical surface aggregates, with 
the exception of bile-salts which yielded only viscous gummy 
matter, and sodium oleate which gave inconstant results. 
These facts strongly suggest that the ability to form good 
bubbles is essentially bound up with the presence of large 
particles at the free surfaces. When air is sucked out of 
saponin bubbles the surface solid actually jams together 
sideways (or folds up) into solid rods almost visible to the 
naked eye—simultaneously, as you see, the film loses nearly 
all its contractility and takes several seconds to re-dissolve 
the surface excess of saponin and recover its original 
spherical shape. 
Taking then the studding of the air surface of most 
of these solutions with solid particles as an established fact, 
the question arises why do certain substances freely soluble 
in water pass spontaneously out of solution at a water-air 
surface? What explanation can be offered of a phenomenon 
so remarkable? ‘The beginnings of an explanation were 
found ready to hand in the work of Willard Gibbs in the 
United States, and of J. J. Thomson in England. These 
workers had shown by reasoning from general principles of 
energetics that any change in the disposition of the 
molecules close to a surface which would result in a lowering 
of the surface-tension was one which would tend to take 
place. ‘This deduction is commonly known as the principle 
