OIL FILMS ON WATER AND MERCURY DEVAUX. 265 



undergo an abrupt closing up between the oil and the paper. Retreat 

 the barrier, and all at once the same grains become free, again floating 

 freely side by side. By means of these sudden changes and by mov- 

 ing the paper slightly back and forth, I can accurately, within a few 

 mfllimeters, find the lunit at which the oil is just slightly contracted, 

 that is at the place where there is the first appearance of change in 

 the tension. At this place I make my measure, determining once for 

 all by my double decimeter rule the length of the film of oil. 



(2) Results.— We thus get the area of the mean surface covered 

 by the film. In the experiment made the 18th of April, 1912, it was 

 363.71 square centimeters. Now, this was produced by two drops of 

 the oil solution; that is, by 400X10-' cubic centimeters of oil. The 

 thickness of the film was therefore : 



V_400XH):^ 

 S~ 363.71 



1.10 ;t/i 



with an approximation between 1.04 and 1.15 [Xfj,. 



We can then state from this that the thinnest film of oil which 

 can exist upon water is one and one-tenth millionths of a millimeter. 

 This thickness, almost identical with that found by Lord Rayleigh, 

 is remarkably small. A simple comparison will give us a better 

 idea of it. 



Let us imagine a film of this thickness covering a globe 50 centi- 

 meters in diameter; let us enlarge in thought this globe until it has 

 the actual dimensions of our earth. The film enlarged in the same 

 proportion will acquire a thickness of only 26 millimeters, while the 

 paper which covers the globe and upon which the world map was 

 made will increase from its original thickness of 0.1 millimeter to 24 

 kilometers ! 



(3) Comparison with rnolecular dimensions. — But we may make 

 better comparisons. In the molecular theory, the thinnest film of any 

 substance which can exist is evidently made of a single layer of 

 molecules; for it is impossible to conceive of a film thinner than a 

 molecule except through the deformation or destruction of the mole- 

 cule itself. 



We possess to-day very numerous and exact determinations of the 

 Avogadro constant, allowing us to calculate molecular dimensions. 

 We have made the calculation for oil, or rather for the trioleate of 

 glycerin. Using Perrin's value for Avogadro's constant, we found 

 1.13 p.[jL for the molecular diameter. The theoretical value of the 

 diameter of a molecule thus calculated is practically identical with 

 1.10 \}\i., the experimentally measured thickness of an oil film at its 

 maximum extension. The difference is only in the hundredths of a 

 micron. 



