268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



We can then in any case choose an oil fihn without globules having 

 the desired thickness, isolate a portion of the surface, S. then enlarge 

 this to its greatest extension, S'. It will then have its minimum 

 thickness. The ratio S'/S will be the ratio of the two thicknesses. 

 Since the minimum thickness is known, we obtain the other thicloiess 

 by multiplying by this ratio. 



(b) Experimental procedure: In order to determine the greatest 

 thicloiess of an oil film without globules, I proceed as follows. By 

 means of a glass fiber, I place upon the water of my tray a drop of 

 several tenths of a cubic millimeter. It expands into a film which 

 contracts very quickly into a multitude of little droplets scattered 

 over a black film. I now place a sheet of paper over the greater 

 part of the surface and move it very slowly toward me. Immediately 

 we see the globules over the rest of the surface grow into brilliant 

 disks which finally break up into smaller drops. Eepeating this par- 

 tial wiping away several times, the globules one by one disappear, 

 each momentarily becoming a disk, multicolored or of brilliant white. 

 Finally the whole surface of the water appears* black. But there are 

 still very small droplets which may be made evident by slight en- 

 largements made by jerking the dish. Each one gives a flash of light 

 and then disappears. The final phase of the phenomenon requires 

 acute observation, especially for some oils which produce particularly 

 fine globules. In such cases I scatter a light veil of talc powder on 

 the film, then extend the film slightly and at once we see the talc 

 thrown out in little circles about each minute globule. 



((?) Results: The following table shows results obtained by the 

 process just described. It gives the ratio between the greatest and 

 least thickness we can have with films without globules. 



Trioleine 1.32 1.27 1.28 



Olive oil 1.27 1.21 1.22 



Linseed oil 1.18 1.15 



Nut oil 1.18 



Cod-liver oil 1.16 



Sheeps-foot oil 1.16 



Castor oil 1.53 



The ratio of the maximum to the minimum thickness for an oil 

 film without globules varies a little from oil to oil, but it is always 

 less than two. It is usually very close to unity, so that a film ex- 

 tended over water can have a maximum thickness but little superior 

 to its minimum thickness. We may otherwise state this. A film 

 thicker than one molecule can not exist without nearly all the excess 

 of oil forming into globules. 



(d) The formation of foam in a very shrunken oil film: The last 

 experiment explains a very curious and interesting fact. If we re- 

 duce an oil film from its maximum extension so as to diminish its 

 surface to one-tenth or one-twentieth of its original area the film 



