OIL FILMS ON WATER AND MEECURY DEVAUX. 267 



the same way (pi. 4), so that finally the film is changed into groups 

 of droplets scattered over the surface of the water, which reappears 

 as if free from oil, and uniformly dark (pi. 5). 



It is evident, however, that the surface of the water is yet covered 

 between the globules by a very thin film of oil ; and the persistence of 

 this final phase shows that it remains in this discontinuous condition 

 because the oil on the water is almost in static equilibrium. It is 

 therefore necessary to distinguish two phases in the development of 

 an oil film — the evolutionary phase, always fugitive, and the final 

 static phase. 



IV. THE STATIC PHASE OF OIL UPON WATER. 



Let us consider especially this last phase, that of a very thin, con- 

 tinuous film extended over the surface of the water and studded or 

 not with globules or disks. We will begin by establishing an im- 

 portant fact : the thickness of this continuous film depends upon the 

 existence and dimensions of the globules. Because we find that when 

 a film with minute globules exists beside one with great ones, the first 

 always contracts at the cost of the second. Since, therefore, the 

 tension is stronger in the former, we must conclude that the film with 

 minute globules is the thinner. 



With regard to the thickness of thin films, we are then led to dis- 

 tinguish four cases: the maximum and minimum thickness of films 

 without globules; the maximum and minimum thickness of films 

 with globules. Practically these reduce to three cases, since the 

 maximum thickness of a film without globules is necessarily the same 

 as the minimum thickness of one with globules. 



(l) MINIMUM THICKNESS OF FILM WITHOUT GLOBULES. 



We have already measured this thickness since it occurs in a film 

 at its maximum extension and it is about 1.10 \i.]x. 



(2) MAXIMUM THICKNESS OF A FILM WITHOUT OR THE MINIMUM 

 THICKNESS OF ONE WITH GLOBULES. 



(a) Principle used in measuring films of a thickness greater than 

 that at the minimum: Wliile the minimum thickness of oil films is 

 easy to obtain and even to measure, because of the sudden and con- 

 siderable change in the surface tension for small variations in thick- 

 ness, this is not the case for thicker films; for when we pass the 

 critical thicloiess, the surface tension scarcely alters even for very 

 great variations in the thickness of the film. It is therefore much 

 easier to measure a film at its minimum thickness than at a greater 

 thickness. 



However, since it is always possible, by enlarging the film, to 

 pass from a thicker to a thinner film, this difficulty can be avoided. 



