270 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1913. 



(3) MAXIMUM THICKNESS AVITH GLOBULES. 



(a) Method of measurement: This measurement is especially diffi- 

 cult. After various attempts, I came to the conclusion that here the 

 only certain method was to proceed by the extension of the film as in 

 the previous case. In order to determine the maximum thickness, I 

 isolate portions of great black spots (4.5 cm. in diameter) which have 

 appeared very slowly from a thick sheet of oil (pi. 6). Then, first 

 lightly powdering the surface, I enlarge it to its maximum extension. 

 This operation is often hindered by the existence of very minute 

 globules. In an instance where the globules were absent I noted that 

 the maximum extension was obtained by about doubling the surface. 

 It certainl}^ was not tripled. We may say, then, that a film of oil 

 at its greatest thickness, when the excess of oil has formed into disks 

 in contact with it, is only about twice its least thickness. 



In other words, no continuous film will be stable on water when 

 its thickness is greater than two molecules^ whatever be the thickness 

 of the masses of oil in contact with it. It will be necessary to await 

 new measures before we truly know whether these films have a real 

 thickness analogous to the maximum thickness without globules. 

 That is, whether they are not formed of a layer of single molecules 

 packed as closely together as possible. 



(b) Discontinuity maximum: We are now in the presence of the 

 maximum of the discontinuity of oil films upon water. We may 

 easily have upon the water disks a millimeter or more in thickness. 

 I have noted, for instance, that a cubic centimeter of olive oil placed 

 upon water already heavily oiled forms a disk 30 mm. in diameter 

 and having an area about 7 square cm. Its mean thickness is there- 

 fore greater than 1 mm. and it is certainly 2 mm. thick at its central 

 part. Despite this thickness, the disk is surrounded by water on all 

 sides, kept in stable equilibrium by an absolutely invisible film of 

 oil having a thickness one-millionth of that of the disk. 



A simple comparison will show how peculiar is this discontinuous 

 equilibrium of oil on water: Let us imagine our film enlarged one- 

 half a million times; then our oil film at its maximum thickness 

 would be 1 mm. thick, and it carries instable equilibrium masses of 

 oil whose thickness can reach and even surpass 1 kilometer 

 (1,000,000 mm.) ! 



(c) Comparison with the black film of soap bubbles: I have 

 already, in calling attention to the evolution of a thick film of oil 

 newly formed upon water, spoken of the constant appearance of 

 black circles which grow larger and larger and merge finally into a 

 continuous surface dotted Avith globules. It is odd that physicists 



