1877.] 



Thickness of Soap Films. 



339 



lated), the ratios of the numbers given in Column X. would not be 

 altered ; the only change would be in the scale on which the thickness is 

 represented. The topmost and lowest points of the curves refer to the 

 parts of the films in contact with the upper and lower cylinders, and the 

 thicknesses at these and all intermediate points are represented by the 

 horizontal distances between • the points on the curves and the vertical 

 lines drawn near them. For the reasons given above, lines representing 

 the thicknesses of the cylinders are magnified 5000 times more than those 

 representing their lengths. 



Figs. I. to V. (p. 340) represent the successive forms assumed by a film 

 at the hours named on them. They serve to illustrate the phenomena gene- 

 rally observed — namely, that after the formation of the black the colours of 

 the portion of the film in contact with it change so as to indicate an in- 

 crease in thickness, and are, as it .were^ absorbed by those immediately 

 below them, although in no case does any portion of the film become thicker 

 than any other part situated at a lower level than itself. At the same 

 time, however, the lower part of the film continues to become thinner, 

 so that at last the whole assumes one uniform tint, which chauges 

 but slowly, and sometimes in such a way as to show that the whole film 

 is become thicker. This phenomenon is probably caused by absorption of 

 moisture from the air, though the thickening of the upper part of the film 

 above referred to may be due, at all events in part, to the fact that the 

 formation of the black part of the film must necessitate the comparatively 

 rapid removal of the superfluous liquid from that portion in immediate 

 contact with it. 



Fig. YI. represents a film in which the lower boundary of the black 

 was, at the time of the observation, rising instead of descending. 



The last four figures represent films which were not sufficiently thin 

 to exhibit the black ; the upper part of that shown in fig. X. was, at the 

 time of the observation, becoming thicker. 



An inspection of these figures is sufficient to prove that, as a general 

 rule, the films did not increase uniformly in thickness from top to bottom, 

 but that regions of comparatively rapid and slow increase of thickness 

 alternated. The inclination of the outside layer of the film to the vertical 

 seems often suddenly to become much greater at or about a thickness 

 corresponding to 20 small divisions on the curve-paper or to the yellow 

 of the second order, as seen through the telescope of the cathetometer, 

 but none of the changes in thickness are so rapid as that which takes 

 place at the lower edge of the black. One of the films observed (not one 

 of those drawn) displayed a ring of black 7*82 millims. in breadth, while 

 the rest of the film appeared to be a uniform green of the third order. 

 If we suppose the upper j)art of the film to be as thick as is possible 

 consistently with its appearing black when seen by light at normal inci- 

 dence, the film must in this case have been increased to fourteen times 



