ON THE THICKNESS AND SURFACE TENSION OF LIQUID FILMS. 
G45 
from Tables I. and II. that the limiting sensitiveness of the experiments was 0'56 in 
the one case and 0 - 83 in the other. Mensbrugghe purposely used segments which 
differed but little from hemispheres, and may therefore have attained very great 
sensitiveness. The highest value to be deduced from the experiments he describes 
is 11-5. 
We used two sets of rings the diameters of which were 2’08 and 3'25 cm. respec¬ 
tively. When the two cylinders were employed the sensitivenesses (see Table III.) 
were 
4-34X 1-04=4*51 and 4‘34X r62 = 7‘03, 
the distance between the rings being in each case 1‘25 times the diameter. 
In like manner when two spheres were used the values were 14*81 with the small 
rings and 23 - 06 with the large ones. Such high sensitiveness was found in practice 
inconvenient, and this arrangement was not often employed. 
We have for simplicity calculated the sensitiveness of the sphere and cylinder on 
the assumption that the ratio of the diameter of the rings used was 1'6. It was 
really 3•25/2*08 = P56. We may, however, without serious error, assume the value 
37 to be correct. 
If, then, we credit Plateau and Van der Mensbrugghe with the most favourable 
arrangements described by 'them, and make the above probable assumptions for 
Lltdtge, we get the following Table of sensitiveness :— 
Plateau . 
076 
Mensbrugghe .... 
11-5 
Ludtge. 
0-83 (?) 
R. & R. Two spheres— 
R. & R. Sphere and cylinder 
37 
Small rings .... 
14-81 
„ Two cylinders—- 
Small rings . 
4-51 
Large rings .... 
23-06 
Large rings . 
7-03 
These numbers express the relative merits of the different experiments regarded as 
null methods. They are inversely proportional to the differences of surface tension 
which could in each case exist without detection. It is, however, proper to point out 
that the more sensitive methods are also more liable to accidental disturbances of 
various kinds, and do not therefore present so great an advantage as the theory 
would indicate. 
This remark does not apply to a method which we employed, and by which the 
sensitiveness is doubled, viz,, keeping each film thick in turn. By this means the 
indications of a difference of surface tension were twice as great as they would other¬ 
wise have been, and the figures given above may be multiplied by two. 
