ON THE LIMITING THICKNESS OF LIQUID FILMS. 
647 
examined, consisting of mixtures of glycerine, water and oleate of soda in varying 
proportions, potassium nitrate being in all cases added to increase the conductivity. 
It was found at last that a plain soap solution not containing any glycerine answered 
the purpose best, and the solution employed in the experiments about to be described 
had the following composition :— 
Grammes. 
Oleate of soda . 1'44 
Potassium nitrate. 2’88 
Distilled water.100 
Specific resistance .—The specific resistance of the soap solution was determined by 
the method described in our previous paper (Phil. Trans., loc. cit.). 
At 12° C the specific resistance was 43'9 ohms per centimetre cube. 
15 „ „ 40-8 
18-8 „ „ 37H 
We may therefore assume 
temperatures : — 
13° 
to be 43 
without important error the 
14° 15° 16° 17° 
42 41 40 39 
specific resistance at the 
18° 
38 
Refractive index. —The refractive index of the solution was 1'337 at the temperature 
16° C. 
Description of the apparatus. —The apparatus constructed for us by Messrs. Elliott 
Bros, is represented, half size, in Plate 47, fig. 1. It consists of a box made of thick 
glass plates, bevelled at the edges, and cemented together. The internal dimensions are 
10 centims. square by 16'25 centims. high. The lid is aglass plate 11‘4 centims. square, 
which closes the box airtight by means of a little grease. To it are attached all the 
essential parts of the apparatus. The lid with its fittings is shown in plan in fig. 3. 
The soap film is supported between two platinum cylinders B and F (fig. 1), each 
32 - 5 millims. in diameter. B is screwed to the end of a brass tube A, with a rack 
running along its length, which passes through a hole in the centre of the lid. It 
can be raised or lowered by means of a pinion C (figs. 1 and 3). That part of it 
which extends above the lid is enclosed in a larger tube in such a way that it can be 
moved up or down without establishing any connexion between the inside of the box 
and the outer air. D is a brass rod terminating below in a stout platinum wire E, 
which is bent at right angles and carries the short cylindrical ring F, perforated with 
holes. The rod D passes through an ebonite sheath G cemented to the lid. H is an 
ebonite milled head by which the ring F can be moved laterally as well as up and down. 
The needles are supported in the following manner :—J is a brass tube accurately fit¬ 
ting the ebonite sheath, K, and terminating in a milled head, L. It supports a rectangular 
4 o 2 
