THE ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 
479 
These compared so unfavourably with observations made when the blotting paper 
was used, that it was again employed—in conjunction with the tray—a corner of the 
paper dipping in the liquid. Occasionally if the paper appeared to be getting dry, it 
was moistened by liquid introduced by means of a fine pipette through a small hole cut 
for the purpose in the back of the case. 
As in spite of these precautions the air would probably remain a little too dry, there 
was evidently good reason for excluding from the comparison with the liquid in mass 
all films for which the reading of the hygrometer was low, and 51° was taken as a 
provisional limit. Another equally important condition of accuracy was that the 
hygrometric state of the chamber should be constant, i.e., that the hygrometer and 
thermometer should be steady during the observations. Changes in the hygrometric 
state of the air might evidently involve the loss or gain of water by the film. We 
found that ten films satisfied the conditions that neither hygrometer nor thermometer 
should vary more than 0°'3 while they were under observation, and that of these four 
were inadmissible, as the hygrometer stood below 51°. The smallest thickness for 
which all of the remaining six were examined was 9 X 10 -5 centims., and the results at 
that thickness are given below :— 
Date and number of film. 
h. 
t. 
Observed 
specific 
resistance. 
P- 9- 
September 27, III. 
53-1 
20-6 
141-9 
145 
28,111. . 
53T 
18-9 
148-2 
143 
28, I. . 
534 
18-8 
151-7 
146 
„ 27, II. . 
53-0 
20-9 
142-4 
146 
,. 30, II. . 
51-2 
17-9 
145-8 
137 
„ 30, I. . 
511 
17-8 
151-6 
142 
Mean 143 
The mean value of p. 20 differs only by 1 ‘8 per cent, from that of the liquid in mass, 
and the probable error is ±1 or ±071 per cent. 
The conclusion to which this result points, viz.: that the specific resistance of a 
liquid film is the same as that of a mass of the liquid of the same constitution, is 
greatly strengthened by another line of argument. 
A film, produced in an atmosphere charged with aqueous vapour at a tension differ¬ 
ing but slightly from that proper to the liquid from which it was formed, would 
rapidly adjust itself to that tension. The extremely small quantity of vapour which 
would have to be absorbed or given off before the condition of equilibrium was attained, 
and the comparatively large changes of temperature which this absorption or emission 
would produce in a body of such small mass, would both conduce to this result. It 
might therefore be that a film the constitution of which differed from that of the liquid 
