THICKNESS AND ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 507 
In the Table the results of the individual experiments are given, each thickness 
being the mean of the thicknesses of about 100 films. 
Table I. 
Percentage of 
KNO 3 . 
Thickness of black films in 
/c/i., measured optically. 
Mean. 
30 
12-6, 12-2, 12-3 
12-4 
10 
15-7, 11-4, 13-7, 13-2 
13-5 
0-5 
13-4, 15-7 
14-5 
0-0 
23-2, 22-3, 20-3, 22-8 
22-1 
It is satisfactory to note the close agreement of the above thickness for the 3 per 
cent, solution with that previously obtained by us (‘ Limiting Thickness,’ loc. cit., 
p. 661), the numbers being 12‘4 and 12’1 respectively. We may note, however, 
that the solution previously used contained less soap (viz., 1 in 70 instead of 1 in 50), 
a point to which we shall refer again. 
The number of observations is hardly sufficient, and the discrepancies between the 
individual measures on 1 per cent, and 0'5 per cent, solutions are too great to warrant 
our insisting on the gradual increase in the thickness at these percentages. There 
can, however, be no possible doubt that when no KNOg is present, the numbers are 
much larger than when the solution contains 3 per cent, of the salt. 
This result is confirmed by the following measurements in which soft soap (oleate 
of potash) was used instead of oleate of soda, and sodium sulphate instead of 
potassium nitrate. The solution became thick when a larger proportion than one per 
cent, of Na 2 S 04 was mingled with it. 
Table II. — Soft soap ; 1 part in 40 of water. 
Percentage of 
NaoSO^. 
Thickness of black films in fi.jn., measured 
optically. 
Mean. 
1-0 
141, 16-3, 14-3 
14-9 
0-0 
20-1, 18-2, 19-8, 21-3, 17-8, 197 
19-5 
A further confirmation is obtained by the use of liquicle glyceriqiie. It is practi¬ 
cally impossible to employ a soap solution in which more than 3 per cent, of potassium 
nitrate has been dissolved, as the liquid becomes “ ropy ” and does not thin to the 
black. With a Uquide glycevique, however, containing two parts by volume of 
glycerine to three of soap solution, the proportion of potassium nitrate can be largely 
3 T 2 
