450 
PROFESSORS A. W. REIXOLD AND A. W. RUCKER OX 
By comparing the results with observations on the liquid in mass, we are able to 
draw certain conclusions as to the light thrown on the molecular constitution of liquids 
by experiments on soap films. 
The investigation divided itself into two parts, viz.: the determination (1) of the 
thickness, (2) of the resistance, of a film at any particular epoch, and as these, though 
carried on at the same time, were to a great extent independent, it will be convenient 
at first to treat of the former alone. 
II. The Liquid. 
The soap films were made in accordance with the receipt given by M. Plateau, 
with the exception that a small quantity of saltpetre was added to increase the 
conductivity. One part by weight of oleate of soda was dissolved in 40 parts of 
water, together with (in general) either 3, 5 or 7 parts of saltpetre to 100 of water. 
Three parts by volume of this liquid were then mixed with 2‘2 parts of Price’s 
glycerine. It will be convenient to call a solution containing n parts of saltpetre to 
100 of water an n per cent, solution. The solutions thus prepared produced in the 
course of a few days a flocculent precipitate which could not be separated by filtering. 
They were therefore placed in long vertical glass tubes furnished with caoutchouc 
stoppers at the upper, and stop-cocks at the lower ends. In time the liquids cleared 
by the precipitate rising to the top, and became sufficiently transparent to allow of 
their refractive indices being measured. They could then be drawn off for use without 
disturbing the impurities on the surface. This operation of clearing occupied several 
months, and was always carefully performed in order to ensure the homogeneity of the 
liquids. It will be convenient to refer to a liquid thus prepared as a standard 
solution; others containing greater or less proportions of glycerine will be called 
derived solutions. 
III. Refractive Index. 
The following table (Table I.) gives the refractive indices, for sodium light, of 
several solutions, the composition of which is indicated in the first three columns. 
Table I. 
Percentage of salt in 
standard solution. 
Parts of standard 
solution by volume. 
Parts of water or 
glycerine by volume. 
Refractive index. 
5 
100 
0 
1-3909 
5 
95 
5 H.,0 
1 3947 
5 
95 
5 glycerine 
1-4002 
0 
100 glycerine 
1-4715 
8-82 
75-6 
24 - 4 glycerine 
1-4155 
