Thin Fabrics and Films to Hydrogen and Helium. 275 



a soap bubble, by blowing a hydrogen bubble in hydrogen 

 and noting the decrease in diameter as time went on, due to 

 the slight excess pressure inside the bubble. What he 

 measured was the excess of the rate of gas diffusion outward 

 over the rate inward through the film, and he found that as 

 the soap bubble became thinner the gas transference became 

 greater. In the present experiment the endeavour was to 

 determine the actual rate of gas flow per square centimetre 

 through the film, keeping the film as nearly constant in 

 composition and thickness as possible. 



V. Description of Apparatus. 



A small cylindrical brass chamber (see fig. 1) was made 

 for the film in two sections with aground brass joint, which, 

 when covered with soft wax and pressed together, made the 

 vessel air-tight. Each section was 4'1 cm. in diameter and 

 7*0 cm. in height, having inlet and outlet tubes as shown in 

 the diagram. The top section A, fig. 1, was closed by a 

 window of plate glass, G, put on with hard wax, so that 

 when a source of light was held directly over the chamber, 

 its image in the film could be distinctly seen and in this way 

 the character of the surface of the film — whether concave, 

 convex, or plane — was known at once by the character of the 

 image produced. Knowing the curvature of the film one 

 could adjust the pressures of hydrogen and air on either side 

 very accurately and so as to keep the film plane and there- 

 fore eliminate the diffusion due to excess pressure on either 

 side. The brass ring 0, fig. 1, supporting the film was 

 4*95 cm. in diameter, and ground down to a sharp edge. 

 An annular channel, D, was made in the outer part of the 

 supporting ring, and the whole soldered in the lower section 

 of the film chamber, leaving about O'G cm. of the brass ring- 

 projecting above the wax surface. In this way the soft wax 

 used in making the joint air-tight was prevented from 

 contaminating the film and destroying its surface tension. 



To overcome the difficulty of evaporation and drainage 

 from the film, that is to keep its composition and thickness 

 constant, the air and hydrogen used were both saturated 

 with water vapour before entering the chamber, and, in 

 addition, a means of adding solution to the film was provided 

 in the following way. A bent tube, T, was inserted in the 

 upper chamber as indicated in the diagram, having a thistle 

 tube connected to the outer end by rubber tubing. A small 

 amount of the same soap solution used in making the film 

 was poured into the thistle tube and a drop of this was 



