278 Prof. McLennan and Mr. Shaver on Permeability of 



VIIT. General Procedure. 



When readings were taken the air and the gas under test 

 were allowed to flow past the film until the katharo meter 

 reading giving the percentage of gas in the air was steady. 

 This required one half hour, and then readings were taken at 

 two minute intervals for forty minutes, and note was taken of 

 the rate of flow of air by means of the gas meter. This ranged 

 from 2 to 10 litres per hour. After one set of readings was 

 taken, if the film still remained intact, the rate of air flow- 

 was changed, and after conditions became steady again 

 another set of results was obtained. In this way as many 

 as four sets of readings were taken without renewing the 

 film. 



IX. Purity of Gases. 



The purity of the gases under test was in both cases 

 comparatively high. The hydrogen was obtained from a 

 commercial supply which was guaranteed to be of 99 per 

 cent, purity. The helium used was first purified by passing 

 it through a set of four charcoal tubes at the temperature of 

 liquid air. Its purity was tested by means of a quartz density 

 balance, and found to be 99*2 per cent. 



X. Soap Solution. 



The soap solution used was one made up according to 

 Boys' formula, and contained 2 per cent, sodium oleate, 

 24 per cent, glycerine, and 71 per cent, water, with a few 

 drops of strong ammonia. 



XI. Results. 



The following are the results obtained. The last column 

 gives the number of cubic centimetres of: gas transfusing 

 through one square centimetre of film per hour. 



The readings were taken at room temperature which varied 

 slightly as shown in the table. However, taking the average 

 values obtained for the two gases, we find the ratio of the 

 transfusion of helium to that of hydrogen to be 0*70. In 

 the case of balloon fabrics, Elworthy and Murray found this 

 ratio to be 0*67. Expressing the average results for hydrogen 

 and helium in the case of a soap film in the same terms as 

 the permeability of balloon fabrics, we find the permeability 

 of films to these gases given by Elworthy and Murray for 



