a 



356 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 



it is plain that the passing material is influenced by the 

 substance of which the pore consists only on those por- 

 tions that come in contact therewith. Through the cen- 

 tral parts of the pore the material will pass by mere 

 leakage. If, then, we desire to determine the physical 

 conditions under which these movements take place, we 

 must make use of barriers the pores of which are not of 

 sensible size. 



I therefore closed the top of a glass tube, A, Fig. 80, 

 otherwise open at both ends, with a disk of 

 paper placed at such a distance down the tube 

 a as to permit a stratum of water, a a, one eighth 

 of an inch thick, to be laid upon it. Conveying 

 the tube to the pneumatic trough, I filled it in 

 succession with various gases, and watched their 

 passage through the water roof, for so it might 

 be called, into the air. In such an experiment 

 a column of hydrogen gas half an inch in length 

 had escaped in twenty-four hours. 



But experiments of this kind may be much shortened 

 by using very thin films instead of a thick stratum. A 

 glass bell-jar, a a, Fig. 81, was filled with 

 hydrogen gas, and by the side of it was 

 placed a small bottle containing atmos- 

 pheric air. A finger dipped in soap- 

 water was passed over the mouth of the 

 bottle, so as to close it with a thin film, 

 and the glass bell -jar of hydrogen was 

 then placed over it, as at b. In the course of two min- 

 utes the film, which was at first horizontal, had become 

 convex, and eventually swelled into a large spherical 

 bubble, c. In sixteen minutes it had become so thin 

 that it was of a dark metallic lustre. 



But the action is much more speedy if, instead of these 

 horizontal films, soap-bubbles are used. Such films are 



