﻿Hydrogen through a Palladium Septum. 207 



the interior of the vessel would be a vacuum to hydrogen ; 

 and as its walls are permeable to hydrogen, pressure should 

 rise by passage of hydrogen into the interior, until the pres- 

 sure of the hydrogen on the interior walls became equal to 

 that on the exterior walls. The effect of this would be to 

 superadd the pressure of the hydrogen to that of the gas 

 originally contained in the vessel ; and if it be supposed that 

 the vessel was originally filled at atmospheric pressure, the 

 entry of hydrogen should increase that pressure by another 

 atmosphere, providing the hydrogen surrounding the exterior 

 walls of the vessel be at atmospheric pressure. 



It has been suggested that when pressure is raised by the 

 passage of water into the interior of a cell with semiper- 

 meable walls containing a solution, the rise of pressure is 

 due, not to the pressure exercised by the molecules of the dis- 

 solved substance, but to that produced by the entering water. 

 Gases present us with an exact analogy. It is idle to inquire 

 what causes the rise of pressure in the interior of such a palla- 

 dium " cell." The total pressure is due to the hydrogen and to 

 the gas with which the cell was originally filled ; the original 

 pressure has undoubtedly been increased by the entry of hy- 

 drogen. But a portion of the pressure — and the effective 

 portion, from the point of view of osmotic pressure — is due to 

 the original gas, whether nitrogen, carbon dioxide, or any gas 

 whatever to which the cell-walls are impervious, and which 

 is not chemically attacked by hydrogen. It is therefore quite 

 correct to ascribe osmotic pressure to the dissolved substance, 

 although it is apparently produced by entry of solvent. 



The experiments to be described were made with the object 

 of submitting this entry of hydrogen through the walls of a 

 palladium cell to quantitative study. After considerable 

 progress had been made, a paper by A. Biltz (Zeitschr. f. 

 phys. Chem. ix. p. 152) was published, describing lecture- 

 experiments devised to show the ordinary diffusion of gases 

 without a septum, diffusion with a leaky septum of porous 

 earthenware, and also diffusion with a semipermeable septum, 

 permitting the passage of hydrogen, but hindering the 

 passage of other gases. Biltz employed for the last-mentioned 

 purpose an iron tube, and made a few rough quantitative 

 measurements ; but he does not appear to have continued 

 any experiment long enough to obtain a maximum pressure, 

 nor was his apparatus designed with the object of quanti- 

 tative measurement, but only for the purpose of class demon- 

 stration. 



