﻿Hydrogen through a Palladium Septum. 215 



It is evident that in all eases the pressure of the hydrogen 

 in the interior of the palladium tube is lower than the baro- 

 metric pressure, i. e. lower than the pressure of the hydrogen 

 on the exterior. It will be convenient here to tabulate the 

 ratios found : — 



a. Mean ratio from hydrogen and nitrogen at 280° . 0*9053 



b. Ratio at 335° . . 0*8984 



c. Ratio with 50 per cent, of hydrogen at 335° . . 0*9362 



d. Ratio with 25 per cent, of hydrogen at 335° . . 0*9344 



e. Ratio from hydrogen and carbon dioxide at 280°. 0*9621 



f. Ratio from hydrogen and carbon monoxide at 280° 0*9545 



g. Ratio from hydrogen and cyanogen at 280° . . 0*9693 



I omit the two last determinations, as uncertainty was 

 introduced owing to the presence of water- vapour. 



On reviewing these results, it would appear that the ratio is 

 not altered by rise of temperature above 280°. The same 

 number was obtained at 280° and at 335°. But at a lower 

 temperature the ratio is much smaller, if, indeed, the end- 

 point was really reached. Dilution of the hydrogen appears 

 to raise the ratio ; and the presence of carbon dioxide, carbon 

 monoxide, or cyanogen in the interior of the palladium tube 

 appears to be favourable to the passage of hydrogen. 



To what are these results to be ascribed ? 



Troost and Hautefeuille have determined the pressures of 

 dissociation of the definite hydride of palladium, Pd 2 H ( Compt. 

 rend, lxxviii. p. 686). At 130 c the pressure is nearly equal 

 to that of the atmosphere, it is 624 millim.; while at 140° the 

 pressure is 812 millim. It rises rapidly with temperature, and 

 at the highest temperature given, 170°, it amounts to the large 

 number 1840 millim. It is impossible to believe, then, that 

 any hydride of palladium should exist even at the lowest of 

 the temperatures employed, 223°. Yet palladium becomes 

 quickly permeable to hydrogen only at a higher temperature. 



Two questions require answer : — (1) Why is palladium 

 permeable to hydrogen ? and (2) Why is the pressure of 

 the internal hydrogen always less than that of the external 

 hydrogen ? 



(1) It is hardly probable that the palladium acts simply as 

 a filter, allowing small molecules of hydrogen to pass while 

 blocking the passage of larger molecules of other gases. 

 Graham, however, appears to regard this as in some measure 

 the cause of the passage. His words are : — " Such phenomena 

 of gaseous penetration appear to suggest a progression in the 

 degree of porosity. There appear to be (1) pores through which 

 gases pass under pressure or by capillary transpiration, as in 



