Mr. T. Graham on the Occlusion of Hydrogen Gas by Metals. 67 



prepared. A solution of this salt, with an excess of sulphuric acid, 

 was precipitated by the hypophosphite of soda ; a black powder fell, 

 which speedily underwent decomposition at 0°, evolving copious 

 volumes of hydrogen gas. The final residue appeared to be pure pal- 

 ladium, of its usual black amorphous appearance, and with no trace 

 of crystallization. It is singular that this palladium precipitate con- 

 tained no occluded hydrogen ; and even when heated, and after- 

 wards exposed to an atmosphere of hydrogen in the usual manner, 

 the palladium black so prepared condensed no sensible quantity of 

 that gas. 



I am inclined to conclude that the passage of hydrogen through a 

 plate of metal is always preceded by the condensation or occlusion 

 of the gas. But it must be admitted that the rapidity of penetra- 

 tion is not in proportion to the volume of gas occluded ; otherwise 

 palladium would be much more permeable at a low than at a high 

 temperature. A plate of that metal was sensibly exhausted of hy- 

 drogen gas at 267°, but continued permeable, and in fact increased 

 greatly in permeability at still higher temperatures, and without 

 becoming permeable to other gases at the same time. In a striking 

 experiment, a mixture of equal volumes of hydrogen and carbonic 

 acid was carried through a small palladium tube, of which the 

 internal diameter was 3 millimetres, and the thickness of the wall 

 0*3 millimetre. From the outer surface of this tube gas escaped 

 into a vacuum, at a red heat, with the enormous velocity of 1017'54 

 cub. centims. per minute for a square metre of surface. This gas did 

 not disturb baryta-water. It was pure hydrogen. 



A still more rapid passage of hydrogen was observed through the 

 substance of a hollow cylinder of palladium 1 millimetre in thick- 

 ness, at a higher temperature, approaching the melting-point of 

 gold. The palladium cylinder being enclosed in a porcelain tube 

 charged with pure hydrogen, was exhausted as usual, and gave 105'8 

 cub. centims of gas in five minutes ; measured with bar. 753 millims., 

 therm. 10°. As the external surface of the palladium tube amounted 

 to 0'0053 square metre, the passage of gas was 



3992*22 cub. centims. from a square metre of surface, per minute. 



The rate of penetration of hydrogen through the same palladium 

 tube, at the lower temperature of 265° C, was previously observed 

 to be 



327 cub. centims. from a square metre of surface, per minute. 



The velocity of penetration thus appears to increase in a rapid ratio 

 with the temperature. 



When carbonic acid was substituted for hydrogen, at the same 

 high temperature, a very minute penetration was perceived, amount- 

 ing to 



1*86 cub. centim. from a square metre of surface, per minute. 



This gives for carbonic acid one twenty-thousandth part of the 

 rate of hydrogen. Whether it is a penetration of the same sort, 

 although greatly less in degree, or rather the consequence of a 

 sensible porosity in the palladium (of which it would become the 

 measure), remains uncertain. 



F2 



