426 



Mr. T. Graham on the Occlusion of 



[June 11, 



tallic characters. Indeed a hydride of palladium itself can be formed, but 

 not preserved, on account of its great instability. Following the process 

 of M. Wurtz for the hydride of copper, nitrate of palladium was boiled 

 ■with sulphuric acid, and the sulphate of palladium (a red crystalline salt) pre- 

 pared. A solution of this salt, with an excess of sulphuric acid, was precipi- 

 tated by the hypophosphite of soda ; a black powder fell, which speedily un- 

 derwent decomposition at 0°, evolving copious volumes of hydrogen gas. The 

 final residue appeared to be pure palladium, of its usual black amorphous 

 appearance, and with no trace of crystallization. It is singular that this 

 palladium precipitate contained no occluded hydrogen ; and even when 

 heated, and afterwards 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 penetration is not in propor- 

 tion 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 hydrogen gas at 26/°, but continued per- 

 meable, and in fact increased greatly in permeability at still higher tem- 

 peratures, 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 m^ore rapid passage of hydrogen was observed through the sub- 

 stance of a hollov/ cylinder of palladium 1 millimetre in thickness, at a 

 higher temperature, approaching the melting-point of gold. The palla- 

 dium 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 



399 2*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, amounting to 

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



