520 Mr. T. Graham on the Absorption and 



seventy-five minutes more, 36*75 cub. centims., making alto- 

 gether 141*46 cub. centims. Of this gas 127*74 cub. centims. 

 proved to be hydrogen, the remainder being nitrogen, derived, 

 no doubt, from the large imperfectly exhausted porcelain tube. 

 The palladium alloy, in the form of a thick plate, appears there- 

 fore to have held 



20*5 vols, hydrogen, measured at 18°'2 and barom. 756millims. 



This alloy of palladium becomes crystalline by heating, and 

 appears to lose much of its absorbent power at the same time. 



The conclusion, then, is that welded palladium, in the condition 

 of thin foil, readily absorbs hydrogen, to the extent of upwards 

 of 600 times the volume of the metal at a temperature under the 

 boiling-point of water, upwards of 500 volumes at 245°, and less 

 at higher temperatures, the metal being always surrounded by 

 hydrogen under atmospheric pressure. Hydrogen is also largely 

 absorbed, although less constantly, at ordinary temperatures. 

 On the other hand, palladium already fully charged with hydro- 

 gen at or under 100°, and under the pressure of the atmosphere, 

 begins to give out gas when exposed either to atmospheric air 

 or to a vacuum at the original temperature of absorption ; and 

 the gas is freely discharged at 200° C. 



It is probable that hydrogen enters palladium in the physical 

 condition of liquid, whether the phenomenon prove to be ana- 

 logous to the imbibition of ether, chloroform, and such solvents 

 by the colloid india-rubber, or whether a certain porosity of 

 structure in the palladium be required. The porosity of the 

 metal is supposed to be of that high degree which will admit 

 liquid but not gaseous molecules. Now the numerous liquid 

 compounds of carbon and hydrogen have all a nearly similar 

 density, generally a little under that of water. There is no 

 reason to suppose that the density of liquid hydrogen would 

 differ greatly from the hydrocarbon class ; but then the surpri- 

 sing lightness of hydrogen gas must cause liquid hydrogen to 

 yield a volume of vapour disproportionately large when compared 

 with the former class of substances, or, indeed, with any other 

 substance whatever. The absorption of hydrogen by palladium 

 will appear, then, less extravagantly great when viewed as the 

 absorption of a highly volatile liquid capable of yielding an ex- 

 ceedingly light vapour, rather than that of a gas. 



An excellent opportunity of observing the penetration by hy- 

 drogen of a compact plate of palladium, 1 millim. in thickness, 

 was afforded by a tube of that metal constructed by Mr. Mathey. 

 This tube was said to have been welded from palladium near the 

 point of fusion of the metal. The length of the tube was 115 

 millims., its internal diameter 12 millims., thickness 1 millim. 



