1868.] 



Hydrogen Gas hy Metals. 



423 



breaking the tube under mercury after two months, the vacuum was found 

 perfect. No hydrogen had vaporized in the cold (about 12°) ; but on 

 the application afterwards of a heat of 100° and upwards, 333 volumes of 

 gas were evolved from the metal. 



A similar result was obtained on making a hollow palladium cylinder, of 

 which the length was 115 millimetres, diameter 1 2 millimetres, and thick- 

 ness 1 millimetre, the negative electrode in an acid fluid, while the closed 

 cavity of the cylinder was kept exhausted by means of a Sprengel aspi- 

 rator. No hydrogen whatever passed through into the vacuous cavity in 

 several hours, although the gas was no doubt abundantly absorbed by the 

 outer surface of the cylinder and pervaded the metal throughout. 



It appears, then, that when hydrogen is absorbed by palladium the vola- 

 tility of the gas may be entirely suppressed ; and hydrogen may be largely 

 present in metals without exhibiting any sensible tension at low tempera- 

 tures. Occluded hydrogen is certainly no longer a gas, whatever may be 

 thought of its physical condition. The same conclusion was indicated by 

 another series of experiments, in which it was found that, to be occluded 

 by palladium, and even by iron, hydrogen does not require to be apphed 

 under much pressure, but, on the contrary, when highly rarefied is still 

 freely absorbed by these metals. 



The occluded hydrogen is readily extracted from palladium by reversing 

 the position of the latter in the decomposing cell of the battery, so as to 

 cause oxygen to be evolved on the surface of the metal. The hydrogen is 

 then drawn out as rapidly as it had previously entered the palladium, and 

 the metal is exhausted in a complete mariner by such treatment. When 

 palladium charged with hydrogen is left exposed to the atmosphere, the 

 metal is apt to become suddenly hot, and to lose its gas entirely by spon- 

 taneous oxidation. 



Platinum may be charged with hydrogen by voltaic action, as well as 

 palladium, but with the usual inferior proportion of gas. The charge 

 of hydrogen taken up in a decomposing voltaic cell by old platinum in the 

 form of a tube, of the thickness of a small crucible, was 2*19 volumes. 

 This absorbed gas was also readily withdrawn from the platinum and 

 oxidized on reversing the place of the metal in the decomposing cell. The 

 platinum acquired its well-known polarizing-power in virtue of the oc- 

 cluded hydrogen. This power was retained by the metal after being 

 washed with pure water and wiped with a cloth, and was brought into 

 action on placing the metal in dilute acid. The temperature required to 

 expel the hydrogen so absorbed by platinum was found to be little short of 

 a red heat, although the gas had entered the metal at a low temperature. 



Soft iron, left some time in a dilute acid, occluded 0-57 volume of 

 hydrogen. This charge of gas was also retained at low temperatures, and 

 did not escape into a vacuum till the temperature was raised nearly to red- 

 ness. This proves that, like platinum, iron is not penetrated through in the 

 cold by hydrogen, the temperature of emission being elevated considerably*. 



* In M. Cailletet's experiment of exposing a tliin sheet of iron to an acid, the metal 



