64 Royal Society : — 



A similar result was obtained on making a hollow palladium cy- 

 linder, of which the length was 115 millimetres, diameter 12 milli- 

 metres, and thickness 1 millimetre, the negative electrode in an acid 

 fluid, while the closed cavity of the cylinder was kept exhausted by 

 means of a Sprengel aspirator. 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 cylin- 

 der and pervaded the metal throughout. 



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

 volatility of the gas may be entirely suppressed ; and hydrogen may 

 be largely present in metals without exhibiting any sensible tension 

 at low temperatures. 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 applied 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 re- 

 versing 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 pre- 

 viously entered the palladium, and the metal is exhausted in a com- 

 plete manner by such treatment. When palladium charged with 

 hydrogen is left exposed to the atmosphere, the metal is apt to be- 

 come suddenly hot, and to lose its gas entirely by spontaneous oxi- 

 dation. 



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 This absorbed gas was also readily withdrawn 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 po- 

 larizing-power in virtue of the occluded 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 ab- 

 sorbed 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 tempera- 

 tures, and did not escape into a vacuum till the temperature was 

 raised nearly to redness. This proves that, like platinum, iron is not 

 penetrated through in the cold by hydrogen, the temperature of emis- 

 sion being elevated considerably*. 



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

 metal is no doubt penetrated through by hydrogen in the cold, but apparently 

 from the penetrating agency of the acid which is insinuating itself into the 

 metal at the same time. — Co>npfes Rcndits, 4 Mai 1868 



