1868.] 



Hydrogen Gas hy Metals, 



425 



it into a vacuum very slowly compared with hydrogen, although the 

 volume of carbonic oxide which the metal is capable of absorbing is very 

 sensible, amounting to 4 volumes, and more considerable than the volume 

 of hydrogen which the same metal can occlude. Carbonic oxide did not 

 sensibly pass through iron of 1*7 millimetre in thickness till the tempera- 

 ture was greatly elevated ; and then the passage of gas was, in a minute — 

 Of carbonic oxide, at a full red heat, 0*284 cub. centim. per square 

 metre of surface. 



Of hydrogen oxide, at a full red heat, 76*5 cub. centims. per square 

 metre of surface. 



The condition of hydrogen as occluded by a colloidal metal may be 

 studied with most advantage in its union with palladium, where the pro- 

 portion of gas held is considerable. In the pulverulent spongy state, pal- 

 ladium took up 655 volumes of hydrogen ; and so charged it gave off no 

 gas in vacuo at the ordinary temperature, nor till its temperature was 

 raised to nearly 100°. Hammered palladium foil has been observed to 

 take up quite as much gas. But the condition in which palladium appears 

 to be most absorptive is when precipitated from a solution of about 1*6 

 per cent, of the chloride, by the action of a voltaic battery, in the form of 

 a compact metal. Palladium is not one of the metals readily thus preci- 

 pitated ; but it may be thrown down upon a thin platinum wire, in bril- 

 liant laminse, by the action of a large single cell. The palladium after a 

 time detaches itself from the wire, exhibiting a bright white metallic sur- 

 face where it had been in contact with the platinum, and a dull surface, 

 suggesting metallic arsenic, on the side exposed to the acid. As so pre- 

 pared, it does not contain any occluded hydrogen. But the metallic films, 

 vvhen heated to 100° in hydrogen, and allowed to cool slowly for an hour 

 in the same gas, were found to occlude 982*14 volumes of gas, measured 

 with thermometer at 11°, and barometer at 756 millimetres. This is the 

 largest absorption of hydrogen observed. From palladium so charged 

 there was a slight indication of the escape of hydrogen into a vacuum, 

 with extreme slowness in the cold. This charged palladium is represented 

 by weight as 



Palladium 1*0020 grm 99*277 



Hydrogen 0*0073 grm '723 



100-000 



It is in the proportion of one equivalent of palladium to 0*772 equiva- 

 lent of hydrogen*, or there is an approximation to single equivalents 

 Pd II. But the idea of definite chemical combination is opposed by 

 various considerations. No visible change is occasioned to the metalhc 

 palladium by its association with the hydrogen. Hydrides of certain 

 metals are known, as the hydride of copper (Wurtz) and the hydride of 

 iron (Wanklyn) ; but tbey are brown pulverulent substances with no me- 



II =1, Pel =106-5, 



