66 Royal Society i — 



bonic oxide did not sensibly pass through iron of 1*7 millimetre in 

 thickness till the temperature 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 proportion of gas held is considerable. In the pulverulent 

 spongy state, palladium 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 chlo- 

 ride, by the action of a voltaic battery, in form of a compact metal. 

 Palladium is not one of the metals readily thus precipitated ; but it 

 may be thrown down upon a thin platinum wire, in brilliant 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 prepared, it does not contain any occluded hydrogen. But the 

 metallic films, when 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 indi- 

 cation 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 

 equivalent of hydrogen*, or there is an approximation to single 

 equivalents Pd H. But the idea of definite chemical combina- 

 tion is opposed by various considerations. No visible change is 

 occasioned to the metallic 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 they are 

 brown pulverulent substances with no metallic characters. Indeed a 

 hydride of palladium itself can be formed, but not preserved, on ac- 

 count of its great instability. Following the process of M. AVurtz 

 for the hydride of copper, nitrate of palladium was boiled with 

 sulphuric acid, and the sulphate of palladium (a red crystalline salt) 



* H=l, Pd= 106*5. 



