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XVII. On the Relation of Hydrogen to Palladium. 

 By Thomas Graham, F.R.S., Master of the Mint*. 



IT has often been maintained on chemical grounds that hy- 

 drogen gas is the vapour of a highly volatile metal. The 

 idea forces itself upon the mind that palladium with its occluded 

 hydrogen is simply an alloy of this volatile metal, in which the 

 volatility of the one element is restrained by its union with the 

 other, and which owes its metallic aspect equally to both consti- 

 tuents. How far such a view is borne out by the properties of 

 the compound substance in question will appear by the following 

 examination of the properties of what, assuming its metallic cha- 

 racter, would have to be named Hydrogenium. 



1. Density. — The density of palladium when charged with eight 

 or nine hundred times its volume of hydrogen gas is perceptibly 

 lowered; but the change cannot be measured accurately by the 

 ordinary method of immersion in water, owing to a continuous 

 evolution of minute hydrogen bubbles which appears to be de- 

 termined by contact with the liquid. However, the linear dimen- 

 sions of the charged palladium are altered so considerably that 

 the difference admits of easy measurement, and furnishes the 

 required density by calculation. Palladium in the form of wire 

 is readily charged with hydrogen by evolving that gas upon the 

 surface of the metal in a galvanometer containing dilute sulphuric 

 acid as usual f. The length of the wire before and after a charge 

 is found by stretching it on both occasions by the same moderate 

 weight, such as will not produce permanent distention, over the 

 surface of a flat graduated measure. The measure was graduated 

 to hundredths of an inch, and by means of a vernier the divi- 

 sions could be read to thousandths. The distance between two 

 tine cross lines marked upon the surface of the wire near each of 

 its extremities was observed. 



Exp. 1. — The wire had been drawn from welded palladium, and 

 was hard and elastic. The diameter of the wire was 462 mil- 

 lim. ; its specific gravity was 12*38, as determined with care. 

 The wire was twisted into a loop at each end and the mark made 

 near each loop. The loops were varnished so as to limit absorp- 

 tion of gas by the wire to the measured length between the two 

 marks. To straighten the wire, one loop was fixed, and the 

 other connected with a string passing over a pulley and loaded 

 with 1*5 kilogramme, a weight sufficient to straighten the wire 

 without occasioning any undue strain. The wire was charged 

 with hydrogen by making it the negative electrode of a small 

 Bunsen's battery consisting of two cells, each of half a litre in 



* Read before the Royal Society January 7, 1869. 

 t Proceedings of the Royal Society, vol. xvi, p. 422 (1868). [Phil. Mag. 

 Ser. 4. vol. xxxvi. p. 63.] 



