4G0 Royal Society: — 



a matter of certainty. This is a curious molecular fact, of which 

 we are unable as yet to see the full import. In illustration, another 

 experiment upon a pure palladium wire may be detailed. This 

 wire, which was new, took up a full charge of hydrogen, namely 

 956-3 volumes, and increased in length from 609*585 to 6 19*354 

 millims. The elongation was therefore 9*769 millims. With the 

 expulsion of the hydrogen afterwards, the wire was permanently 

 shortened to 6001 15 millims. It thus fell 9'4/0 millims. below its 

 normal or first length. The elongation and retraction are here within 

 0*3 millim. of equality. The two changes taken together amount to 

 19*239 millims., and their sum represents the increase of the wire 

 in length due to the addition of hydrogenium. It represents a 

 linear expansion of 3*205 on 100, with a cubic expansion of 9*827 

 on 100. The composition of the wire comes to be represented as 

 being, 



In volume. 



Palladium 100*000 or 90*895 



Hydrogenium 9' 827 or 9*105 



109*827 or 100*000 



The specific gravity of the palladium was 12*3, the weight of the 

 wire 1*554 grm., and its volume 0*126 cub. centim. The occluded 

 hydrogen measured 120*5 cub. centims. The weight of the same 

 would be 0*0108 grm., and the volume of the hydrogenium 0*012382 

 cub. centim. (100: 9*827 : : 0*126 : 0*01238). The density of the 

 hydrogenium is therefore 



Q - Q108 =0*872. 

 0*01238 



This is a near approach to the preceding result, 0*854. Calculated 

 on the old method, the last experiment would give a density of 1*708. 



It was incidentally observed on a former occasion that palladium 

 alloyed with silver continues to occlude hydrogen. This property 

 is now found to belong generally to palladium alloys when the second 

 metal does not much exceed one half of the mixture. These alloys 

 are all enlarged in dimensions when they acquire hydrogenium. It 

 was interesting to perceive that the expansion was greater than 

 happens to pure palladium (about twice as much), and that, on after- 

 wards expelling the hydrogen by heat, the fixed alloy returned to 

 its original length without any further shortening of the wire. The 

 embarrassing retraction of the palladium has, in fact, disappeared. 



The fusion of the alloys employed was kindly effected for me by 

 Messrs. Matthey and Sellon — when the proportion of palladium was 

 considerable, by the instrumentality of M. Deville's gas-furnace (in 

 which coal-gas is burned with pure oxygen), or by means of a coke- 

 furnace when the metals yielded to a moderate temperature. The 

 alloy was always drawn out into wire if possible ; but if not suffi- 

 ciently ductile, it was extended by rolling into the form of a thin 

 ribbon. The elongation caused by the addition of hydrogenium was 

 ascertained by measuring the wire or ribbon stretched over a gra- 

 duated scale, as in the former experiments. 



1. Palladium, Platinum, and Hydrogenium. — Palladium was fused 



