78 HYDROGENIUM; THE RELATION OF HYDROGEN TO PALLADIUM. 
Density of hydrogenium.1-708 
The density of hydrogenium, then, appears to approach that of magnesium, 1-743, by 
this first experiment. 
Further, the expulsion of hydrogen from the wire, however caused, is attended with 
an extraordinary contraction of the latter. On expelling the hydrogen by a moderate 
heat, the wire not only receded to its original length, but fell as much below that zero 
as it had previously risen above it. The palladium wire first measuring 609-144 
millims., and which increased to 9‘77 millims., was ultimately reduced to 599-444 
millims., and contracted 9-7 millims. The wire is permanently shortened. The density 
of the palladium did not increase, but fell slightly at the same tiqie, namely from 12*38 
to 12*12, proving that this contraction of the wire is in length only. The result is the 
converse of extension by wire-drawing. The retraction of the wire is possibly due to an 
effect of wire-drawing in leaving the particles of metal in a state of unequal tension, a 
tension which is excessive in the direction of the length of the wire. The metallic par¬ 
ticles would seem to become mobile, and to right themselves in proportion as the hydro¬ 
gen escapes ; and the wire contracts in length, expanding, as appears by its final density, 
in other directions at the same time. 
A wire so charged with hydrogen, if rubbed with the powder of magnesia (to 
make the flame luminous), burns like a waxed thread when ignited in the flame of a 
lamp. 
Expt. 2.—Another portion of the same palladium wire was charged with hydrogen in 
a similar manner. The results observed were as follows :— 
Length of palladium wire. 
The same with 867-15 volumes of occluded gas 
Linear elongation ......... 
Linear elongation on 100. 
Cubic expansion on 100.. 
Weight of palladium wire. 
Volume of palladium wire. 
Volume of occluded hydrogen gas . . . . 
Weight of same. 
Volume of hydrogenium.. 
From these results it is calculated 
Density of hydrogenium. 
488-976 millims. 
495-656 „ 
6-68 „ 
1-3663 „ 
4-154 „ 
1-0667 grm. 
0-08072 cub. centim. 
75"2 
0-00684 grm. 
0-003601 cub. centim. 
1-898 
Expt. 3.—The palladium wire was new, and on this occasion was well annealed be¬ 
fore being charged with hydrogen. The wire was exposed at the negative pole for two 
hours, when it had ceased to elongate. 
Length of palladium wire. 556-185 millims. 
Same with 808-303 volumes hydrogen . . . 563-652 „ 
Linear elongation. 7'467 „ 
Linear elongation on 100 . 1-324 „ 
Cubic expansion on 100 . 4-025 „ 
Weight of palladium wire. 1-1675 grm. 
Volume of palladium wire. 0-0994 cub. centim. 
Volume of occluded hydrogen gas .... 84*3 cub centims. 
Weight of same. 0-007553 grm. 
Volume of hydrogenium . . . i . . . 0-003820 cub. centim. 
These results give by calculation 
Density of hydrogenium. 1-977 
It was necessary to assume in this discussion that the two metals do not contract nor 
expand, but remain of their proper volume on uniting. Dr. Matthiessen has shown 
that in the formation of alloys generally the metals retain approximately their original 
densities.* 
In the first experiment already described, probably the maximum absorption of gas by 
wire, amounting to 935-67 volumes, is attained. The palladium may be charged with 
any smaller proportion of hydrogen by shortening the time of exposure to the gas (329 
* Philosophical Transactions, 1860, p. 177. 
