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VII1.—On the Physical Constants of Hydrogenium. By JAMES DEWAR. 
(Plate XII.) 
(Read January 20, 1873.) 
In March 1869 I communicated to the Society a paper entitled “ Motion of 
a Palladium Plate during the Formation of Granam’s Hydrogenium,” which 
appears in the Proceedings for Session 1868-69. When engaged with this 
subject many points of interest regarding the behaviour of palladium containing 
occluded hydrogen, suggested themselves for investigation, and in concluding 
the paper I remarked “that careful determinations must be made of the electro- 
motive force, latent heat, &c., of hydrogenium” before we could arrive at any 
conclusion regarding the condition of the absorbed hydrogen. Subsequently 
Professor Tait made a series of determinations on the “ Electrolytic Polarisa- 
tion of Palladium Electrodes,”* devising a new and ingenious method for the 
purpose. Although at different times subsequent to my first communication, 
the problem of determining the physical constants of hydrogenium recurred, as 
my attention was in the meantime directed to the specific heat of carbon at high 
temperatures, no progress was made with the investigation until September 
1872, when the results of my preliminary experiments were communicated to 
the “ Philosophical Magazine,” under the title, ‘‘ Note on the Specific Heat of 
Hydrogenium.” In that note it is stated that by means of a specially con- 
structed calorimeter the specific heat of hydrogen in palladium is found to be 
31 per atomic weight, nearly identical with that of gaseous hydrogen. The 
present paper deals with some of the physical constants of hydrogenium, more 
especially with the specific gravity, specific heat, and co-efficient of expansion. 
: GRAHAM, in his celebrated paper on hydrogenium, made many determina- 
tions of the specific gravity of the occluded hydrogen by observing the increase 
of length of palladium wire after being fully charged, thus finding the 
cubical expansion, and from it deducing the weight of unit volume of the 
absorbed hydrogen. 
From experiments made in this way he found the specific gravity to be 
nearly 2. Afterwards he discovered the value was about three times what it 
ought to be from a contraction of length occurring when palladium wire is used. 
This he confirmed by the use of alloys of palladium that resist this contraction, 
and finally regarded the specific gravity as 0°733. No determinations were 
made when the palladium was partially saturated, and he rejected the ordinary 
* Proceedings of the Royal Society of Edinburgh, Session 1868-9. 
VOL. XXVII. PART I. 2x 
