Constants of Hydrogenium. 335 



of hydrogenium" before we could arrive at any conclusion 

 regarding the condition of the absorbed hydrogen. Subse- 

 quently Professor Tait made a series of determinations on the 

 "Electrolytic Polarization of Palladium Electrodes"*, devising 

 a new and ingenious method for the purpose. Although at dif- 

 ferent 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 pre- 

 liminary experiments were communicated to the Philosophical 

 Magazine, under the title " Note on the Specific Heat of Hy- 

 drogenium." In that note it is stated that by means of a spe- 

 cially constructed calorimeter the specific heat of hydrogen in 

 palladium is found to be 3*1 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 coefficient of expansion. 



Graham, in his celebrated paper on hydrogenium, made many 

 determinations 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 gra- 

 vity 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 con- 

 firmed by the use of alloys of palladium that resist this contrac- 

 tion, and finally regarded the specific gravity as 0*733. No 

 determinations were made when the palladium was partially 

 saturated ; and he rejected the ordinary process of taking specific 

 gravities, because of the continual evolution of gas preventing 

 exact weighings being taken in water. 



In the experiments to be detailed a cubical mass of the metal 

 was charged with hydrogen by electrolysis, taken at different 

 times during the progress of the saturation and weighed in air 

 and in water. If the mass was allowed free exposure to the air 

 for several hours, little difficulty arose from the evolution of gas 

 when immersed in water, and accurate results could be obtained. 



From the experimental numbers the specific gravity of the 

 absorbed hydrogen was calculated by the well-known formula 



^1 + w c . 



= S; 



■■*-* H — * 



S 2 S 2 



Proceedings of the Royal Society of Edinburgh, Session 1868-69. 



