Constants of Hydro genium. 337 



in the plate, the density of the hydrogen was 0*633; and where 

 only 0'0495 grm. was left, the value was 0*615. The mean 

 specific gravity of the three different alloys of plate was 0*623, 

 nearly the same as previously found for the bar. After palla- 

 dium has been treated with hydrogen and heated, the metal 

 becomes porous, and ofteu blisters, thus rendering specific-gra- 

 vity determinations very uncertain. The curious discovery of 

 Graham, that the strain on the particles of palladium produced 

 by wire-drawing induces such a curious shrinking in the length 

 after the hydrogen has been expelled, suggests the importance 

 of investigating how a saturated piece of bar would resist the 

 action of sustained pressure or tension. If the formation of the 

 alloy is attended with an increase of volume in excess of the sum 

 of the volumes of the constituents regarded as in the solid state, 

 then partial decomposition ought to occur under great pressure. 

 Experiments on this subject are left over for the present. After 

 palladium has been used in the above experiments, and the last 

 trace of hydrogen is removed by heating, the specific gravity is 

 found to have diminished from 12*0359 to 11*9546. If this 

 final value is taken in calculating the mean density, then the 

 average result of the three hydrides of plate is 0*707 for the 

 specific gravity of the condensed hydrogen. The mean of the 

 first and second series of experiments is thus 0*664. A piece 

 of palladium, weighing 31 grins., was fused in a lime crucible 

 with the oxyhydrogen-flame ; and the specific gravity was now 

 found reduced to 10*5549. When this sphere of palladium was 

 hydrogenized for a very long time (twenty-four hours) only 

 0*0684 grm. of hydrogen was absorbed, and by further treat- 

 ment nothing was added. The specific gravity of the hydrogen 

 in this case was 0*655, nearly identical with the mean of the 

 values found from the plate experiments. 



Specific-Heat Observations. 



The apparatus devised for this purpose is represented in the 

 figure (p. 338) ; the calorimeter used in the experiments had the 

 form A. It held conveniently 100 grammes of water, and was 

 inserted in the middle of a stout brass envelope, thoroughly 

 exhausted of air, the whole being placed in the middle of a large 

 cylindrical tin vessel (E), having an outer annular compartment. 

 The tin vessel was filled with water • and a constant current from 

 the town supply was kept circulating by means of a siphon 

 through the outer chamber. To the thermometer a ring of thin 

 sheet india rubber was attached as a stirrer, and, when it was 

 removed before the immersion of the palladium, was placed in 

 the little tin tube represented at JB. Immediately before an 

 observation the whole apparatus was moved below the steam- 



Phil. Mag. S. 4. Vol. 47. No. 313. May 1874*. Z 



