[ 459 ] 

 LVII. Proceeding* of Learned Societies* 



ROYAL SOCIETY. 



[Continued from p. 399.] 



June 17, 1869. — Lieut. -General Sabine, President, in the Chair. 



'THIIE following comrnunieation was read : — 

 -*- "Additional Observations on Ilvdrogcnium." By Thomas 

 Graham, F.R.S., Master of the Mint. 



From the elongation of a palladium wire, caused by the occlu- 

 sion of hydrogen, the density of hydrogenium was inferred to be a 

 little under 2. But it is now to be remarked that another number 

 of half that amount nay be deduced with equal probability from 

 the same experimental dafa. This double result is a consequence of 

 the singular permanent .shortening of the palladium wire observed 

 after the expulsion of hydrogen. In a particular observation formerly 

 described, for instance, a wire of 609*14 millims. increased in length 

 to 618*92 millims. when charged with hydrogen, and fell to 599 '44 

 millims. when the hydrogen was extracted. The elongation was 

 9*78 millims., and the absolute shortening or retraction 9*7 millims., 

 making the extreme difference in length 19*48 millims. The elon- 

 gation and retraction would appear, indeed, to be equal in amount. 

 Now it is by no means impossible that the volume added to the 

 wire by the hydrogenium is represented by the elongation and re- 

 traction taken together, and nut by the elongation alone, as hitherto 

 assumed. It is only necessary to suppose that the retraction of the 

 palladium molecules takes place the moment the hydrogen is first 

 absorbed, instead of being deferred till the latter is expelled ; for 

 the righting of the particles of the palladium wire (which are in 

 a state of excessive tension in the direction of the length of the 

 wire) may as well take place in the act of the absorption of the 

 hydrogen as in the expulsion of that element. It may indeed 

 appear most probable in the abstract that the mobility of the pal- 

 ladium particle is determined by the first entrance of the hydrogen. 

 The hydrogenium will then be assumed to occupy double the space 

 previously allotted to it, and the density of the metal will be reduced 

 to one half of the former estimate. In the experiment referred 

 to, the volume of hydrogenium in the alloy will rise from 4*68 per 

 cent, to 9*36 per cent., and the density of hydrogenium will fall 

 from 1*708 to 0*854, according to the new calculation. In a series 

 of four observations upon the same wire, previously recorded, the 

 whole retractions rather exceeded the whole elongations, the first 

 amounting to 23*99 millims., and the last to 21*38 millims. Their 

 united amount would justify a still greater reduction in the density 

 of hydrogenium, namely to 0*8051. 



The first experiment, however, in hydrogenating any palladium 

 wire appears to be the most uniform in its results. The expulsion 

 of the hydrogen afterwards by heat always injures the structure of 

 the wire more or less, and probably affects the regularity of the ex- 

 pansion afterwards in different directions. The equality of the ex- 

 pansion and the retraction in a first experiment appears also to be 



2 H2 



