Mr. W. C. Roberts on the Expansion of Palladium. 51 



to be proved to be the expression of an actual law. In Gauss's 

 theory analogous results are obtained on the hypothesis of two 

 magnetic fluids, which are assumed to be capable of separation, 

 and to be such that, when separated, like fluids mutually repel, 

 and unlike mutually attract, according to the law of the inverse 

 square. But what are we to understand by the separation of 

 dissimilar fluids, and the dependence of mutual attractions and 

 repulsions on this condition ? It is as hard to conceive of rea- 

 sons for these hypotheses as to account for the magnetic facts 

 proposed to be explained by them. The present theory tends to 

 show that there is no physical foundation for such hypotheses, 

 the facts admitting of explanation on the supposition that a single 

 fluid (the aBther) acts in a manner conformable to hydrodynamical 

 principles. The argument contained in this communication I am 

 entitled, I think, to regard as confirmatory of the hydrodyna- 

 mical theory of magnetism. 

 Cambridge, May 22, 1869. 



VI. Note on the Experimental Illustration of the Expansion of 

 Palladium attending the Formation of its Alloy with Hydroge- 

 nium. By W. Chandler Koberts, F.C.S., F.G.S* 



\ TTENTION has recently been directed to the experimental 

 *™ demonstration of the absorption of hydrogen by palladiumf. 



As the present writer has had the privilege of being con- 

 nected with Mr. Graham's recent researches, he ventures to 

 offer a description of the special arrangements that, from some 

 experience, appear to him best suited to the purpose of illus- 

 tration. 



It will be remembered that Mr. Graham finds palladium, by 

 the occlusion of 936 volumes of hydrogen, to sustain an increase 

 in its linear dimensions of 1*605 on the 100; or assuming the 

 expansion to be equal in all directions, the cubic expansion will 

 be 4-908 on the 100, equal to sixteen times the dilatation of pal- 

 ladium when heated from 0° C. to 100° C. A simple illustra- 

 tion, well adapted for lecture-experiments, consists in arranging 

 two fine palladium wires on the same plane, but slightly inclined 

 towards each other ; these are placed in a cell filled with acidu- 

 lated water, which may be illuminated by an electric or other 

 lamp, and the image of the wires thrown upon a screen. The 

 wires are to be connected with either element of a small battery, 

 a commutator intervening. 



* Communicated by the Author. 



t James Dewar, F.R.S.E.," On the Motion of a Palladium Plate during 

 the Formation of Graham's Hydrogenium ;" and Poggendorff, " On the 

 Voltaic Deportment of Palladium :" Phil. Mag. No. 251, pp. 424 and 474. 



E2 



