Indices of Refraction of Substances for the Electric Ray. 293 



Hoitsema lias shown that Troost and Hautefeuille's deduction that 

 a compound exists having the formula Pd 2 H is not warranted. The 

 constancy of the heat of occlusion over the whole range of absorp- 

 tion is also opposed to the view that such a compound is formed. 



The composition of fully charged palladium hydrogen corresponds 

 closely with the formula Pd 3 H 2 first suggested by Dewar. The 

 principal and almost only evidence, up to the present, in favour of the 

 formation of such a definite chemical compound is to be found in the 

 approximation of the above atomic ratios to the theoretical value 1'5, 

 required by the formula Pd 3 H 2 . Although Hoitsema's arguments 

 may be equally well directed against the existence of this compound, 

 the authors consider that additional and independent evidence is 

 desirable, and hope to be able to provide it. 



It is also shown that the heats of occlusion of hydrogen in platinum 

 and palladium black are not in favour of the view which has some- 

 times been put forward that the heat of occlusion of a gas repre- 

 sents the heat of condensation or liquefaction of the gas in the 

 capillary pores of the absorbing substance, or the heat of solidifica- 

 tion or fusion. 



•* On the Determination of the Indices of Refraction of various 

 Substances for the Electric Ray. II, Index of Refraction 

 of Glass." By Jagadis Chunder Bose, M.A., D.Sc, Pro- 

 fessor of Physical Science, Presidency College, Calcutta. 

 Communicated by Lord Rayleigh, F.R.S. Received 

 October 1 ,— Read November 25, 1897. 



In my previous paper, read before the Royal Society on October 20, 

 1895,* I described a method of determining the indices of refraction 

 of various substances for electric radiation, the principle of which 

 depends on the determination of the critical angle at which total 

 reflection takes place. A semi- cylinder of the given substance was 

 taken, and the angle of incidence gradually increased till the rays 

 were totally reflected. The experiment was repeated with two semi- 

 cylinders, separated by a parallel air-space. The advantage of the 

 latter arrangement was that the image cast by the two semi-cylinders 

 remained fixed. The image underwent extinction when the angle of 

 incidence attained the critical value. 



The determination of the indices of refraction for long electric 

 waves derives additional interest from Maxwell's theoretical relation 

 between the dielectric constant and the refractive index for infinitely 

 long waves. The relation K=/i. 2 has, however, been found to be 

 fulfilled in only a few instances. The value u x is usually deduced 



* Vide ' Roy. Soc. Proc.,' vol. 59, p. 160. 

 VOL. LXII. r 



