Chemistry and Physics. 227 



theoretical value 1*5, required by the formula Pd 3 H 2 . Although 

 Hoitaema'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 favor of the view which 

 has sometimes been put forward, that the heat of occlusion of a 

 gas represents the heat of condensation or liquefaction of the gas 

 in the capillary pores of the absorbing substance, or the heat of 

 solidification or fusion. 



17. Refractivities of Air, Oxygen, Nitrogen, Argon, Hydrogen 

 and Helium. — The results obtained by Ramsey and Travers in 

 an investigation of the refractivities of a series of gases are given 

 in the following table : 



Refractivities of Gases, Air equal to Unity. 



Through 



Directly , A ^ 



compared. Oxygen. Nitrogen. Hydrogen. ■ Argon. 



Hydrogen 0-4733 0-4737 0-4727 



Oxygen 0-9243 0-9247 0-9237 0*9261 



Nitrogen 1-0163 0-0155 1-0170 1-0191 



Argon 09596 0-9577 0*9572 



Carbon dioxide 1*5316 



The values are compared here directly and also through the 

 other gases mentioned. The greatest care was taken to obtain 

 pure material in each case. The special method of experiment is 

 stated to have been the same as that described by Lord Rayleigh 

 (Proc. Roy. Soc, lix, 203): 



The authors have also calculated the refractive index of air 

 from the determinations obtained with the individual gases pres- 

 ent, viz. 



Oxygen 0*9243 



Nitrogen 1*0163 



Argon 0*9596 



and the densities of the constituent gases, and obtained the 

 following numbers : 



(1*0163X78*15) +(0*9243X20-91) + (0-9596X0-94) = 99-653 



They hence conclude that the refractivity of the mixture, air, is 

 somewhat less than that of the sum of the refractivities of its con- 

 stituents taken in the proportion in which they occur. This dis- 

 crepancy is not sensibly altered by such minor changes in the 

 data given as seem to be admissible. 



To test the same point similar calculations were made with a 

 mixture of hydrogen and helium, selected as being particularly 

 favorable since their critical points are so low and they hence 

 approximate closely to the ideal gas. The result was to obtain a 



