On the Intimate Structure of Crystals. 299 



gonal axis of the primitive cube, and thus (7'416 4'206)/2 = 1-605, 

 the true diameter of an atom which could just occupy the central 

 interspace of the primitive cubelet. The cube of this number will 

 give us the gross volume of the atom of occluded hydrogen ; it is 

 4-134. If now we turn to Riicker's address on "The Range of 

 Molecular Forces," * we find the most probable estimates given for the 

 volume of hydrogen (H) are as follows : From K (Boltzman) 4'4, 

 (Klemencic) 4'4 ; from n (Mascart) 4'65 ; from 6 (Van der Waals 

 and O. Meyer) 4*4. Between these numbers and that we have just 

 obtained there is a very remarkable concordance. 



It may further be observed that the number of such interspaces as 

 we have considered is, to the number of atoms among which they lie, 

 in the ratio of 1 : 1, so that from purely geometrical considerations 

 it might be inferred that the limiting value for the absorption of 

 hydrogen by palladium would be reached with the formation of the 

 substance Pd 2 H 2 . Observation shows that this limit is never ex- 

 ceeded, never even attained, while that which is reached may fairly 

 be represented by the formula Pd 3 H 2 . It is obvious that purely 

 geometrical considerations are not all -that are involved, and to dis- 

 cuss other factors would be to trespass beyond our province. There 

 is one point in direct connection with our inquiry which must not, 

 however, be disregarded. The value we have found for the diameter of 

 hydrogen was obtained on the assumption that all the central spaces 

 were occupied by hydrogen, which would only be the case if Pd 2 H 2 

 were formed ; the observed ratio, Pd 3 H 2 , would lead us to believe that 

 only two- thirds of the spaces are so occupied. This renders neces- 

 sary a correction in our estimate, which would slightly increase the 

 dimensions of the hydrogen atom. It is not possible, however, to 

 introduce this correction, on account of the absence of information 

 regarding the crystalline form assumed by Pd 3 H 2 . If crystals of 

 palladium be capable of taking a charge of hydrogen, there should 

 be no difficulty in ascertaining whether a change in crystalline 

 form accompanies occlusion. On the assumption that the maximum 

 expansion of palladium due to occlusion is confined to two-thirds of 

 the volume of the metal experimented upon, I find that the diameter 

 of the hydrogen atom should be 4*395. Possibly the assumption is 

 not defensible, but in any case it would appear that the amount of 

 coincidence we have already obtained between the dimensions of the 

 hydrogen atom, as calculated from the crystalline structure we have 

 assigned to palladium (along with other metals) and the dimensions 

 which follow from other modes of inquiry, affords strong confirma- 

 tion of our hypothesis. 



The absorption of hydrogen by potassium might easily take place 

 without producing any marked expansion, i.e., so far as the relative 

 * ' Trans. Chem. Soc.,' vol. 63, p. 257, 1888. 



