( 96 ) 



found by van E verdingen ') with Perrot's prism, namely — ^1,68, 



and witli the vahie found bv the Swedish investigators for haematite • 

 1,78 at 50" C. 



7. If there were a perfect concordance between theory and 



observation, we should have in both cases — =: 1. Tlie said values 



1,128 and 1,480 are, therefore, in a certain sense a measure for 

 the extent of the divergence between the observation and the con- 

 clusion which is rendered prol)able by the electron theory. 



In the first place it will be observed that the agreement is much 

 better with bismuth than with haematite. However, this may be 

 expected if we consider that the theory has been proposed, in the 

 first instance, for metallic conductoj'S. Tiie influence of the peculiar 

 nature of the o,vide when compared with the true vietal is shown 

 very plainly in this case. 



The question may be raised whether, perhaps, there may be 

 shown to exist some connection between the crystal structure and 



the chemical nature on one side, and the given values of — on the 



other side. 



Such a connection would have some significance because it may 

 be, probably, a guide for the detection of special factors situated in 

 the crystalline structure, which stand in the way of a complete 

 agrement of electron theory and observations. 



§ 8. First of all, it must be observed that we are easily led to 

 compare the structures of the two phases. Both substances inves- 

 tigated crystallise ditrigonally and have an analogous axial quotient; 

 for bismuth: a : c = 1:1,3035 (G. Rose); for haematite a : c = 

 1 : 1,3654 (Melczer). In both substances, the habit is that of the 

 rhomboid, which in each of them approaches very closely to the 

 regular hexahedron. The characteristic angle « is 87°34' for bismuth 

 for haematite 85^42'. Particularly in bismuth the pseudo-cubic 

 construction is very distinct; the planes of complete cleavage which 

 answer the forms |111| and |111| approach by their combination the 

 regular octahedron in a high degree. Although haematite does not 



1) VAN EvERDiNGEN, Avclilves Néerlaiid. (1901) 371 ; Vers). Akacl. v. Wet. (1895— 

 1900); Gomm. Ptiys. Lab. Leiden, 19, 26, 37, 40 and 61. See Arcliiv. Nêerl. 

 p 452 ; rods No. 1 and No. 5. 



