i 3 6 SCIENCE PROGRESS. 



cannot occupy much less surface, because then another 

 R would have room to attach itself to C.) The first 

 arrangement has the symmetry of the cubic system, the 

 second that of the rhombic system ; and for CR 3 R' we 

 should have a rhombohedral arrangement. And in fact 

 we find CI 4 to be tetrahedral, CBr 4 rhombic, and 

 CHBr 3 rhombohedral. But in the present state of our 

 knowledge of the relation between chemical constitution 

 and crystalline form, little weight can be attached to such 

 coincidences. 1 



That the carbon atom really possesses a repulsive force, 

 Le Bel infers from the ratio of the specific heats of marsh 

 gas, which shows that the atoms do not touch ; and he 

 points out that, for carbon at least, the hypothesis of the 

 repulsive zone dispenses entirely with the necessity for a 

 valence-hypothesis. His objection to the valence-hypo- 

 thesis is that, as ordinarily held, it attributes to the carbon 

 atom, not merely an attractive force, but a directive force 

 acting at right angles to this, and causing the attracted 

 groups to take the tetrahedral position. 



It will be seen from this paper, written by Le Bel to 

 show how his views differ from those of Van't Hoff, that 

 this difference is not so great after all. Le Bel's " re- 

 pulsive zone" is represented by Van't Hoff's " force pro- 

 ceeding from the carbon atom, and tending to bring the 

 groups connected with it into positions as far removed from 

 one another as possible, that is " (but here Le Bel would 

 add the qualification " unless the ratio of the repulsive 

 zones is unfavourable") "into the tetrahedral position ". 



And, to quote another passage of Van't Hoff : "His- 

 torically the difference lies in this, that Le Bel's starting- 

 point was the researches of Pasteur, mine those of Kekule. 

 . . . My conception is a continuation of Kekule's law of 

 the quadrivalence of carbon, with the added hypothesis 



1 For an account of the work of Sollas, Muthmann, Tutton, and Pen- 

 field on the arrangement of atoms in crystals, the reader is referred to the 

 article by H. A. Miers in "Science Progress" (iii., 129). 



