THE SPACE RELATIONS OF ATOMS. 493 



Pasteur observed that when a body is optically active 

 in solution, its crystals possess a hemihedry corresponding 

 to the activity (right or left-handed) of the solution, and he 

 deduced the theory of the asymmetric structure of the mole- 

 cules of bodies active in solution. We cannot admit that the 

 rotatory power is due to the particular mode of aggregation 

 of the chemical molecules forming the crystal molecule, for 

 it would be necessary to admit also that this association 

 persists in solutions and in vapours, whereas determinations 

 of freezing-points and of vapour-densities show that the 

 crystal molecule is dissociated in these conditions. Pasteur 

 concluded that " the right- and left-handed isomers are like 

 right- and left-handed spirals or staircases " in their atomic 

 arrangement. As dissymmetry is impossible without 

 internal stability, we have here the fundamental principle 

 of stereochemistry. 



Now Sarrau has given mathematical proof that when 

 in a body through which polarised light is passing there is 

 dissymmetry, it must betray itself by rotation of the plane 

 of the light. This being so we may proceed to consider 

 in what cases dissymmetry would be expected to occur, 

 and we may then compare the results with the facts by 

 testing the substances in question for optical activity. 



Evidently every arrangement of four or more different 

 bodies lacks symmetry, unless it is a plane arrangement. 

 In general, then, every molecule in which a radical A is 

 joined to three or more radicals, R' R" R'", etc., differing 

 from it and from one another will be asymmetric. But 

 there will be exceptions, ( 1 ) when the radicals are not 

 relatively fixed ; (2) when the arrangement is plane ; (3) 

 when one radical, as R", is a duplicate of the rest of the 

 molecule, so that a symmetrical molecule A R' R" R" R' A 

 results. 



( 1 ) Molecules internally unstable. An exception of 

 this class occurs according to Le Bel [Revue Scientifiqtie, 

 xxviii., 609), whenever the radical A is not a simple radical. 

 That is to say, the link between two groups is movable so 

 that indirectly connected atoms have no definite orientation. 

 For consider ethane, C 2 H 6 ; the simplest arrangement of the 



