OCTOBBE 12, 1900.] 



SCIENCE. 



543 



his indebtedness to Pasteur, while van't 

 Hoif made no special mention of him and 

 proceeded to develop his theory on a priori 

 grounds, though he has elsewhere told us 

 that his interest was aroused by Wislicenus' 

 discovery of the physically isomeric lactic 

 acids. Beginning with the assumption that 

 the four valences of the carbon atom are 

 directed towards the apices of a tetrahedron, 

 van't Hoff showed that if any two of the 

 four combined atoms or groups are iden- 

 tical (Caabc) but one form can result, while 

 if they are all different (Cabcd) there must 

 result two forms, identical in so far as their 

 plane structural formulas are concerned, 

 and identical in all chemical and physical 

 respects, save that the one is to the other 

 as an unsymmetrical object and its reflec- 

 tion in a mirror ; they would constitute 

 right- and left-handed figures, their influ- 

 ence on polarized light and on the form of 

 the crystal would be the same, but in op- 

 posite senses, that is, they would be dextro- 

 and Igevo-rotatory, and if showing hemihe- 

 drism, this, too, would be opposite in the 

 two forms. Proceeding to apply this hy- 

 pothesis, he showed that every optically 

 active organic compound, the constitution 

 of which was then known, contains one or 

 more such asymmetric carbon atoms, carbon 

 atoms combined with four different atoms or 

 groups ; the dextro- and Isevo-tartaric acids 

 are identical when the ordinary formulas 

 are considered, but different when the space 

 relations are taken into account. It was 

 further shown that two or more asymmetric 

 carbon atoms in the same molecule might 

 reinforce or neutralize each other with re- 

 spect to rotatory power, in the latter case 

 giving bodies like inactive tartaric acid, 

 which differs from racemic acid in not being 

 separable into optical antipodes. 



Van't Hofif further applied his theory of 

 the tetrahedral carbon atom to other 

 obscure cases of isomerism, found only in 

 bodies having doubly united carbon atoms, 



such as fumaric and maleic acids, which 

 are not chemically identical, but the inter- 

 pretation of which on the current views of 

 structure had not been satisfactorily accom- 

 plished. Fittig regarded fumaric acid as 



CH.CO.H 



CH.CO.H 



and the isomeric maleic acid as 

 CH,.CO,H 



I , . 



= C— CO,H 



the latter formula, however, not being in 

 harmony with the facts, while Anschiitz 

 held maleic acid to be 



CH.C(OH), 

 II >0 . 

 CH.CO 



On van't Hofl"'s hypothesis, the doubly com- 

 bined carbon atoms are incapable of free 

 rotation, the combined groups or atoms 

 being, therefore, compelled to retain their 

 relative positions, thus giving stability to 

 the two configurations 



,b 



b>c = c<- 



and ^>C = C<" 



We have thus two distinct types of geo- 

 metrical isomerism ; in the one, the type 

 with asymmetric carbon atoms, the chem- 

 ical properties are identical and likewise 

 the physical, except in so far as they in- 

 volve space relations, as optical rotation 

 and hemihedrism; in the other, that of the 

 doubly united carbon atoms, the chemical 

 properties, while not absolutely identical, 

 are so nearly so that they can be expressed 

 by the same structural formula of the old 

 style. 



Van't Hoflf and Le Bel's explanation of 

 physical isomerism long attracted but little 

 attention among chemists, partly because 

 such cases were then comparatively rare, 

 partly because of the inertia of the chem- 

 ical mind, which preferred to seek an ex- 



