205 



angle in one direction, while the other acid under analogous circum- 

 stances made it deviate through the same angle, but in just the 

 opposite direction. The dextrogyratory acid was proved to be identical 

 with the ordinary, already well- 

 known, tartaric acid; the laevo- 

 gyratory acid appeared to be 

 another, isomeric tartaric acid, 

 unknown up to that date. Not 

 only both acids themselves, but 

 also the various salts derived 

 from them, appeared in each 

 case to have enantiomorphous 

 crystal-forms, while their che- 

 mical behaviour was always 

 identical. However, the solu- 

 tions of the left-handed crys- 

 tals made the plane of polarisa- 

 tion of a traversing polarised 

 light-beam constantly rotate in 

 just the opposite direction, to 

 that which solutions, prepared 

 from the right-handed crys- 

 tals, did 1 ). 



Seldom has a scientific discovery had such far reaching conse- 

 quences, as this one had. The connection between the non-super- 

 posable mirror-images of the crystal-forms of the isomeric substances 

 and the oppositely directed rotatory power of their molecules, seemed 

 proved beyond all doubt. 



Neither could there be any doubt as to the structural identity 

 of the two tartaric acids, and the explanation given by Pasteur 

 himself 2 ) : that the arrangement of the atoms in the molecules 

 of both acids must necessarily be supposed to be related to each 

 other, as that of two "non-superposable" stereometrical figures, - 

 led not only to the conception of a new kind of isomerism, but it 

 marked the very starting-point of our views concerning the spatial 

 arrangement of the atoms of chemical molecules in general; and 

 a quarter of a century later it opened the way to that new branch 



x ) L. Pasteur, Ann. de Chim. et Phys., (3), 24, 442, (1848); Compt. rend, 

 de 1'Acad. d. Sciences Paris, 26, 535; 27, 101, 367, (1848). 

 2 ) L. Pasteur, loco cit. 



L. Pasteur. 18221895. 



