I MM 



antjr in our dm rtinn, \\hilr the other acid umli-r ana! icum- 



M.IIHVX made it deviate through the same angle, but in just the 

 opposite direction. The dextrogyratory acid was j>n>\v<l to be idenn< al 

 with the ordinary, already well- 

 known, tartaric acid; the lue\<>- 

 -vr.itory acid appeared to be 

 another, isomeric, turtaric ncid. 

 unknown up to that date. Not 

 i ml Y both acids themselves, but 

 also the various salts deriv. d 

 in >m them, appeared in each 

 case to have enantiomorphous 

 r\ >tal-forms, while their che- 

 mical behaviour evidently was 

 always identical. However the 

 solutions of the left-handed 

 crystals made the plane of 

 polarisation of a traversing po- 

 larised light-beam constantly 

 rotate in just the opposite di- 

 rection, to that which solutions, 

 prepared from the right-handed 

 crystals, 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 special 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 special 

 arrangement of the atoms of chemical molecules in general; and 

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



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

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

 -) L. Pasteur, loco cit. 



L. Pasteur. 18221895. 



