24 LOUIS PASTEUE. 



part at the end of their own careers, these learned men 

 observed with pleasure the incipient ray which had 

 not yet become a glory but which was the precursor 

 thereof. 



' My young friend,' said M. Biot to Pasteur, when 

 presenting him to Mitscherlich somewhere about that 

 time, ' you may boast of having done something great, 

 in having discovered what had escaped such a man as 

 this.' 



' I had studied,' replied Mitscherlich, not without 

 a shade of regret, addressing himself to Pasteur, 'I 

 had studied with so much care and perseverance, in 

 their smallest details, the two salts which formed the 

 subject of my note to the Academy, that, if you have 

 established what I was unable to discover, you must 

 have been guided to your result by a preconceived 

 idea.' 



Mitscherlich was right, and this preconceived idea 

 might have been formulised thus : A dissymmetry in 

 the internal molecular arrangement of a chemical sub- 

 stance ought to manifest itself in all its external pro- 

 perties which are themselves capable of dissymmetry. 



If this theoretic conception was correct, Pasteur 

 might expect to find that all the substances in which 

 M. Biot had observed the power of rotating the plane of 

 polarisation would possess the crystalline di^- 



