XVI INTRODUCTION. 



previously extracted from the two tartrates, acids which, 

 in regard to polarised light, behaved like themselves. 

 Such was the worthy opening of M. Pasteur's scientific 

 career, which has been dwelt upon so frequently and 

 emphatically by M. Eadot. The wonder, however, 

 is, not that a searcher of such penetration as Pasteur 

 should have discovered the facets of the tartrates, 

 but that an investigator so powerful and experienced 

 as Mitscherlich should have missed them. 



The idea of molecular dissymmetry, introduced 

 by Biot, was forced upon Biot's mind by the discovery 

 of a number of liquids, and of some vapours, which 

 possessed the rotatory power. Some, moreover, turned 

 the plane of polarisation to the right, others to the 

 left. Crystalline structure being here out of the 

 question, the notion of dissymmetry, derived from 

 the crystal, was transferred to the molecule. ' To 

 produce any such phenomena,' says Sir John Herschel, 

 ' the individual molecule must be conceived as un- 

 syminetrically constituted.' The illustrations employed 

 by M. Pasteur to elucidate this subject, though well 

 calculated to give a general idea of dissymmetry, will, 

 I fear, render but little aid to the reader in his at- 

 tempts to realise molecular dissymmetry. Should 

 difficulty be encountered here at the threshold of this 

 work, I would recommend the ivader not to be daunted 

 by it, or prevented by it from going further. He may 

 comfort himself by the assurance that the conception 



