26 PASTEUR: THE HISTORY OF A MIND 



arrangement of the atoms in the molecule, and thus 

 we come quite naturally to represent to ourselves the 

 molecules of the two tartaric acids, the right- and the 

 left-handed, not only as dissymmetrical individually and 

 \^ith a non-superposable dissymmetry^ but also as 

 having an inverse dissymmetry one with the other. If 

 one is a right hand the other is a left hand. If one is a 

 corkscrew with a dextrorse spiral, the other is a corkscrew 

 with a sinistrorse spiral. In short, we know nothing 

 and we shall know nothing probably for a long time of 

 the mode of arrangement of the atoms in these two 

 molecules, but we remain faithful to logic and the laws 

 of induction in acknowledging that these two arrange- 

 ments, individually dissymmetrical, are reciprocally sym- 

 metrical in relation to a plane. 



Once this conception is admitted, we can easily 

 represent to ourselves the effect of these groupings in a 

 water solution on a ray of polarized light which tra- 

 verses it. Let us suppose that this solution contains only 

 identical and superposable tetrahedrons, for example, 

 those of boracite: these molecules are present in very 

 great numbers in the path of the ray, even when 

 the solution is not of much thickness; they occupy, in the 

 medium in which they are free, all possible positions. 

 Consequently, if we suppose that one of them is inclined 

 in a certain direction with regard to the ray of light and 

 acts on it in a given direction, there will always be 

 another one, identical with it and in the inverse position, 

 which will produce an effect in the opposite direction. 

 The molecular effects will always counterbalance there- 

 fore in pairs, that is to say, the light ray will leave 

 the solution just as it entered it, except for the small 

 amount of absorption undergone in traversing it. If, on 

 the contrary, the tetrahedrons in solution are not 

 identical, if they cannot be superposed, it would be 



