FIEST DISCOVERIES. 15 



before a mirror ; the image faithfully reproduces the 

 chair. The mirror also reproduces the human body 

 considered as a whole. But place before the mirror 

 your right hand and you will see a left hand. The 

 right hand is not superposable on the left, just as 

 the glove of your right hand cannot be fitted to your 

 left, and inversely.' 



Then reverting to the beginnings of his studies in 

 crystallography, Pasteur recounted to me briefly that, 

 after having gone through the work of M. de la Pro- 

 vostaye, he perceived that a very interesting fact had 

 escaped the notice of this skilful physicist. M. de la 

 Provostaye had failed to observe that the crystalline 

 forms of tartaric acid and of its compounds all belong 

 to the group of objects which have not a plane of 

 symmetry. Certain minute facets had escaped him. 

 In other words, Pasteur discerned that the crystal- 

 line form of tartaric acid, placed before a mirror, pro- 

 duced an image which was not superposable upon the 

 crystal itself. The same was found to be true of 

 the forms of all the chemical compounds of this acid. 

 On the other hand, he imagined that the crystal- 

 line form of paratartaric acid, and of all the com- 

 pounds of this acid, would be found to form part 

 of the group of natural objects which have a plane of 

 symmetry. 



Pasteur was transported with joy by this double 

 result. He saw in it the possibility of reaching by 



