THE PARATARTRATES 17 



sometimes finds in the cavities between the large crystals 

 of this acid some little needle-like crystal forming 

 tufts which are visible as an opaque white mass on the 

 surface of the semitransparent tartaric acid and some- 

 times so much resemble oxalic acid crystals that in the 

 factory of Thann, where they were formerly very abun- 

 dant, they have been taken for oxalic acid crystals and 

 an attempt made to sell them as such. It was soon 

 recognized that they were formed of a particular kind 

 of tartaric acid, giving salts entirely similar to the tar- 

 trates. Mitscherlich, who made a comparative study 

 of the known tartrates and of these new salts, 

 which he called paratartrates, found them identical 

 in all their relations. They had the same crystal- 

 line form, the same specific gravity, the same double 

 refraction in the crystalline state, the same angle 

 of the optical axes, the same index of refraction when 

 they were dissolved in the same proportion of water. 

 In short, no method, either physical or chemical, made it 

 possible to distinguish them, and they seemed identical 

 in every respect, save this, that the tartrates acted on 

 polarized light while the paratartrates were entirely 

 without action. 



Having arrived at this stage in his researches, Pasteur 

 could not fail to be impressed by this apparent contradic- 

 tion. "Mitscherlich was deceived," he said, "in affirm- 

 ing that the crystals of the tartrates and the paratartrates 

 resemble each other. There must be some external differ- 

 ences between them as regards the hemihedral facets. 

 Mitscherlich, preoccupied with his ideas on isomorphism 

 which made much of all the crystalline resemblances 

 between the different forms, would not have seen these 

 differences which he did not seek, but I, who have the 

 preconceived idea of their existence, am in a good posi- 

 tion to find them if they are there." 



