16 LOUIS PASTEUR. 



experiment the explanation of the difficulty which 

 the note of Mitscherlich had thrown down as a kind 

 of challenge to science, when it signalised an optical 

 difference between two chemical compounds affirmed 

 to be otherwise rigorously identical. Pasteur reasoned 

 thus : Since I find tartaric acid and all its tartrates 

 without a plane of symmetry, while its isomer, para- 

 tartaric acid, and its compounds have such a plane, I 

 will hasten to prepare the tartrate and the paratartrate 

 of the note of Mitscherlich. I will compare their 

 forms, and in all probability the tartrate will be found 

 dissymmetrical that is to say, without a plane of 

 symmetry while the paratartrate will continue to 

 have such a plane. Henceforward the absolute iden- 

 tity stated by Mitscherlich to exist between the forms 

 of these two compounds will have no existence. It 

 will be proved that he has erred, and his note will no 

 longer have in it anything mysterious. As the optic 

 action proper to the tartrates spoken of in his note 

 manifests itself by a deviation of the plane of polarisa- 

 tion to the right, we have here a kind of dissymmetry 

 which has nothing incompatible with the dissymmetry 

 of form. On the contrary, these two dissymmetries 

 can be referred to one and the same cause. In like 

 manner, the absence of dissymmetry in the form of 

 the paratartrate will be connected with the optical 

 neutrality of that compound. 



The fulfilment of Pasteur's hopes was only partial. 



