10 LOUIS PASTEUK. 



specific weight, the same double refraction, and con- 

 sequently the same inclination of the optic axes. 

 Dissolved in water, their refraction is the same. But 

 while the dissolved tartrate causes the plane of polar- 

 ised light to rotate, the paratartrate exerts no such 

 action. M. Biot has found this to be the case with the 

 whole series of these two kinds of salts. Here (adds 

 Mitscherlich) the nature and the number of the 

 atoms, their arrangement, and their distances apart 

 are the same in the two bodies.' 



Imbued as he was with the teachings of Haiiy and 

 Delafosse, and full of the ideas of M. Dumas in mole- 

 cular chemistry, Pasteur asked himself this question : 

 * How can it be admitted that the nature and number 

 of the atoms, their arrangement and distances apart, 

 in two chemical substances are the same ; that the 

 crystalline forms are equally the same, without con- 

 cluding that the two substances are absolutely iden- 

 tical ? Is there not a profound incompatibility between 

 the identity affirmed by Mitscherlich and the dis- 

 crepancy of optic character manifested by the two 

 compounds, tartaric and paratartaric, which form the 

 subject of his note ? ' 



This difficulty rested in Pasteur's mind with the 

 tenacity of a fixed idea. Eeceived as agrege of physical 

 science at the end of his third year at the Ecole, and 

 then keeping near his master, M. Balard, he had 

 begun the study of crystals and the determination of 



