EXPERIMENTS IN CHEMISTRY 31 



vented him from following out the clue to his 

 original discovery. If he finally abandoned his 

 tentative view that there might be a constant re- 

 lation between crystalline form and rotary power, 

 he held firm to the idea of molecular asymmetry as 

 the basis for the action of dissolved substances on 

 polarized light. 



Here are two substances, identical in chemical 

 composition, and in numerous physical properties, 

 but having opposite effects on the polarized ray. 

 How can such a relationship be explained? The 

 molecules themselves, thought Pasteur, must have 

 asymmetry, the one being, so to speak, the mir- 

 rored image of the other. This conception formed 

 the foundation upon which was built one of the 

 most important branches of chemical science, 

 namely, stereo-chemistry, which is concerned with 

 the relative arrangement in space of the constituent 

 parts of the chemical molecule. Is it possible, 

 asked Pasteur, to change one of these asymmetrical 

 forms into the other? Paratartaric or racemic acid, 

 which was a relatively rare form which had been 

 obtained as a sort of by-product of the manufac- 

 ture of tartaric acid, had suddenly ceased to appear 

 where Pasteur had previously obtained it. In 

 order to collect other samples, Pasteur visited 



