THE POLAEIMETER 75 



on a circular scale. Further, those substances whose solutions 

 give a right-hand twist to the plane of polarisation looked at 

 from the eye of the observer are known as dextro-rotatory, 

 those which twist it in the opposite direction are known as 

 Iccvo-rotatory. The property of rotating the plane of polarisa- 

 tion in this way is known as optical activity. 



The Relations between Optical Activity and 

 Molecular Structure 



The effect of a solution of tartaric acid and sundry other 

 organic substances upon the plane of polarisation was observed 



FIG. 16. EXANTIOMOEPHOUS CRYSTALS. 



first by Biot in 1838, and he also showed that racemic acid, 

 which has the same composition as tartaric acid, does not 

 possess this optical property. This was confirmed by Mitscher- 

 lich in 1844, but it was Pasteur in 1848 who made the first 

 great step in unravelling the cause of this difference. He found 

 that by careful crystallisation of sodium ammonium racemate, 

 a salt which in itself has no action upon polarised light, it was 



