ARTICLES 245 



wave-length of the light used, and upon the temperature and 

 the thickness of the layer of substance through which the 

 light passes. 



About the time of Biot's discovery (181 5) which first 

 brought the phenomena of polarisation under the consideration 

 of chemists, Mitscherlich observed the close connection exist- 

 ing between crystalline form and chemical composition, a 

 circumstance which naturally directed attention also to crystal 

 measurements. 



Hauy had early divided quartz crystals into two varieties 

 showing hemihedral facets inclined to the right or the left 

 when the crystals were orientated in a given manner. The 

 observations of Hauy and Biot were brought into relationship 

 by Herschel, 1 who showed that the two kinds of hemihedral 

 crystals were respectively dextro- and lsevo-rotatory, and 

 concluded that the hemihedral facets were produced " by the 

 same cause which determines the displacement of the plane 

 of polarisation of a ray traversing the crystal parallel to its 

 axis." Our knowledge of these obscure phenomena was a very 

 little later advanced almost to the position in which it stands 

 at the present day. About the beginning of the second quarter 

 of the nineteenth century the interest of chemists was mainly 

 centred on substances of organic origin, and one of the most 

 interesting problems of the time was the relationship of the 

 " acid from the Vosges," or racemic acid, to the long- known 

 tartaric acid. A careful investigation by Berzelius had em- 

 phasised their close resemblance, the chief difference being that, 

 whilst tartaric acid and its salts were dextro-rotatory, racemic 

 acid and its salts were optically inactive. 



A crystallographic examination of the double sodium 

 ammonium salts of the two acids by Mitscherlich only served 

 to render the difference less explicable, for he stated in a com- 

 munication to the French Academy in 1844 that the two salts 

 had the same chemical composition, the same crystalline form 

 with identical angles, the same density and the same double 

 refraction. With the exception of the presence or absence of 

 optical activity the salts were as similar as seemed possible 

 without being identical. 



Mitscherlich concluded from this similarity of properties 

 that the nature and the number of the atoms in the molecule, 



1 Trans. Cattib. Phil. Soc. 1882, 1, 43. 



