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Connectimi 0/ Chemical Forces with Polarization of Light 67 



formed, was explained^ as also the nature of the development 

 of Such a crystal by the modifications placed on the edges and 

 angles according to the law before mentioned. But the excep- 

 tion to that law ^vas not less remarkable for its generality of char- 

 acter than the law itself This exception consists in the fact 

 that very often crystals are found in which not every similar edge 

 or angle was modified, but where every alternate similar edge or 

 alternate similar aagle was so. This circumstance was then 

 Illustrated by the actual truncation of some models ; and it was 

 shown that such alternately developed or hemihedral cvystah may 

 uG; right and left^ the upper terminal modification being to the 

 nght in the one case, and the correspoi>ding and similar lower 

 terminal modification being then to the left, while in the other 

 case they are exactly the converse. Hence, one crystal is, as 

 Pasteur describes it, '^ non siiperposaby^ to the other: the one 

 IS as the image of the other reflected in a mirror, as the right hand 

 IS compared to the left. 



A beautiful connection was then pointed out, as established 



long ago by the acute observation of Sir J. Herschel, that the 



plagihedral facettes of quartz indicated, by their relative positions 



on the crystal, the direction in which the crystal would rotate 



|( the plane of polarization. The crystal lographic character of these 



lacettes was then pointed out, and their connection shown with 

 this hernihedrism. But the most beautiful instance of the con- 

 J^ection of hernihedrism in crystals with the direction of the ro- 

 tatory power of the substance of which they are composed is 

 afforded by the recent discoveries of Pasteur, which may now be 

 almost traced to a law, enunciated thus : that where a substance 

 IS hemihedric when crystallized, and possesses the rotating char- 

 acter, the direction of the rotation is indicated by the nature of 

 the hernihedrism. 



Paratartaric acid was then introduced. It was described as ex- 

 tremely like tartaric acid in its chemical reactions and identical 

 ^Uh it in composition. It was similarly formed to the latter, and 

 ^as found only one year in the cream of tartar of the wine of the 

 osges. Pasteur separated the crystals of the Paratartrate of soda 

 and ammonia into two several sets; the one set he showed to be 

 hemihedric to the right, the other set to the left. The former 



in cio^A rntnfino- in fht^ vx^ht. the Xatiet of 



; 



^»e rotating to the left. On examining these acids they were 

 ^^nd iu every single property, but this rotation, identical with 

 ^^e another and with tartaric acid. Yet when mixed they 

 'orrned again the Paratartaric acid, which, like the amethyst, is 

 ^vithout any action of a rotatory character, and the difference of 

 which from the other two acids was then exhibited by the pre* 

 cipitation by th# latter of a salt 

 either of the former turbid 



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