Royal Institution. 431 



in which not every similar edge or angle was modified, but where 

 every alternate similar edge or alternate similar angle was so. This 

 circumstance was then illustrated by the actual truncation of some 

 models ; and it was shown that such alternately developed or hemi- 

 hedral crystals may be right and left, the upper terminal modification 

 being to the right in the one case, and the corresponding 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-superposable" to the other; the one is as the 

 image of the other reflected in a mirror, as the right-hand is com- 

 pared to the left. 



A beautiful connexion 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 polari- 

 zation. The crystallographic character of these facettes was then 

 pointed out, and their connexion shown with this hemihedrism. 

 But the most beautiful instance of the connexion of hemihedrism in 

 crystals with the direction of the rotatory 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 character, the direction of the rotation is indicated by 

 the nature of the hemihedrism. 



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

 tremely like tartaric acid in its chemical reactions and identical with 

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

 found only one year in the cream of tartar of the wine of the Vosges. 

 Pasteur separated the crystals of the paratartrate of soda and ammo- 

 nia into two several sets : the one set he showed to be hemihedric 

 to the right, the other set to the left. The former proved to be the 

 salt of an acid rotating to the right, the latter of one rotating to the 

 left. On examining these acids they were found in every single 

 property, but this rotation, identical with one another and with tar- 

 taric acid. Yet when mixed they formed again the paratartaric 

 acid, which, like the amethyst, is without an)', action of a rotatory 

 character, and the difference of which from the other two acids was 

 then exhibited by the precipitation by the latter of a salt of lime 

 which did not render either of the former turbid. 



Mr. Maskelyne then detailed the experiments of M. Pasteur on 

 malic and aspartic acids and asparagine, and showed how all of these 

 could be understood to contain chemically a molecular unit common 

 to all these and perhaps to tartaric acid, and only modified a little 

 by the superposition as it were of other substances, in combination 

 with it, upon the extremities of its molecule. 



He also dwelt on the ])ossibility of the paratartaric acid being a 

 quadribasic as the tartaric acid is a bibasic acid ; it being on this 

 view a conjugate acid consisting of the two united tartaric acids. 

 He then invited attention to the interesting nature of M. Biot's in- 

 vestigation of the action of tartaric acid in solution in water, and he 



