48 NOTICES OF THE MEETINGS [March 28, 
sort of nucleus or primitive form existing on which the crystal was 
formed, was explained, as also the nature of the developement of 
such a crystal by the modifications placed on the edges and angles 
according to the law before mentioned. But the exception to that 
law was not less remarkable for its generality of character 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 
angle wasso. This circumstance was then illustrated by the actual 
truncation of some models; and it was shown that such alternately 
developed or hemihedral crystals may be right and /eft, 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 compared to the left. 
A beautiful connexion was then pointed out, as established long 
ago by the acute observation of Sir J. Herschel, that the plagiedral 
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 pos- 
sesses 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 ammonia 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 tartaric acid. Yet when mixed they formed again the Pa- 
ratartaric acid, which, like the amethyst, is without any 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 
lS 
