738 EVENING DISCOURSES. 
the vagaries of the style, the crystal class. Sohncke has shown that there are 
sixty-five such vagaries possible, which he terms regular point systems, and 
these coincide with sixty-five of the 230 possible modes of partitioning space. 
These are the broad simple facts, now proved up to the hilt, which 
explain the majority of crystal structures, all, in fact, but a very few of the 
more complicated classes of the thirty-two. For the remaining 165 ways of 
appropriating space all fall into a very small number of crystal classes. 
They are of very great interest, however, and involve an entirely new prin- 
ciple, that of ‘reflective’ or ‘mirror-image’ symmetry, enantiomorphism as 
it is technically termed, and include those crystals which possess the remark- 
able property of rotating the plane of polarised light. These are the cases 
whose geometrical possibility has been accounted for by the simultaneously 
independent work of Schénflies, von Fedorow, and Barlow, and to which we 
were experimentally introduced by the discovery of the right- and left- 
handed varieties of tartaric acid by Pasteur. The latter has since been 
followed by the revelation of many similar cases of two forms of the same 
chemical substance, related crystallographically and structurally like a 
right-hand to a left-hand glove, and optically differing by the direction in 
which they rotate a beam of plane polarised light. 
With their discovery and explanation the elucidation of the seven styles 
of crystal architecture and their thirty-two subdivisions becomes un fait 
accompli, and although many difficult problems still confront the crystal- 
lographer, problems of vast importance to chemistry, the groundwork is now 
securely laid, the memorable achievement of the last twenty years. The 
results, moreover, are in entire accordance with the now well-proved fact 
that the chemical atom is composed of electronic-corpuscles. For the definite 
orientation of the atom and its sphere of influence within the molecule and 
the crystal is thereby accounted for, the motion in the solid state so fre- 
quently hitherto attributed to the atom being a myth, such motion relating, 
in fact, to the corpuscles within the atom. 
[The rest of this discourse was devoted to experimentally demonstrating, 
with the aid of a projection Nicol-prism polariscope of original construc- 
tion, firstly, the optical behaviour of the simpler kinds of crystal structure, 
and, secondly, that of the interesting cases of mirror-image symmetry. | 
Quartz in particular will afford us not only some magnificent phenomena 
by reason of its right- or left-handed structure, but also a most instructive 
example, in its repeatedly twinned and all-but-molecularly alternating 
variety of amethyst, of the phenomenon of ‘ pseudo-racemism.’ For it is the 
display of this phenomenon which often renders a crystalline inactive 
substance so difficult to distinguish from a truly ‘racemic’ substance, which, 
as in the case of racemic acid itself, the optically inactive variety of 
tartaric acid, is a truly molecular compound of the right- and left-handed 
active varieties, the optical activity being neutralised and destroyed by the 
act of combination. 
TUESDAY, AUGUST 31. 
Our Food from the Waters. By Professor W. A. Herpman, F'.R.S. 
At the last meeting of the British Association in Canada (Toronto, 
1897) I was able to lay before Section D a preliminary account of the 
results of running sea-water through four silk tow-nets of different degrees 
of fineness continuously day and night during the voyage from Liverpool 
to Quebec. During the eight days’ traverse of the North Atlantic, the 
nets were emptied and the contents examined morning and evening, so 
that each such gathering was approximately a twelve-hours’ catch, and 
each day and each night of the voyage was represented by four gatherings. 
