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TRANSACTIONS OF SECTION B, 431 
often occur in temperature-rotation curves, sometimes in such as show also a 
maximum, and by piecing together the evidence collected from an examination 
of a fair number of optically active substances it seems probable that the varia- 
tion of the rotation of an active substance with change of temperature may be, 
and very probably is, a periodic phenomenon, doubtless irregularly periodic— 
such that several maxima and minima may be expected to occur in the curve 
representing it. Owing to experimental difficulties, however, it is not possible 
to trace these curves through any very wide range of temperature. 
Now it seems legitimate to assume that a point of maximum rotation 
indicates that condition of the substance in which one of the groups attached to 
the asymmetric atom attains to a maximum influence—a singular condition of the 
substance. When such singular points are found in the curves of condition for 
a number of fairly closely related compounds if seems reasonable to suppose that 
the maxima represent the rotations of these different compounds in, at least, 
fairly similar conditions. The great merit of a maximum rotation is its recog- 
nisability and the possibility it affords of tracing some particular state of the 
compound as the external conditions are varied. Maxima are found at different 
temperatures for the various members of an homologous series, but the discussion 
of this field, since it involves the relationship between the rotation and the con- 
stitution of a series of active compounds, may be passed over until the variation 
of rotation of a single active compound with change of external conditions is 
more fully understood. 
Here the first matter to which attention may be directed is that the maximum, 
in certain cases at any rate, occurs at a different temperature for light of 
various refrangibilities, whence it would appear that the irregularly periodic 
temperature-rotation curves are probably retarded on each other; and since 
the curve for violet light has the greatest amplitude, it follows that these curves 
cut one another throughout a certain region, and in this region the rotation- 
dispersion of the substance must necessarily be anomalous. Hence we arrive at 
once, not, it is true, at an explanation of anomalous rotation-dispersion, but at 
a reason why anomalous rotation-dispersion should exist at all. 
It is doubtful whether any substance will really show normal rotation- 
dispersion, but if such a substance be found then it seems probable that the 
temperature-rotation curves for the dilterent colours of light will intersect at a 
single point, the rotation-dispersion being positive on one side of this point and 
negative on the other. 
Rotation in Solution.—A study of such data as are available appears to show 
that when such a compound as shows a maximum rotation—for example, ethyl 
tartrate—is dissolved in some indifferent liquid, this maximum rotation is 
displaced towards a lower or a higher temperature, as the case may be, with a 
corresponding alteration in value, solvents differing very much in regard to the 
displacement which they bring about. Now it is also found that the region in 
which abnormal rotation-dispersion takes place is shifted, on solution, in a very 
similar way to that in which the maximum rotation is displaced, and therefore 
it seems clear that the effect. of solution is to displace the whole temperature- 
rotation curve. It then appears at once why some substances which show 
abnormal rotation-dispersion at a certain temperature for the homogeneous 
compound show a normal rotation-dispersion when dissolved in some solvent 
which considerably alters the rotation. The solvent has the effect of shifting 
the family of temperature-rotation curves in such a manner as to bring the parts 
of the curves in the neighbourhood of the maximum into view, and in this 
neighbourhood the rotation-dispersion appears to be normal. 
(vi) A Partially Corrected Fluor-Quartz Lens System for Spectrum 
Photography. By Lieut-Colonel W. Grirrorp. 
(vil) Crystalline-Liquid Substances. By Dr. J. Huimy, 
