ON DYNAMIC ISOMERISM. 141 
stance may retain its colour and lose its fluorescence by crystallising out 
from solution), but on a mere analogy, the value of which is highly 
problematical. 
Colour.—In contrast to these cases it is noteworthy that colour is not 
as a rule affected in any marked way by crystallisation. Certainly the 
passage from the liquid to the solid state is accompanied by nothing at 
all analogous to the abrupt bleaching which would almost inevitably take 
place if colour were really due to any concrete form of oscillatory chemi- 
cal change, and which forms the most commonplace of observations 
when dealing with fluorescent colour. It is indeed true that colour is 
often intensified at high temperatures and reduced by cooling, but these 
alterations proceed continuously ; in this respect they are in direct con- 
trast to the abrupt arrest of chemical change which takes place when 
nitrogen peroxide is frozen or when nitrocamphor is crystallised out from 
solution ; there is therefore nothing here to justify the contention that 
colour is due fo chemical change rather than to oscillations or vibrations 
of a ‘ physical ’ character not involving any real alteration of structure. 
On the contrary, the effects of crystallisation are such as to confirm the 
conclusions arrived at from general considerations and from the effects 
produced by impurities that colour, unlike phosphorescence and fluor- 
escence, is a physical phenomenon in which chemical change plays no 
essential part. 
The Study of Isomorphous Sulphonic Derivatives of Benzene.— 
Report of the Committee, consisting of Principal MIERs 
(Chairman) and Professors H. E. Armstrona (Secretary), 
W. J. Pope, and W. P. WYNNE. 
In continuance of previous work a number of members of several series 
of sulphonic derivatives of para-dihalogen derivatives of benzene have 
been prepared and crystallographically examined. The substances for 
which data are given below crystallise in the monosymmetric system :— 
Br 
(Be ERIS der) Melting point 
S0,Cl 
1. 24760 : 1 : 1:1439, B=95° 26’ . ; : + Tele} 
Br 
Br 
SO,Br 
2 2°4792 : 1: 111448, B=96° 49’ . : . . 114°. 
Br 
I 
£0,Br 
3. 24689 : 1 3 1:1537, B=95° 50’ . ; ’ » 102°, 
Cl 
