ON DYNAMIC ISOMERISM. 137 
B. Catalytic Action of Impurities. 
It cannot be insisted too strongly that no actual case of ‘ intramole- 
cular change’ is known to chemistry. The part which a chain or 
circuit of molecules plays in bringing about chemical action is now very 
generally recognised; it is unnecessary to do more than mention, as 
instances, the part played by moisture in determining the combusti- 
bility of gases, the importance of impurities in conditioning the dis- 
solution of metals by acids and the need of a catalyst to bring about 
the isomeric change of phenylchlorimide; it is, however, permissible to 
emphasise the fact that neither the dissociation of a molecule of am- 
monium chloride nor the transference of a hydrogen atom from carbon 
to oxygen in a molecule of nitrocamphor—changes which, if such a 
thing were possible, might be expected to provide excellent examples of 
single-molecule transformations—can be effected without the aid of a 
foreign substance, and the establishment of a complex heterogeneous 
molecular circuit. It is therefore legitimate, in considering the con- 
nection between luminous phenomena and chemical change, to apply 
as a critical test to any particular relationship that may be proposed 
the question as to whether the action can or cannot be arrested by the 
elimination of impurities. If the phenomenon can be proved to be 
manifest only in presence of a catalyst, not when pure materials are 
used, there is good evidence that it may be dependent on chemical 
change; if the effect can be proved to be independent of impurities, it 
must be regarded as inherent in the physical nature of the material. 
Positive evidence that the presence of a catalyst is essential has been 
forthcoming in the case of the phenomena of mutarotation and in the 
case of phosphorescence, as both manifestations may be arrested by 
suitable methods of purification ; these may, therefore, be correctly attri- 
buted to chemical rather than to purely physical changes. Refraction, 
dispersion, and optical rotatory power, on the other hand, are properties ~ 
which do not appear to be dependent in any way on the presence of 
foreign substances and must therefore be referred to physical and not 
to chemical characteristics of the molecule. In an intermediate group 
may be placed (1) colour, (2) fluorescence, and (3) triboluminescence ; 
it is in reference to these three phenomena that controversy and discus- 
sion have for the most part been carried on. 
Of the three phenomena quoted, that of triboluminescence is the one 
in which the clearest evidence is available, crystals of saccharin, for 
instance, showing the phenomenon very irregularly, highly purified 
crystals giving no ‘ flash’ whatever'; there is, therefore, good reason 
to adhere to the view put forward in 1903 that the flash of light which 
appears when the crystal is crushed is due to the sudden liberation of 
chemical energy stored in the crystal, for instance by the separation 
of a certain quantity of a labile form during rapid crystallisation. The 
case of fluorescence is less clear, as no investigation of a critical 
character on the influence of impurities in determining fluorescence 
appears to have been carried out. _It is therefore impossible at pre- 
sent to do more than to point out that fluorescence is often manifest 
only under special chemical conditions, e.g., in presence of an alkali, or 
' See Armstrong and Lowry, loc. cit., p. 261, 
