138 REPORTS ON THE STATE OF SCIENCE, 
after dissolution in concentrated sulphuric acid*; these conditions are 
often precisely those which determine the occurrence of oscillatory 
isomeric change: it is therefore permissible to adhere provisionally to 
the theory that fluorescence is dependent on change of structure until 
definite evidence to the contrary is forthcoming. 
The phenomenon of colour is on an altogether different footing, since 
exliaustive experiments have frequently been made in the hope of 
removing colour by careful purification of the material. These experi- 
ments have led to results of two types: in the case of some compounds, 
such as picric acid and p-nitrophenol, the colour of the crude material 
has actually been removed by purification or by crystallising out under 
special conditions; in the case of other compounds, such as quinone and 
o-nitrophenol, no indication whatever has been obtained which would 
even suggest the possibility of bleaching the compound. It is therefore 
impossible to resist the conclusion that colour, in the case of the latter 
group of substances, is an essentially physical phenomenon of the same 
general type as the closely related phenomenon of refraction and that 
it does not depend on any fluctuation of structure to which the ordinary 
laws of chemical change can be applied. The case of substances which 
can be rendered colourless by suitable methods of purification requires 
further consideration : it might be suggested that an impurity is able to 
develop colour throughout the material in much the same way as that 
in which the phosphorescence of calcium sulphide is developed by the 
combined action of a trace of bismuth and a trace of a sodium salt,” 
but the view that has been universally adopted is that the mass of the 
material is merely stained by a trace of some highly coloured dye-stuff 
and is itself essentially colourless ; it is indeed frequently an easy matter 
for an experienced eye to detect the staining-process by the feeble and 
variable development of colour which it produces, as contrasted with the 
intense and uniform colour of materials which are in themselves absorp- 
tive. If this view be adopted, it is clear that in these cases also the 
colour is due, not to change of structure in the original material, but to 
some characteristic absorptive-power inherent in the (fixed) structure of 
the staining material. 
Note in reference to ‘ stained materials.’—In considering the pro- 
perties of stained materials it is important to distinguish three cases. 
Sometimes the stain is produced exclusively during the preparation of 
the substance, and may be permanently removed by distillation, by 
contact with animal charcoal or by similar straightforward methods. 
There are, however, many substances which cannot be purified in this 
easy way, since they undergo change continually in contact with the 
air and give rise persistently to coloured oxidation-products (e.g., indigo- 
blue from indigo-white); in such instances purification can only be 
effected under special conditions, and a material partly bleached by 
purification may easily revert to its original colour if the essential pre- 
cautions are in any way relaxed. The third case, in which the 
substance undergoes reversible chemical change, is more puzzling than 
either of the others, and may easily give rise to erroneous conclusions. 
Fortunately, the conditions governing such cases are now well under- 
A : Few and Tervet, ‘Oxonium Salts of Fluorane,’ Trans. Chem. Soc., 1902, 
, 664, 
2 Ne Visser, Rec. Trav. Chim., 1901, 20,'435; 1903, 22, 138, 
