140 REPORTS ON THE STATE OF SCIENCE. 
Phosphorescence.—The storage of energy by phosphorescent bodies 
is apparently confined to the solid state, though in this category it is 
necessary to include both vitreous and crystalline solids. Its gradual 
liberation finds a close analogy in the slow discharge of the residual 
current from a Leyden jar, and both processes may be regarded as 
depending on the retarded electrolysis of a viscous medium. It is note- 
worthy that a great number of compounds which are not phosphorescent 
at ordinary temperatures become so at the temperature of liquid air,* and 
that conversely the after-glow of many substances which are phos- 
phorescent when cold, disappears when they are heated. In other 
cases the energy stored up in the crystal is liberated in the form of light 
only on warming (thermophosphorescence) or on crushing the crystal 
(triboluminescence or tribophosphorescence). In all these cases the limi- 
tation of the phenomenon to the solid state and the profound influence 
exerted by temperature changes are fully in accord with the view that 
the energy is stored by means of reversible chemical change, being 
released only when the rigidity of the material is sufficiently relaxed to 
permit of electrolysis and chemical change.* 
Fluorescence, like phosphorescence, is profoundly influenced by the 
state of aggregation of the material. It is of frequent occurrence in 
liquids, including solutions in concentrated sulphuric acid, dilute alkalis, 
water and organic solvents; it is very rare indeed amongst solids. 
Indeed, in spite of the existence of a few well-known exceptions (anthra- 
cene, uranium salts, etc.), itis probable that out of a total of some two thou- 
sand fluorescent substances, less than one per cent. are fluorescent in the 
solid state. There is therefore ample support to be found for the view that 
fluorescence is due to oscillatory chemical changes of the same general 
character as those which take place in solutions of nitrosobutane or 
enitrocamphor, since these are usually (but perhaps not invariably) 
arrested on passing from the liquid to the solid state. This view is also 
strongly supported by the close relationship which has been proved to 
exist between fluorescence and phosphorescence. Thus Wiedemann has 
shown * that eosin, fluorescein, aesculin, quinine sulphate, etc., show a 
weak after-glow when the solutions are rendered plastic by gelatine 
and a little glycerine, that they become definitely phosphorescent when 
the solution is ‘set’ with gelatine, and that a still stronger phos- 
phorescence is developed when glue is used. Observations of this kind 
indicate clearly that phosphorescence is essentially identical with 
fluorescence and differs from it only in the fact that the energy absorbed 
during insolation is liberated gradually instead of instantaneously : it is 
therefore legitimate to argue that the strong evidence, obtained in- 
dependently, that these two phenomena are due to reversible chemical 
change, becomes doubly strong when they are proved to be merely two 
varieties of the same type of activity. It may also be pointed out that 
recent attempts to correlate fluorescence with colour, by introducing the 
ideas of ‘ fluorophor ’ and ‘ fluorogen’ groups (compare ‘ chromophor ’ 
and ‘ chromogen ’) do not rest on any direct experimental basis (a sub- 
1 Dewar, Chem. News, 1894, 70, 252-253. 
? Wiedemann and Schmidt, Wied. Ann., 1895, 56, 201-244. 
e 
® The relaxation of molecular forces during crushing is well illustrated by Beilby’s 
work on the flow of metals during polishing and under the influence of mechanical 
forces generally. 
* Wied. Ann., 1888, 34, 446-163. 
q 
k 
