W. LeConte Stevens— Recent Progress in Optics. 283 
and various kinds of glass. Some of these after exposure give 
intense colors when heated, even after the lapse of days or 
weeks. That the vibratory motion corresponding to the 
absorption of luminous energy should maintain itself for so 
long a time as a mere physical process is highly improbable if 
not unparalleled. That it should become locked up, to be sub- 
sequently evoked by warming, certainly indicates the storing 
of chemical energy, just as the storage battery constitutes a 
chemical accumulator of electrical energy. Other indications 
that luminescence is as much a chemical as a physical phenome- 
non are found in the fact that the sudden solution of certain 
substances is accompanied by the manifestation of light, if they 
have been previously subjected to luminous radiation, but not 
otherwise ; that alteration of color is brought about by such 
exposure ; and that friction or crushing may cause momentary 
shining in such bodies as sugar. There is no conclusive direct 
evidence thus far that such luminescence as vanishes instantly 
upon the withdrawal of light is accompanied by chemical 
action. But Becquerel demonstrated long ago with his phos- 
phoroscope that there is a measurable duration of luminous 
effect when to the unaided eye the disappearance seems instan- 
taneous.* Wiedemann now shows that when this duration is 
considerable there is generally chemical change. Since dura- 
tion is only a relative term, it seems highly probable that even 
cases of instantaneous luminescence, commonly called fluo- 
rescence, are accompanied with chemical action on a very minute 
scale, and that all luminescence is therefore jointly physical 
and chemical in character. We have thus color evoked by the 
direct action of light, which disturbs the atomic equilibrium 
that existed before exposure, and the manifestation of such 
color continues only until the cessation of the chemical action 
thus brought into play. 
The influence of very low temperature upon luminescence 
and photographic action has been studied by Dewar.t The 
effect of light upon a photographic plate at the temperature of 
liquid air, —180° CO, is reduced to only a fifth of what it is at 
ordinary temperature; and at —200° the reduction is still 
greater, while all other kinds of chemical action cease. In like 
manner, at —80° calcium sulphide ceases to be luminescent ; 
but, if illuminated at this low temperature and then warmed, 
it gives out light. At the temperature of liquid air many 
substances manifest luminescence which ordinarily seem almost 
incapable of it; such are gelatine, ivory, and even pure water. 
A crystal of ammonium platinocyanide, on the other hand, 
when immersed in liquid air and illuminated by the electric 
* Becquerel, Comptes Rendus, xcvi, 121. 
+ Chemical News, Ixx, p. 252, 1894. 
