282 W. LeConte Stevens—Lecent Progress in Optics. 
energy, quite analogous to the radiation of heat from a surface 
on which sunlight is shining, has been generally accepted. 
But whether the phenomena of fluorescence and phospho- 
rescence are in general physical or chemical has for the most 
part remained unknown or at least very uncertain. I. Wiede- 
mann, who suggested the term luminescence to inelude 
all such phenomena, has within the present year* pub- 
lished in conjunction with Schmidt a part of the outcome 
of an extended investigation undertaken with a view of 
clearing up these uncertainties. He has shown that it is 
often possible to distinguish between cases in which the 
emission of light springs from physical processes and those in 
which it is due to chemical action, or at least invariably accom- 
panied by this. We have here, as in photography, a trans- 
formation of radiant into chemical energy, to which is super- 
added the re-transformation of chemical into radiant energy of 
longer period, and this either at the same time or long after 
the action of the exciting rays. Indeed between this process 
and that of photography in colors, the analogy is quite striking. 
What has generally been called phosphorescence is well known 
to be the effect of oxidation in the case of phosphorus itself, 
and in that of decaying wood or other organic matter which 
under certain conditions shines in the dark. Wiedemann has 
shown that the shining of Balmain’s luminous paint, and gen- 
erally of the sulphides of the alkaline earths, is accompanied 
with chemical action. A long period of luminosity after the 
removal of the source renders highly probable the existence of 
what he now calls chemi-luminescence. A large number of 
substances, both inorganic and organic, have been examined 
both by direct action of light and by the action of kathode 
rays in a controllable vacuum tube through which sparks from 
a powerful electrical influence machine were passed. Careful 
examination with appropriate reagents before and after 
exposure was sufficient to determine whether any chemical 
change had been produced. Thus the neutral chlorides of 
sodium and potassium, after being rendered luminous by action 
of kathode rays, were thereby reduced to the condition of 
subchloride so as to give a distinctly alkaline reaction. Many 
substances, moreover, which manifest no luminescence at 
ordinary temperatures after exposure, or which do so for only 
a short time, become distinctly luminescent when warmed. 
This striking phenomenon is sufficient to warrant the use of a 
special name, thermo-luminescence Among such substances 
may be named the well-known sulphides of the alkaline earths, 
the haloid salts of the alkali metals, a series of salts of the zine 
and alkaline earth groups, various compounds with aluminum, 
* Wiedemann’s Annalen, April, 1895, p. 604. 
