87 
invisible parts of the spectrum, which furnished an addition 
to those employed, viz., the thermometer, silver compounds, 
etc. ; these sulphides may be compared to chords which have 
the power of absorbing vibrations whose wave-lengths are 
too small to affect the organs of hearing — too shrill to be 
heard — and of changing them into vibrations of greater 
wave-lengths and emitting them, so that they may be dis- 
tinctly heard. 
When these sulphides are held before some source of 
light, the sun’s rays for instance, and then placed in a 
dark room, they gradually, after many hours, lose their 
phosphorescence — if, however, they be then heated a few 
degrees above the temperature at which they have remained, 
say from 60° to 80° Fah., they again become faintly 
luminous, and if kept for some time at that temperature 
they gradually lose their phosphorescence again, if allowed 
then to cool in the dark to 60° Fah., and again heated to 
the same temperature they show no phosphorescence, but if 
heated to a still higher degree of temperature, say 100° Fah., 
they again become luminous, and so on ; this power may be 
regained and lost any number of times by placing them for a 
few seconds before any bright source of light and then re- 
moving them to a dark room — an electric current from a 
Ruhmcoff’s coil passed through vacuum tubes containing 
these sulphides also developes their different colours. 
The interesting point with respect to the sulphides which 
I show here this evening, is that they have been prepared 
by a special mode of manipulation by M. Andrd, and that 
the luminosity or phosphorescence culpable of being pro- 
duced by them is greater than that from any which hitherto 
have been prepared. 
The following gives the compositions of the different 
sulphides which show the differently- coloured phospho- 
rescences : — 
Green is composed of sulphide of calcium. 
