186 PHOSPHORESCENCE. 
which differs in certain properties, from that pro- 
duced by a burning or luminous gas, as shown by 
Arago; the former gives indications of polariza- 
tion when viewed at an acute angle, whilst the 
latter never shows any traces of polarization at 
whatever angle it 1s viewed. 
All these experiments remain to be made with 
phosphorescent bodies. We are therefore very 
much in the dark as to the nature of phosphoric 
hight. 
We are so accustomed to associate light and 
heat, as in the flame of a candle, for instance, that 
it is difficult to bring the mind to reflect upon a 
fact beyond doubt, namely, that the one may be 
generated without the other. When a wire be- 
comes heated by an electric current, it often be- 
comes luminous also; but in other cases a body 
may become luminous without any sensible degree 
of heat. With combustible substances used in 
candles, lamps, etc., the greater the light the less 
the heat, and reciprocally. The flame of an oil 
lamp is very hot; that of a camphine lamp, which 
is far brighter, is very much cooler; whilst the 
flame of a spirit lamp, quite invisible in the sun- 
shine, produces a great amount of heat. Let us 
inquire how these things occur. 
We know that whenever any one of those spe- 
cific motions of matter which we term force (light, 
heat, electricity, etc.) ceases to manifest itself, it is 
