186 PHOSPHORESCENCE. 



which, differs in certain properties, from that pro- 

 duced by a burning or luminous gas, as shown by 

 Arago j 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 is 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 

 light. 



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 



