122 KECENT PROGRESS IN OPTICS. 



siuni, after being rendered luminous by action of cathode rays, are 

 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 phe- 

 nomenon is sufficient to warrant the use of a special name, thernio- 

 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 zinc and alkaline earth groups, various com- 

 pounds with aluminium, 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 in, to be subsequently evoked by warm- 

 ing, 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 

 phenomenon 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 phosphoroscope that there is a measurable duration of 

 luminous effect when to the unaided eye the disappearance seems 

 instantaneous (Becquerel, Oomptes rendus 96-121). Wiedemann now 

 shows that when this duration is considerable there is generally 

 chemical change. Since duration is only a relative term it seems 

 highly probable that even cases of instantaneous luminescence, com- 

 monly called fluorescence, are accompanied with chemical action on a 

 very minute scale, and that all luminescence is therefore jointly physi- 

 cal 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 pho- 

 tographic action has been studied by Dewar (Chemical News, LXX, 

 p. 252, 1894). The effect of light upon a photographic plate at the 

 temperature of liquid air —180° C. 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 



