458 



Mr. J. B. B. Burke on the 



has been reached by compression or by rarefaction, in the latter 

 case the phosphorescence is produced by heating the substance. 

 But in both cases a certain amount of energy is stored up, 

 in the substanco, from the spark. 



Thermo-luminescence is another instance of the part played 

 by slight impurities in the production of phosphorescence. 

 It is found to occur chiefly in substances which van 't HofT 

 has styled " solid solutions/' formed when two salts, one greatly 

 in excess of the other, are precipitated from the same solution. 

 The connexion between them seems greater than that of an 

 ordinary mixture. 



It is conceivable, as we have pointed out, that in a gas a 

 somewhat similar semi-chemical mixture may be brought 

 about under certain conditions, such, for instance, as the 

 passage of electricity through it; and this view of the matter 

 is strengthened by the fact that the passage of electricity 

 through a gas is most easily effected at the pressures at which 

 the phosphorescence appears. 



(16) A strong beam of sunlight w T hen passed through a gas 

 does not communicate to it the energy requisite for the glow 

 (see Newall, loc. cit.) ; Wiedemann has shown that thermo- 

 luminescence excited by a spark is not due to ultra-violet 

 light ; since if a thin plate of quartz be interposed between 

 the spark and the therm o-luminescent substance the effect 

 does not take place. He has shown that the thermo- 

 luminescence is excited by a radiation from the spark which 

 he has called "Kntladungstralden" to which solid bodies 

 are opaque. They are not much absorbed by gases except 

 carbon dioxide, but they produce ionization in gases through 

 which they pass (J. J. Thomson, ' Camb. Phih Soc. Proc' 

 1899). J.J. Thomson has also shown that this radiation does 

 not proceed from the cathode or dark space, but either from 

 the positive column of the discharge or. from the negative 

 glow. He has suggested that this radiation is analogous to 

 Eonto-en rays. The Rontgen rays being the thin pulses 

 produced when charged particles are stopped, and the 

 " Entladungstr allien >} the much longer pulses produced when 

 charged particles are put into motion. 



There is, however, apart from the "Entlachmgstralden" a 

 radiation of particles in the neighbourhood of the anode, as in 

 the case of the cathode (see J. J. Thomson, ' Recent Re- 

 searches,' pp. 175-177). J. J. Thomson considers that the 

 difference between the emission of particles from the spark 

 and that of cathode-rays is that in the former, the pressure 

 being greater, the molecules communicate their momentum to 

 the surrounding gas instead of retaining it until they strike 

 against the walls of the discharge-tube. 



