516 Mr. G. T. Bcilby. PkospJwrescence caused l>y [Feb. 9, 



41. When the stimulus of the rays is removed, electrolytic 

 dissociation will cease, but a certain number of ions continue to 

 find their way to each other and to combine, producing secondary 

 phosphorescence. This state may last only for a moment or it may 

 continue for days (paragraph 28). The insulating power of the 

 molecules depends on their mobility, and this in turn depends on 

 temperature. For every rise in temperature a new equilibrium point 

 must be established, and a further combination of ions with a fresh 

 outburst of luminescence will occur. At whatever stage a rise of 

 temperature takes place, the effect must be the same, whether it be at 

 the first stage while dissociation is active, at the second stage while 

 recombination is taking place, or at the third stage when equilibrium 

 at the lower temperature had been reached and luminescence had 

 entirely ceased. 



42. From this point of view it becomes possible to explain the 

 comparatively conflicting observations on the behaviour of the same 

 substance in the cathode rays or in the radium rays. To take only 

 one example of this: In 1881* Sir W. Crookes stated that calcite 

 showed the residual glow longer than any substance he had 

 experimented with. "After the current has been turned off the 

 crystals shine in the dark with a yellow light for more than a minute." 

 This short period of secondary phosphorescence as compared with the 

 four days recorded in paragraph 28 becomes perfectly intelligible when 

 the temperature is taken into account. No mention of the actual 

 temperature is made in Crookes' observation, but it may fairly be 

 assumed that the temperature of a substance exposed to a powerful 

 cathode discharge would be very much raised. Even a temperature of 

 100 would break down the insulation of the calcite molecules to such 

 an extent that only a very brief secondary phosphorescence could occur. 



43. The effects of low temperature in developing and increasing 

 secondary phosphorescence, as shown by Crookes and Dewar, fall 

 naturally in line with the dissociation hypothesis. By the increased 

 insulating power of the molecules at the lower temperature the capacity 

 of a substance to store dissociated ions may be increased to an enormous 

 extent, so that the break-down of insulation by a rapid return to the 

 ordinary temperature will set free a great store of energy and produce 

 an outburst of phosphorescence. It is evident that substances which 

 have too little insulating power at ordinary temperatures to enable 

 them to produce secondary phosphorescence may at the lower tempera- 

 tures develop this power to a high degree. Even the much feebler 

 stimulus of the light waves may thus be enabled to record dissociation 

 effects which have altogether escaped notice at ordinary temperatures. 



44. The conclusions drawn in the foregoing papers may be briefly 

 summarised under the following divisions : 



* ' Chem. News,' 1881, p. 237. 



