1903.] Luminosity and its Correlation with Radio- Activity. 259 



flash of luminosity was seen to cover the screen over the place where 

 the crystal was broken. 



(6) A screen of hexagonal blende was taken and experiments (4) and 

 (5) were repeated with it. The results were negative, except that I 

 fancied there might be a trace of light when the crystal was broken 

 close to the sensitive surface of the screen." 



Although it may be argued from experiment (5) that saccharin 

 is radio-active when crushed, we have been unable hitherto to detect 

 any effect on the electrometer. Through the kindness of Dr. Lapworth 

 we have been able also to examine the menthyl phenylformylacetato 

 recently described by him, a substance capable of existing in isodynamic 

 forms, which is not only intensely triboluminescent but gives out 

 most brilliant flashes of light as it separates from solution, its behaviour 

 in this respect being, if possible, even more striking than that of 

 radium in face of a screen of hexagonal blende to which Sir William 

 Crookes has directed attention. As a small quantity of this substance 

 was crystallising out from petroleum spirit, no fewer than ninety-two 

 flashes were observed in broad daylight in the course of ten minutes. 

 Even this substance, however, has not hitherto given positive results 

 when tested by the electrometer. 



Tribohiminescence. 



We have now to consider the nature of the change involved in the 

 production of the luminous flash, in order that it may be clear why, 

 in our opinion, if radio-activity were observed in such a case, it would 

 have been as the concomitant to chemical change. 



There is distinct evidence that the phenomena of triboluminescence 

 may be correlated with the occurrence of the form of isomeric change 

 which attends the passage of a compound into the isodynamic* form 

 of lower potential. Tschugaeff, who has examined over 500 inorganic 

 and organic compounds,! found that about 25 per cent, of the latter 

 gave a more or less intense flash when crushed ; of these a consider- 

 able proportion appear to be such as could exist in isodynamic forms. 

 Only about 5 per cent, of the inorganic substances flashed. 



To take the case of saccharin, the two conceivable forms are : 



* This term lias been introduced by one of us as being more appropriate than 

 the term tautomeric to connote substances which are apparently almost equi- 

 potential as they so closely simulate one another in behaviour under ordinary 

 circumstances and are easily interconvertible. 



t ' Ber. Deut. Chem. Oes.,' 1901, vol. 34, p. 1820. 



