1903.] Luminosity and its Correlation with Eadio- Activity. 261 



the solid, reversion to the stable form would take place very slowly. 

 Presumably, however, sudden crushing of the crystals would afford 

 opportunity for the change to take place and for the sudden liberation of 

 energy hence the momentary flash. 



In support of this view we have the fact : 



1. That the phenomenon of triboluminescence occurs irregularly in 

 the case of saccharin : being sometimes shown by one crop of crystals 

 and not by another, the highly purified substance Professor Pope 

 informs us being inactive. 



2. That whereas the phenomenon is observed in the cases of three 

 derivatives of camphor which are obviously capable of existing in 

 isodynamic forms,* viz., 



PTT ,CH.N0 2 rw /CH 2 PTT .CHBr 



< 6o Hl4< 6:NOH Hl4< co 



ir-Bromonitrocamphor. Camphoroxime. a-Bromocamphor. 



it is not seen in the case of camphor derivatives which, although 

 otherwise similar, cannot exist in isodynamic forms, e.g., a-chloro- 

 a-bromocamphor : 



CClBr 

 CsHl4< CO 



It is not, at present, necessary to assume that the phenomena are 

 limited to cases of isomeric change ; obviously, changes such as those 

 above considered, may be regarded broadly as dissociative or reversible 

 changes; and from this point of view, it is sufficient to regard 

 the phenomena as the outcome of a loss of potential consequent 

 on the passage from an unstable to a stable system. Thus, if 

 Tschugaeff's statement that a salt such as aniline hydrochloride is 

 triboluminescent be confirmed by observation with the highly purified 

 substance, it will be justifiable to assume that when it is crushed the 

 manifestation of luminosity may be due to the re-formation of salt 

 momentarily dissociated by pressure. 



From the point of view here advocated, it would be impossible to 

 construct a condenser from a pure dielectric ; and if the dielectric of a 

 charged condenser were suddenly smashed under suitable conditions, 

 it might exhibit the phenomenon of triboluminosity and perhaps radio- 

 activity. 



Fluorescence. 



It was originally suggested by one of us, in discussing the origin of 

 visible colour, that fluorescence is the " beginning of colour." Subse- 

 quently, Dr. J. T. Hewitt, in a paper on the relation between 



* Nitrocamphor does not flash when crushed ; perhaps this is because of its 

 texture. It may be that the effect will be observed at, lower temperatures. 



