Mechanics of Luminosity. 383 



tious A in the violet must show the same order as the modifi- 

 cations B in the infra-red. 



The action of very small additions of chromium oxide &c. 

 on the brightness of the phosphorescent light may be ex- 

 plained by their simultaneous action in increasing the sensi- 

 tiveness. They take up the rays which bring about the modi- 

 fication, and carry them on to the molecules associated with 

 them. 



Other phosphorescent phenomena also may be connected 

 with such molecular transformations, such as the tribolumi- 

 nescence of the ketones prepared by Prof. Krafft*, e. g. 

 pentadecylparatolylketone &c, which the discoverer was so 

 good as to hand to me for investigation. When these sub- 

 stances are rubbed or broken, there is a remarkably powerful 

 evolution of light. 



In the triboluminescence of these bodies we have to do first 

 of all with phosphorescence, i. e. with a tolerably slow after- 

 luminosity. If a fragment of the substance be broken in the 

 dark, and moved quickly through the air, a line of light of 

 considerable length is seen. The ketones also shine brightly 

 when examined in the phosphoroscope. They may further 

 occur in two modifications — one stable at high temperatures, 

 the other at low temperatures. Under the influence of the heat 

 produced by friction or under the influence of light, as the 

 case may be, there is formed a very small quantity of the 

 modification unstable at low temperatures but stable at high 

 temperatures, which then, after the modifying influences have 

 ceased to act, changes again into the stable form with evolu- 

 tion of light. But if such a transformation produces lumi- 

 nosity, the triboluminescence of these bodies is a chemi- 

 luminescence, as is also the phosphorescence which is seen. 



The occurrence of luminosity is in this case very interest- 

 ing, since it is here a well-defined body which phosphoresces 

 and not a mixture as with the compounds of calcium sulphide. 



36. In connexion with these measurements I have en- 

 deavoured to determine the ratio of the energy radiated in the 

 phosphorescence of Balmain's luminous paint to the energy 

 received which produces the 'phosphorescence. Both energies 

 may be measured by the corresponding brightnesses if we 

 take account of the different physiological action of the colours. 

 In consequence of this very similar colour-impression of the 

 exciting and excited light, they can be compared with the 

 blue comparison-star of Zollner's photometer, produced by 

 the same method. The measurements, which were carried 

 put with the arrangement described in § 35, are divided into 

 * Chew. Ber. xxi, p. 2265 (1888) ; Beibl. xiii. p. 19. 



