Now with this method we can only get a very rougli delenninatioii 

 of the position of the active region, still it appears that the active 

 ravs are presumably situated in the violet and have a wavelen<>th 

 smallei' than ±430;tj[<. In order to sec whether in the beginning 

 of the ultraviolet active rays were still present, a small jacketed 

 tube was constructed from quartz, the alcoholic benzophenone solution 

 was put into the inner tube and in the jacket a cold saturated 

 solution of nitrosodimethylaniline ^) which absorbs all visible violet 

 and blue rays and transmits ultra-violet ones of 400 — 'iSOfiji. 



Neither in sun-light, nor in front of the quartz lamp did any 

 reduction set in ; from this we could conclude that the active rays 

 were not situated in the ultra violet, but in the visible spectrum 

 <430 and > 400 fifi. 



A fortunate incident now came to our aid when we were engaged 

 in determining the correct position of the active light. 



We had noticed that the ketones were converted with compara- 

 tively great rapidity into pinacones by means of the Heraus quartz- 

 mercury lamp. 



The mercury spectrum must thus contain a great quantity of the 

 chemically active rays. This spectrum exhibits a very intensive blue 

 Jine at 436 — 434 fxfi and two violet ones at 407,8 fift and 404.7 pf*')* 



Photo-tilter V completely removes the violet lines and leaves the 

 blue ones unchanged ; as this tilter in sunlight as well as in front 

 of the quartz lamp prevents all conversion of benzophenone as well 

 as of o-chlorobenzophenone, and as we have noticed that the ultra- 

 violet light of the lamp is inactive we may conclude that the active 

 light for the photochemical reduction of the aromatic ketones is 

 situated in the extreme end of the visible violet. 



The fact that the nature of the source of light has no principal 

 influence on the reduction process is shown from the subjoined table^ 

 in which are given the ratios of the quantities of pinacone that are 

 formed from diverse ketones when exposed either to sun-light or 

 mercury-light. 



The exposure to mercury-light was carried out by placing a solution 

 of 0.5 gram of ketone in 15 cc of ethyl alcohol at a distance of 

 5 cm from and parallel to the quartz lamp and exposing these for 

 10 hours ; hence, the quantity of light was approximately the same 

 for all ketones. 



I'he close agreement of these ratios also renders it probable that 



1) Compare Plotnikow etc. p. 19. 



2) Lehmann, Phys. Zeitschr. 11, 1039 (1910). 



57 

 Proceedings Royal Acad. Amslcidam. Vol. XVII. 



