852 



II. The photo-reaction : ketone -\- ]iyclrogen\=zj)inacone. 



In order to obtain a relatively-quantitalive resuU wc could make 

 nse of a constant source of light and allow this to act on the differ- 

 ent solutions under the same conditions; for this purpose a small 

 7 cm. quartz-mercury lamp was at our disposal. Yet we ha\e em- 

 ployed this method but rarely, for instance in continuous dark 

 weather, because on account of the unequal distribution of the liglit, 

 at most two little tubes could be placed in front of the lamp in 

 such a manner that it might be assumed that they existed under 

 equal conditions. 



When it had been ascertained by us that the reduction took place 

 quite as well in ordinary white glass as in quartz, from which it 

 appeared that a very large part of the aclinic rays was situated in 

 the visible spectrum, the experiments intended for comparison were 

 carried out as follows: 



A number of equally wide tubes of the same kind of glass and 

 having walls of approximately the same thickness were filled with 

 the same quantity of solution, and all placed at the same distance 

 in front of a white screen, which was placed close to a large labora- 

 tory window. 



In this manner it was attained that the quantity of light that 

 fell in the same time on each solution was practically the same, 

 and perfectly comparable results were thus obtained. 



It speaks for itself that even then only the figures of a same 

 experimental series were mutually comparable. ^). 



A photo-reaction is distinguislied from a reaction in the dark by 

 two points. 



It is of a lower and frequently of the order in regard to the 

 substance which is being activated and the temperature coefficient 

 is small. ^) 



As we found that the active light was situated in the visible 

 spectrum and that the alcohols do not absorb visible rays, the 

 ketones are in this reaction the sensitive substances, and so we could 

 expect that the quantities of pinacone would be independent of the 



^) Also comp. 0. Gross Z. phys. Cli. 37, 168 (1901) and E. Goldberg Z phys. 

 Gh. 41, 1 (1902). 



~) The first property is due to the activation occurring in the outer layer; from 

 the sensitive substance only a limited number of molecules can be raised by the 

 same quantity of light to the same degree of activity; even at a moderate dilution, 

 the reaction becomes, on this 'account, independent of the concentration óf the 

 sensitive substance and therefore of the order. This applies to slowly progress- 

 ing reactions where the sensitive substance can be rapidly supplied by diffusion 

 from the dark interior to the fight zone. 



