1147 



Broadly speaking all the solids that absorb light sufficiently and 

 are good conductors of electricity, become photo-electrical. 



On the other hand liquid substances, according to Halt.wachs are 

 none of them very active. He found only two liquids distinct in 

 their positive reaction to radiation, viz. anilin and formic acid. 



Nor does the literature make mention of liqnids as being distinctly 

 photo-electric. 



Both the liqnids just mentioned belong to the odorous snbstances 

 and we succeeded in detecting among this category of materials 

 several that were photo-electric in the liquid state, amongst others 

 the liqnid terms of the anilin series; toluidin and xylidin. Also 

 guaiacol, cressol, engenol, anethol, etc. The sensitivity of anethol 

 e.g. comes near to that of the most active metals. 



Aqneons solutions of all the above solid and liquid substances 

 generally proved not to be photo-electrical. An excej^tion was formed 

 by the solutions of anilin dyes, mentioned before. 



This was supposed *) to be the consequence of the formation of a 

 pellicle on the surface, constituting a very thin layer of solid matter. 

 Photoelectricity rises in proportion to the thickness of the layer lo 

 a certain optimum. This superficial pellicle is coimected with the 

 colloidal slate of the dye-solution. Oxidation does not come into play 

 here. Inorganic colloids such as arsenic and antinionium-trisulphide 

 act likewise*). 



However, Hauavachs himself already records that the photoelectric 

 sensitivity is not always associated with, nor runs parallel with the 

 formation of this pellicle. 



In our investigation of odorous substances we found out that a 

 solution of any substance, whether solid or liquid, is photo-electrical 

 only when the three following conditions are satisfied: 



1. The dissolved substance must be photo-electric. 



2. The solution must largely absorb ultraviolet light. 



3. The solution must be colloidal. 



In these experiments with liquid substances and with solutions 

 we used filterpaper saturated with the liquid and suspended on a 

 stand that had been insulated by amber. 



Filterpaper when having been kept carefully shut off from all 

 contact, shows only little sensitiveness for photo-electricity. Howexer, 

 through adsorption of odorous substances it soon becomes sensitive 



1) RoHDE (Ann. der Physik. 19, p. 935—959. 1906). 



«) Plogmeier, Deutsche Phys. Gesellschaft, Verb. 11, p. 382—396 (1909). 



