447 



legs of the manometer being verified by a minor behind it, on 

 whicii iiorizoiital lines are drawn at distances of 1 mm. When the 

 apparatus has been |)roperly cleaned and filled with distilled n)ercnry, 

 an accnracy of adjustment can ()e attained of 1 or 2 hundredtiis, 

 which as well as the influence of the temperature lies within the limits 

 of the error of observation. The calibration error of the apparatus 

 was so small that it could be neglected. 



After having been weighted with a copper ring, the reaction 

 vessel is placed on a glass table, which itself rests on the bottom 

 plate of llie thei'inostat. The table can be put in a horizontal position 

 by means of three adjusting screws, through which the thin liquid 

 layer entirely covers the bottom surface of the reaction vessel. 

 Aflei- being lit the incandescent body of the lamp is always 

 placed in a horizontal position; the lamp burns at a terminal voltage 

 of 110 Volts and a series resistance of about 20 Ö constant at 2,7 

 Am|)./40 Volt. Lamp and reaction vessel are always at the same 

 distance from each other; in my experiments the distance from the 

 bottom side of the reaction vessel to the window was 20 mm., and 

 from the upper side of the lamp to the window 25 mm.; taking 

 into account the thickness of the glass, the mutual distance from 

 lamp to object was about 50 mm. 



The measurements. 



a. The preparations. They were prepared for the grealei- part by 

 myself or under my supervision, and purified as carefully as possible. 

 As the way of preparing is known for all of them, we may refer 

 for this to the records of the literature published on this subject. When 

 it was possible, at least two preparations of difteient origin were 

 examined, or the prepaiation was again recryslallized or distilled 

 after the measurement; the values found wei'e not considered as 

 definitive until they were perfectly (constant and reproducible; save 

 for a single exception this was always the case. 



b. As solvent, resp. liquid that is to be oxidized, was used absolute 

 ethylalcohol, not because its iteing absolute was quite indispensable 

 for the success of the reaction — for water is formed during the 

 reaction — but in order to start always from a solution of 

 constant proporties. In my earlier investigations I had come to the 

 conclusion that water would be a strong anii-catalyst, at least for 

 the photo-chemical reduction '). At the time 1 did not yet know 

 the photo-catalytic alcohol oxidation by molecular oxygen, nor that 



1) Cohen Rec. 39, 244 (1920). 



