592 



SCIENCE PROGRESS 



current, having been of considerable theoretical and practical 

 importance. 



Azoxybenzene 

 C 6 H 5 -N-N-C 6 H 5 < 



o 

 I 



Azobenzene 

 C 6 H 5 -N = N-C 6 Hs 



Hydrazobenzene 

 C 6 H 5 -NH-NH-C 6 H 5 



+ . 



Benzidine 



NH,-C 8 H 4 -C 6 H,-NH a 



Nitrobenzene 

 C 6 H 5 -N0 2 



* 



Nitroso-benzene 



C 6 H 5 - NO 



\ 



Phenyl-hydroxylamine 



C 6 H 5 -NH-OH 



^ 



V 



A niline 

 C 6 Hs-NH 2 



Amidophenol 

 rH /NH 2 



We turn now to the second class — the electrolytic reduction 

 of difficultly reducible substances. This subject is a more recent 

 one. The first such reduction was carried out by Tafel 1 on 

 strychnine in 1898, to be followed in rapid succession by a large 

 number of other compounds, so that the method has become one 

 of great importance not only as a preparative method, but also 

 in view of the many physico-chemical problems involved. 



The advantages of the electro-reduction of organic substances 

 — more particularly of difficultly reducible substances — will be 

 considered briefly at the conclusion of the article. 



A short description of the method and apparatus employed 

 by Tafel for the quantitative investigation of these reductions 

 must now be given. 2 Experiments must be made on a small 

 scale to see firstly if the substance is capable of being electro- 

 lytically reduced at all, and secondly to determine the influence 

 on the time required for the reduction of variations of current 

 concentration, 3 current density, 4 etc. The time required is an 

 important factor, as many of the products obtained are easily 

 decomposable. Such preliminary experiments are made in the 

 closed apparatus shown in fig. 1. 



The lead beaker (a) serves as anode. The porous cell (b) 

 separates anode chamber from cathode chamber. The cathode 



1 Anna/en, 301 (1898), 285. 



2 Ber. d. deut. chem. Gesell. 33 (1900), 2209 ; Zeitsckr. f. phys. Cheat. 34 

 (1900), 187. 



3 Current concentration = current per litre of cathode liquid. 



4 Current density = current per square centimetre of cathode surface. 



