1450 
mediate product, cannot serve as an explanation in the reduction of 
nitro compounds, aldehydes, and similar substances. *) 
Besides for the knowledge of the phenomena of reduction and 
undoubtedly also for the knowledge of the phenomena of oxidation, 
more in particular in organic chemistry, this phenomenon of co- 
action is of importance in that it is in close relation with catalysis, 
and can easily pass into it. 
The possibility that three substances which in couples do not 
react on each other or in a very small degree, when brought together, 
react all three on each other follows from the theory of the mutual 
activation; the catalysis can then be considered as a special case of 
the coaction, viz. in which two of the three possible reactions 
between the three components do not take place or only in a very 
small degree. At the same time this theory explains how such 
coactions can be realized, and can be changed into a catalytic 
action in some cases. If two compounds A and B are placed over 
against a third one, C, and C is built up of two parts in such a 
way that one part can be attacked by A, the other by B, a coaction 
may be expected between A, B and C. 
It was now the intention to examine a number of systems, con- 
sisting of metal, acid or alkali and reducible compound, first of all 
qualitatively with regard to the influence that the reducible compound 
has on the velocity of solution of the metal, either with generation 
of hydrogen or without. 
It now appeared that not only nitrobenzene, but also an aldehyde 
as benzaldehyde exert a surprisingly accelerating influence on the 
velocity of solution. It is often, especially in the case of nitrobenzene, 
so great that the metal dissolves from five hundred to a thousand 
times more rapidly when nitrobenzene is present. Besides it appears, 
what was also to be expected, that though nitrobenzene has almost 
always an accelerating action, benzaldehyde is more selective. 
(See table). 
It is further remarkable that the greatest acceleration was observed 
in those cases in which in the presence of the reducible compound 
solution of the metal took place, but no generation of hydrogen, 
which is probably related to the fact that the contact between 
metal and reducible compound is prevented by the forming hydrogen, 
which would also point to the probability that the reduction does 
1) SKRABAL, Die induzierten Reaktionen. Samm. Chem. und chem. techn. Vorträge. 
XIII. Bnd. 10. Heft. 1908. 
