no I 



2. By tills liappy choice of catalyst, Zf.linsky sliowcl that beii'/.ciio 

 and cyclo-liexaiie, at ± 200° and at the ordinary pressure, are 

 related according to the mobile equilibriiini : 



which, above 300° is shifted entirely in the direction of benzene 

 and hydrogen, and below 170° wholly in the direction cyclo-hexane. 



From this follows that at 300"^ cyclo-hexane is .5W/7(9;^satu rated, 

 that six carbon atoms which are linked together are no longer 

 tctra\?\Q\\i but ^r/valent in regard to the hydrogen. It is true the benzene 

 at the ordinary' temperature is unsaturated for hydrogen, but if \ve 

 take into consideration that the decomposition of cyclo-hexane starts 

 at 170° that unsaturation cannot be consideralde. 



The knowledge of this equilibrium enables us to understand why 

 the aromatic compounds are reduced with difficulty and why they 

 behave somewhat like saturated hydrocarbons ; without a catalyst 

 like palladium black we could not have elucidated the matter experi- 

 mentally. 



Nickel would have been less suitable because below 100° it is 

 but little hydrogen-active and also because not far above 300^ it 

 becomes too much carbon-active. 



One of us ^) has pointed ont that the great general stability of 

 the aromatic compounds, to which belongs this little-pronounced 

 unsaturation, can be deduced from the "Werner principle of affinity 

 distributed evenly in space" when we take into consideration the 

 extraordinary favourable steric distribution of the atoms. This stability 

 can differ very little from that of the hydroaromatic compounds 

 because the distribution in space is more favourable. 



He also considered the question why, in substiiutions in the 

 benzene nucleus the additive products to be expected have as yet 

 not been isolated. These ought to be derivatives of di- or tetrahydro- 

 benzene. 



If we consider these hydrocarbons themselves we notice that here 

 is found an unequal distribution of the unsaturation as well as of 

 the atoms in space ; in the presence of suitable catalysts they ought 

 to be converted more readily into benzene or cyclo-hexane than the 

 two latter into each other. 



Guided by these considerations we have studied in the tirst place 

 the behaviour of cyclo-hexene. As stated above, this hydrocarbon 



1) Proc. March 1912. 



