1324 
Scheikunde. — De Heer Ernst Cohen biedt eene mededeeling aan 
van den Heer Nilratan Dhar : -‘ün Catalijsis.” (^Part II.) 
(Mede aangeboden door den Heer P. van Romburgh). 
(2) Plioto-cJiemicnl, catalysis. 
Photo Chemical reaciioiis may ronglily be divided into two raain 
groiips: 
(1) Reversible reactions i.e. formation of an unetable system 
reverting to the initial state wheri light is removed. Here work is 
done against affmity. 
(2) Irreversible reactions i.e. acceleralion of the cliajige to a more 
stable state; these may be divided into two subgroups: 
(n) Com[)Iete reactions; 
{b) Pseudo-reversible reactions. 
Examples. 
Lnlnt 
(1) Reversible reactions: The Symbol A D typifies this, of which 
Ltark 
the polymerisation of anthracene is an example. 
WriGERT (Ann. Phys. 190-7 (IV) 21, 55) has shown that the in- 
flaence of light on the reversible reaction of CO -j- CI 2 CO Cl, is 
piirely catalytic and the position of equilibrium suffers no displacement. 
(2) (a) A case of a complete irreversible reaction is that of 
+ Cl, 2 HCl in light, HCl being stable. 
(6) As a psendo-reversible light-i-eaction, we may take the reduc- 
tion of feriic oxalate; 
F. (C,0 ^ 2 Fe C,0, + 2 CO, 
Li;ht 
to ferrons oxalate. In the dark, fer-rons oxalate solntion is again 
oxidized by oxygen of the air to ferric oxalate, so we have : 
Fe'" — » Fe" -j- x 
I.i ht 
Fe'" ^ Fe" + y 
Dark 
In their experiments on the photo-chemical corabination of hydrogen 
and chlorine, Roscoe and Bunsen (Ostwald’s Klassiker N“. 34) observe 
that the activity of the rays from a definite soiirce of light is diminished 
to a much greater extent in passing through a layer of the reacting 
gases than it is when the light is allowed to pass throngh an equi- 
valent layer of pure chlorine. Since the absorption due to admixed 
hydrogen is negligibly sniall, it is appai'ent that the photo-chemical 
change, which occurs in the mixed gases, is accompanied by the 
absorpiion of light energy. This transformation of light energy may 
