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Chemistry. “On Catalysis’. By NmraTHAN Duar. (Communicated 
by Prof. Ernst Coney). Part II. 
(Communicated in the meeting of January 29, 1916), 
(2) Photo-chemical catalysis. 
Photo chemical reactions may roughly be divided into two main 
groups : 
(1) Reversible reactions i.e. formation of an unstable system 
reverting to the initial state when light is removed. Here work is 
done against affinity. 
(2) Irreversible reactions i.e. acceleration of the change to a more 
stable state; these may be divided into two subgroups: 
(a) Complete reactions ; 
(b) Pseudo-reversible reactions. 
Examples. 
Light 
(1) Reversible reactions: The symbol AD typifies this, of which 
Dark 
the polymerisation of anthracene is an example. 
Wricert (Ann. Phys. 1907 (IV) 24, 55) has shown that the in- 
fluence of light on the reversible reaction of CO + Cl, 2 CO Cl, is 
purely catalytic and the position of equilibrium suffers no displacement. 
(2) (a) A case of a complete irreversible reaction is that of 
H, + Cl, ~ 2 HCI in light, HCI being stable. 
(6) As a pseudo-reversible light-reaction, we may take the reduc- 
tion of ferric oxalate : | 
F, (C,0,); — 2 Fe C,O, + 2 CO, 
Light 
to ferrous oxalate. In the dark, ferrous oxalate solution is again 
oxidized by oxygen of the air to ferric oxalate, so we have: 
Fel” Fe! + x 
Light 
Fe” Fe” + y 
Dark 
In their experiments on the photo-chemical combination of hydrogen 
and chlorine, Roscor and Bunsen (Osrwarp’s Klassiker N°. 34) observe 
that the activity of the rays from a definite source 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 through an equi- 
valent layer of pure chlorine. Since the absorption due to admixed 
hydrogen is negligibly small, it is apparent that the photo-chemical 
change, which occurs in the mixed gases, is accompanied by the 
absorption of light energy. This transformation of light energy may 
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