1122 
acts as a positive catalyst i.e. more iodine is liberated in presence 
of light than in darkness. (See table p. 1121.) 
From a series of reactions investigated in this connection it will 
be seen that bromine and iodine are reduced and pass into ionic 
state i.e. beeome bromidion (Br) and iodidion (I’) respectively 
more rapidly in light than in darkness. Moreover, it has been seen 
that in the action of sodium persulphate on potassium bromide or 
potassium iodide, light acts as a positive catalyst i.e. in presence of 
light bromidions (Br’) and iodidions (I’) are oxidized to free bromine 
and íodine more rapidly. Thus this process is the reverse of the 
former and this action of light forms an instance of reversible 
catalysis. It is well known that platinum black absorbs free oxygen 
and yields it again immediately to oxidizable substances, but it can 
also, by virtue of the same affinity, take oxygen from substances 
capable of readily losing it. When platinum black is dropped into 
hydrogen peroxide, the platinum immediately takes up oxygen, probably 
forming an unstable oxide, which decomposes at once. Platinum 
black thus acts as a positive catalyst in processes involving the 
addition or removal of oxygen, according to circumstances. 
SABATIER (Rec. Gén. Chim. pure et appl. 1914 17, 185) in his 
lecture on catalysis states that finely divided copper or nickel placed 
in an atmosphere of hydrogen, fix the gas temporarily and pass it 
on to other substances conversely in the presence of substances 
capable of giving up hydrogen, these metals take up hydrogen, 
which is immediately set free. 
Thus SaBATIER holds the view that the unstable intermediate com- 
pounds formed by catalysts can determine the reaction in either 
direction and the author attempts to explain reversible catalysis 
by the hypothesis of “unstable intermediate compound” formation. 
As has already been observed, in the catalytic action of light on 
the oxidation of bromidion (Br’) and iodidion (I’) by persulphates 
and the reduction of free bromine and iodine by various reducing 
agents (e.g. oxalic acid, oxalates, hydroxylamine hydrochloride ete.) 
we have an instance of reversible catalysis; but SABATIER’s explana- 
tion cannot be extended to these actions, since light is not a material 
substance as used in the ordinary sense. 
The study of the influence of light was extended to various other 
reactions. 
The mechanism of the reduction of mercuric chloride by formic 
acid and formates has been investigated by Portes and RuyssENn 
(Compt. rend. 1876), 82, 1504) Scana (Gazetta (1890), 20, 393), 
LirBeN (Monatsh. (1893), 14, 750) ete. Recently Finpray and Davins 
