1349 
acts as a positive catalyst i.e. more iodine is liberated in presence 
of light than in darkness. (See table p. 1348.) 
From a series of reaetions invesfigated in this connection it will 
be seen that bromine and iodine are rediiced 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 potassinm bromide or 
potassinm iodide, light acts a positive catalyst i.e. in presence ot 
light bromidions (Br') and iodidions (I') are oxidized to free bromine 
and iodine more rapidly. Thns this process is the reverse of the 
forrner and this action of light forms an instance of reversible 
catalysis. It is well known that platinnm black absorbs free oxygen 
and yields it again immediately to oxidizable snbstances, bnt it can 
also, by virtue of the same affinity, take o.xygen from snbstances 
capable of readily losing it. When platinnm black is dropped into 
hydrogen peroxide, the platinnm immediately fakes np oxygen, fórming 
probably an nnstable oxide, which decomposes at once. Platinnm 
black thns acts as a positive catalyst in processes involving the 
addition or removal of oxygen, according to circnmstances 
Sabatier (Ree. Gén. Chim. pnre et appl. 1914 17 , 185) in his 
lectnre 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 snbstances conversely in the presence of snbstances 
capable of giving np hydrogen, these metals take up hydrogen, 
which is immediately set free. 
Thns Sabatiér holds the view that the uristable intermediate com- 
pounds formed by catalysts can determine the reaction in either 
direction and the anthor attempts to explain reversible catalysis 
with the hypothesis of “nnstable intermediate compound” formation. 
As has already been observed, in the catalytic action of light on 
the oxidation of bromidion (Br') and iodidion (1') by persnlphates 
and the reduction of free bromine and iodine by varioiis redneing 
agents (e.g. oxalic acid, oxalates, hydroxylamine hydrochloride etc.) 
we have an instance of reversible catalysis; bnt 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 
reaetions. 
The mechanism of the reduction of mercnric chloride by, formic 
acid and formates has been investigated by Portes and Rüyssen 
(Compt. rend. 1876), 82 , 1504) Scala (Gazetta (1890j, 20 , 393), 
Lieben (Monatsh. (1893), 14 , 750) etc. Recently Findlay and Davies 
