ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 145 
shall see presently. ‘It can hardly be necessary to give any reasons 
why platina does not show this effect under ordinary circumstances. 
It is then not sufficiently clean (617), and the gases are prevented 
from touching it, and suffering that degree of effect which is needful 
to commence their combination at common temperatures, and which 
they can only experience at its surface. In fact, the very power 
which causes the combination of oxygen and hydrogen is competent, 
under the usual casual exposure of platina, to condense extraneous 
matters upon its surface, which, soiling it, take away for the time 
its power of combining oxygen and hydrogen, by preventing their 
contact with it’ (598). Numerous facts are described which show 
that the only condition necessary is a perfectly clean surface, however 
produced. One way of doing this is of theoretical importance as 
showing that the action is not due to chemical combination of either 
oxygen or hydrogen with platinum. Platinum can be made active 
by making it eithér anode or cathode of an electrolytic cell (617). 
Chemical action is also excluded by the fact that nitrons oxide and 
hydrogen can be made to combine (572), and also by the fact that 
the ‘effect is produced by most, if not all, solid bodies’ in varying 
degree (618). It may be said that gas reactions only are contemplated 
in this theory, but statements in pars. 623, 625, and 657 indicate 
that Faraday had in mind the possibility of similar reactions in 
liquids. Indeed, the following statement is remarkably like an 
anticipation of Van Hoff’s theory of solutions, ‘An analogy in 
condition exists between the parts of a body in solution and those of 
a body in the vaporous or gaseous state.’ 
Denham (1910) brings powerful evidence in support of this 
theory of catalysis in inorganic heterogeneous systems as being 
due to the high surface concentration caused by molecular forces 
at the surface, and the high velocity of reaction as being due to 
the increased mass action thus brought into play. Bancroft (1915) 
gives a valuable discussion of contact catalysis. 
Denham, however, although he is of the opinion that some © 
enzyme effects may be accounted for in a similar way, hesitates tc 
apply the theory to all enzymes. We may, therefore, proceed to 
examine the evidence bearing on the question. But before doing 
this, we should remember that it is possible, as Hardy points out 
(see paper by Drury, 1914), that there may be, in addition to the 
simple mass effect, an increased chemical potential of the reacting 
molecules brought about by the actual process of condensation 
itself and the stresses which it involves. 
That the rate of reaction is controlled by adsorption is evident 
from various facts. The law expressing the way in which the 
concentration of enzyme and of substrate is related to the rate 
of change is not one that could be deduced from mass action. Up 
to a particular concentration of substrate, which varies with different 
enzymes, the law followed is that of a parabolic curve, similar to 
that of adsorption, although this, of course, does not in itself prove 
that we have to do with adsorption. Beyond this particular con- 
centration, the rate is either constant, whatever the concentration 
_of the substrate, or, owing to some secondary effects of the enzyme, 
is actually diminished. This fact is easily accounted for by the 
