6 
DR. W. A. BONE AND MR, R. V. WHEELER ON THE COMBINATION OF 
without any permanent oxidation or reduction of the surface and ordinary chemical 
processes which do involve such permanent changes. The experiments with reducible 
oxides, and more particularly those with copper oxide, show that the two kinds of 
change are quite distinct in character. It is usually not difficult in such cases to find 
a range of temperature within which the purely catalytic process goes on uncompli¬ 
cated by the other kind of change, and it is with the mechanism of the former that 
we are alone concerned in this paper. 
Although the research has revealed important minor differences between the action 
of the various surfaces examined, the results, as a whole, leave no room for doubt but 
that the catalytic process depends primarily on a condensation of one or other (and, 
in some cases, possibly both) of the reacting gases on the heated surface. Any purely • 
chemical explanation of the phenomena, such, for instance, as the theory of a rapidly 
alternating series of oxidations and reductions of the catalysing material, seems quite 
inadmissible. Equally certain is it that the rate at which the gases combine over a 
given surface is governed neither by the “ order ’ of the reaction, nor by diffusion 
factors simply. 
The catalysing power of a new surface usually increases up to a steady maximum 
when successive charges of electrolytic gas are circulated over it. After the attain¬ 
ment of this steady state, the rate of steam formation is always directly proportional 
to the pressure, provided that the gases are present in their combining ratios, and 
that the product is rapidly removed from the sphere of action. In other words, 
the velocity curve for electrolytic gas is always of a “ monomolecular ” type, after the 
surface has acquired its “ normal ” degree of activity. This applies to all the surfaces 
examined. 
When one or other of the reacting gases is present in excess, the rate of combination 
is nearly proportional to the partial pressure of the hydrogen, which thus becomes the 
determining factor in any given experiment. The case of copper oxide, and to a 
certain extent that also of silver, proved to be exceptional in this respect; these cases 
will be fully discussed later, The whole evidence of the research shows that, except 
in the case of copper oxide, the hydrogen plays an all important role in the catalytic 
process, being rendered “ active ” by association with the surface. In the majority of 
cases the hydrogen is merely “ occluded,” but in the case of silver there is evidence of 
chemical combination, that is to say, of the formation of an unstable hydride at the 
surface. In conformity with this view, the catalysing power of many of the surfaces 
examined (e.g., porcelain, magnesia, silver, gold, platinum, and nickel) could be 
stimulated, often in a very high degree, by previous exposure to hydrogen at 
moderately high temperatures. 
In this connection reference must be made to the recent researches of MM. Paul 
Sabatier and Jean Senderens, # which have drawn attention to the remarkable 
* ‘ Comptes Rendus,’ (1899), 128, 1173; (1900), 130, 1559, 1761; (1901), 132, 210, 1254; (1901), 133, 
321; (1902), 134, 514, 689, 1127; (1902), 135, 87, 225; (1903), 137, 301. 
