120 
ME. P. V. SEVAN ON THE COMBINATION OF HYDROGEN 
assume that the normal course is of the ordinary mono-, hi-, or polymolecular type. 
By varying the conditions of the action, the relative values of the constants are 
changed, and the result may not agree with any of those deduced from the simple 
types of equation. 
Tlie hypothesis of the existence of intermediate compounds, formed with the water- 
vapour present in the gas mixture, leads us to equations which represent the 
phenomena observed, at any rate qualitatively. The catalytic action of the water- 
vapour in this case is due to its taking an essential part in the process of combination. 
The action is therefore on this theory more than a mere quickening of the action 
which would go on slowly without the catalyser. It is possible that the action takes 
place very slowly in dry gases, but it seems probable that, if so, it occurs in a different 
way, as for example, by the direct combination of dissociated atoms. The function of 
the water-vapour we have assumed to be to form a centre about which the hydrogen 
and chlorine molecules can assemble, so that the atoms come into each other’s spheres 
of action and can re-arrange themselves, giving rise to a system of greater stability. 
This process we have supposed to take place by the formation of molecular compounds, 
and we have seen that a great many of the facts are explained on this hypothesis. 
We can regard this catalytic action as not essentially different from the catalytic 
action of such substances as jDlatinum black. In these cases the gases in the action, 
helped by such a catalyser, probably form condensed layers on the catalyser, and thus 
the actual molecules of the combining gases are brought into very intimate contact, 
and re-arrangement of the atoms may take place. 
In the particular case of the hydrogen and chlorine action, we have evidence of an 
unstable water-chlorine molecule, or, at any rate, aggregate, formed at the expense of 
the energy of the light, and it seems probable that in the case of other actions 
conditioned by the presence of water-vapour, the action proceeds by the aid of similar 
intermediate compounds. If this be so, we have seen how the action may not proceed 
in the way indicated by the ordinary theory, and so we have an explanation of the 
many anomalous results obtained in experiments on the rate of combination in 
gaseous systems. 
Summary. 
I. Exj^erimental. 
(I.) The initial expansion is the result of heat developed in the combination of 
hydrogen and chlorine. 
(2.) The initial expansion is an effect different from that observed when chlorine 
alone is acted on by light. This last effect is due to a rise in temperature, the energy 
for which is supplied by the light. 
(3.) The action ceases almost instantaneously when the light is cut off. 
