108 
MK. W V, BEVAN ON THE COMBINATION OF HYDROGEN 
sample of" gas. After the expansion had l)een made, the piston of the expansion 
apparatus could be allowed to return to its original position, and then, on ojDening the 
tap communicating with the capillary tube, the effect of the expansion on the rate of 
combination could be observed. 
With this apparatus it was shown that the nuclei, on which condensation took 
place on expansion, appeared before any hydrochloric acid was produced. The effects 
were very much like those on p. 107. The hydrochloric acid is formed only after 
about five times the illumination required to produce the maximum cloud. 
If after hydrochloric acid has begun to be formed an expansion is made, the 
formation of hydrochloric acid stops if the expansion is large enough to produce a 
cloud. If no cloud is formed, the expansion being less than corresponding to 
Pj/P^ = P2 about, then the hydrochloric acid formation is not affected. In an 
experiment of this kind, the inert period was found to be about 30 seconds. A cloud 
producing expansion was then made and, after allowing this cloud to settle, the 
piston was returned to its original position, and communication made with the 
capillary index tube ; after the first motion of the index due to the adjustment of 
pressure between the capillary and tlie bulb the index remained at rest for 
16 seconds, and after this began to show formation of hydrochloric acid again. The 
hull) was darkened for a short time after the expansion to allow the cloud to settle, 
but tlie effect of the dai*kening on the succeeding inert period was very small. An 
exactly similar experiment, excepting in the amount of the expansion, showed that 
when no cloud was formed there was no inert period in the second instance. 
There is, therefore, some substance produced by the action of the light on the 
chlorine and water vapour which acts as condensation nuclei. The formation of this 
intermediate substance is necessary for the production of hydrochloric acid, and the 
removal of this substance by expansion and consequent precipitation stops the 
action when hydrochloric acid is being produced, and tbe induction peilod has to 
begin ovei' again. 
§ 6. Experiments vjitli Dried Gases, and other Experiments. 
(1.) Experiments ivith Dried Gases .—With dry gases the action is much less rapid 
than with damp gases. But the general phenomena of induction are the same when 
sufficiently intense light is used. There is a period of inertness and of acceleration. 
Absolutely dry gases have probably never been prepared, but the rate of combination 
diminishes as the gas mixture is made less moist. In a bulb with phosphorus 
pentoxide and the gas mixture combination is v^ery slow even in direct sunlight. 
(2.) Effect of Eontijen Rays. —Rontgen rays were found to have no effect on the 
combination. In the dark they produce no action ; and no effect was found when 
the rays fell on the gas when combination was taking place under the influence of 
liglit. 
