AND CHLOKINE UNDER THE INFLUENCE OF LIGHT. 
108 
No difference in the initial rate of combination conld be detected if the hydrogen 
Avas first illuminated, and then mixed Avitb chlorine in the dark, and the mixture 
exposed to light. 
If the chlorine be exposed to light in a separate vessel, and then introduced into 
the burette through the water and mixed with hydrogen, the behaviour was as if it 
had not been exposed to light at all. The process of bubbling through the water thus 
washes out the substance in the chlorine which causes the difference between the 
insolated and non-insolated chlorine. 
The anomalies observed in the final rates of formation of hydrochloric acid in these 
experiments are to be attributed to the differences in the mixtures in the two burettes. 
It is impossible to obtain with this method two exactly similar mixtures, and the 
effects of small changes in the composition of the mixture are large. If an excess of 
chlorine be present in the mixture, the results with different tubes are fairly 
comparable, much more consistent results being obtainable than with an excess of 
hydrogen. This fact agrees with Bunsen and Koscoe’s result, that a small excess of 
hydrogen makes a very much greater difference in the rate of combination than that 
produced by the same excess of chlorine. 
The effect of insolating the chlorine alone passes off if the chlorine and hydrogen 
mixture be left in the dark for some time. The behaviour is then similar to that of 
the ordinary insolated mixture. 
There is, therefore, a change produced by the action of light on moist chlorine alone, 
which is apparent when the chlorine is caused to combine with hydrogen. Draper 
considered that an allotropic modification of chlorine was produced, which combined 
more readily with hydrogen than ordinary chlorine. It seems more probable that the 
effect is due to some action between the chlorine and the water vapour, which is 
always present in these experiments. For water vapour, if not absolutely necessary 
to the action, assists it very materially. 
This vieAv is also supported by the fact discovered by Baker, ^ that perfectly pure 
and dry chlorine does not expand when exposed to light, while damp chlorine does so 
expand. We have already seen that the expansion of damp chlorine is attended by 
a development of heat. There is then, at any rate, some evidence that the first 
stage of the action to form hydrochloric acid is an action between chlorine alone and 
water vapour. 
§ 5. Expansion Experiments. 
To obtain further evidence of the existence of intermediate bodies in the initial 
stage of the action between hydrogen and chlorine, use was made of the expansion 
method of C. T. Pt. Wilson to discover if any cloud-forming nuclei were produced hy 
the influence of light on the gas mixture. The apparatus was that described by 
* 
Baker, ‘Brit. Ass. Rep.,’ 1894. 
