Prof. Roscoe on the Influence of Light upon Chlorine. 505 
On first thoughts one would naturally conclude that the cause 
of this peculiar phenomenon was to be sought in some allotropic 
change which one or both gases undergo on exposure to light, 
in which state the combining powers are heightened, and the 
subsequent combination on insolation thus rendered possible. 
This is, in fact, the explanation given by Dr. Draper. He supposes 
that during the first insolation it is the chlorine which undergoes 
a change of properties ; and he convinced himself by various ex- 
periments that this is the case, and that the chlorine retains this 
condition of heightened chemical activity when preserved in the 
dark for a long period, extending in some cases over several weeks. 
We have with great care frequently repeated these experiments, 
avoiding the errors to which they were subject, and we have 
not in any one case succeeded in corroborating Dr. Draper’s 
statements. We found, that to whatever kind or amount 
of light the gas had been subjected, it rapidly returned to its 
inactive condition on exclusion of light ; that on standing in the 
dark for about half an hour, the gas did not possess any proper- 
ties different from those which it possessed before it had been 
exposed to light. Hence Dr. Draper’s position, as regards a 
permanent allotrepic condition of either gas, becomes untenable. 
That Dr. Draper arrived at these conclusions by help of the ex- 
periments mentioned in his former papers in your Journal, will 
not surprise us when we become acquainted with the difficulty 
of obtaining by any means accurate results in this subject, and 
the impossibility of so doing by methods so imperfect as those 
which Dr. Draper employed. 
We have shown, in our second paper read before the Royal 
Society, that the presence of a trace of foreign gas, not exceeding 
jon Ath of the total volume, is sufficient to alter in the most 
marked degree the photo-chemical sensibility of the mixture. 
Dr. Draper derives the proof of the existence of a permanent 
allotropic modification of chlorine from (amongst others) experi- 
ments in which that gas was collected over a solution of chloride 
of sodium, then exposed to the sun, and afterwards mixed with 
its own volume of hydrogen, and the rate observed at which the 
mixture combined on subsequent insolation, compared with a 
similar mixture not previously exposed. The limits of error in- 
troduced by this and the other modes of experimentation adopted 
by Dr. Draper to determine this question, extend, as we have 
shown in our paper, even beyond the differences in the action 
arising from the phenomenon itself, and hence his conclusions 
cannot be relied upon. 
In order to determine whether either gas undergoes on expo- 
sure any change in properties, whether permanent or not, we 
passed (with all due precautions) the gases, evolved electrolyti- 
