PEOrESSOE BimSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 389 
increase of the induction on exposure gradually becomes greater until a maximum is 
attained, after which the rapidity considerably diminishes. 
(3) That the increase of the induction from exposure to light takes place much more 
rapidly than the diminution of the same on darkening. 
The cm’ves Nos. 1, 2, 3, 4 and 5, fig. III., Plate XX., from observations A, represent the 
law according to which the restoration of the induction proceeds when it has gradually 
diminished by increasing duration of the period of darkening. The abscissae show the 
times of insolation, and the ordinates the amount of action in these times measured by 
the volume of hydrochloric acid formed in one minute. The curve No. 1 represents 
the increase of the induction when the gas brought to the maximum action was darkened 
for two mmutes ; and the following cmwes. Nos. 2, 3, 4 and 5, when the gas was 
darkened for four, eight, sixteen, and thirty minutes respectively. Observations B of 
the same series of expeiiments give an indication of so short a duration that the form of 
the curves is not rendered very striking. 
The maximum increase of the action is shown in all these curves by the sudden 
change of dkection which the curve takes. The ascending curves in Plate XX. fig. IV., 
A and B, represent the increase of the chemical action with the insolation, and the descend- 
ing cuiwes the diminution on Avithdi’awal of the light from series of observations A and B. 
The horizontal lure represents, as in the figs. I., II., III., the maximum action, which, 
as we shall hereafter show, is proportional to the time of exposure. 
We have explained the fact, that the mixture of chlorine and hydrogen does not 
combine in the dark to form hydrochloric acid, by the supposition of the existence 
of a certain resistance to combination which is overcome when the gaseous mixture is 
exposed to light. This resistance to combination can be considerably increased by a 
variety of cu’cumstances. The presence of a very small quantity of foreign gas in the 
normal mixture of chlorine and hydrogen is sufficient to cause this resistance to be 
increased to a veiy great extent. 
Experiments made for the purpose of determining the influence of such impurities on 
the sensibility of the gas are subject to a great difficulty, owing to the fact, that in 
order to avoid all the distuihing phenomena of absorption and diffusion, it is not possible 
merely to add the foreign gas to the normal mixture in the insolation-vessel, but it is 
necessary to allow the impurity as well as the chlorine and hydrogen to pass for hours 
through the liquids of the apparatus until the absorptiometric equilibrium is completely 
established. It is perfectly useless to collect the electrolytic gas in a large vessel, then 
to mix it Avith the impurity, and afterwards to allow the mixture to pass through the 
apparatus, for there is no possible means of collecting this pure mixture of chlorine and 
hydrogen in large quantities and again forcing it out in a given dkection. We have 
overcome this difficulty by evolving the gas, which was to serve as the impm’ity, electro- 
lytically with the same current which gave us our normal gas, so that the foreign gas 
was allowed to pass through the whole apparatus in company with the chlorine and 
hydrogen. By separating a secondary current, which was led through two separate 
