222 Royal Society :— 
but in this communication we restrict ourselves to the consideration 
of these phenomena in so far as they influence the action of photo- 
chemical induction, intending on a future occasion to enter more 
fully into the new field of research indicated. 
The contact action of foreign gases is still more strongly seen in 
the case of small quantities of oxygen. This gas, when present in 
quautities amounting only to 7355 of the total volume of gas, 
diminishes the action from 100 to 4°7, whilst 43, reduced the 
action from 100 to 1:3. Excess of chlorine acted in a similar 
manner, though not to so great an extent, 4%, of this gas reducing 
the action from 100 to 60°2, and 38° from 100 to 41°3. On 
exainining the effect of small quantities of hydrochloric acid gas 
upon the induction maximum, we found, fortunately for the accuracy 
of the indications of our instrument, that an amount of ,43, of this gas 
does not produce any appreciable effect on the action of the induction. 
Uninsolated gas was found to act similarly on the normal mixture, 
the admission of ;8;; of non-insolated gas reducing the action from ° 
100 to 55. Curves have been drawn, representing the relation 
between the action and the amount of foreign impurity introduced. 
Several series of experiments also showed that a mixture of chlorine 
and hydrogen, which was so nearly pure that no alteration of the 
maximum action was observable, was longer in attaining the maximum 
than the perfectly pure gas; hence the duration of the induction 
serves as an exact measure of the absence of all foreign gases in the 
standard mixture. 
An explanation of the laws of photo-chemical induction derived 
from the above-mentioned experiments, might easily be found in the 
assumption that the chlorine or the hydrogen, or both gases, undergo 
upon exposure to light a change similar to that between common 
and ozonized oxygen, or that these two gases can, under certain 
cirenmstances, be invested with active, and, under other circum- 
stances, with passive properties. If this hypothesis be true, each 
gas must undergo this peculiar modification when separately exposed 
to the action of the light. That this is not the case was shown by 
the following experiment :—The two gases were separately evolved, 
and each led through a long glass tube, in which they could be 
separately exposed to the action of diffuse and direct sunlight. After 
this exposure, the gases passed through a connecting tube into the 
apparatus, in which a constant source of light gave the duration of the 
induction. Thus alternately insolating and darkening the separated 
gases, we observed the effect on the gases subsequently mixed and 
exposed to lamplight. No difference was perceptible in the duration 
of the induction between the gases previously insolated and those 
evolved in the dark. Hence we may conclude, that the light does 
not effect a permanent modification, either of the chlorine or hydrogen, 
but that the combination produced by the light must depend on 
photo-chemical action affecting only the mcreasing attractions of the 
chemically active molecules. 
All the curves representing the increase of the induction under 
various conditions have a common form, and a point of flexure at 
