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the case of small quantities of oxygen. This gas, when present in 

 quantities amounting only to 10 S 00 of the total volume of gas, 

 diminishes the action from 100 to 4*7, whilst 1 ^ reduced the 

 action from 100 to 1*3. Excess of chlorine acted in a similar 

 manner, though not to so great an extent, t * of this gas reducing 

 the action from 100 to 60'2, and -^^ from 100 to 41'3. On 

 examining 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 1 1 g 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 10 6 00 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 BO 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 

 circumstances, 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 



