PHOTOCHEMICAL RESEARCHES. ^ 



evolved from the slow combustion of the chlorine and hydrogen exerted any 

 perceptible action upon the instrument. 



On suddenly cutting ofF the light from the sensitive gas, the action is 

 found not to cease immediately. This absorption, after the exclusion of the 

 light, may be owing to three causes. 



1. The combination of the gases may continue for a short time after the 

 removal of the light. 



2. The hydrochloric acid formed may not be instantaneously removed by 

 solution in the water. 



3. The decrease of volume may be produced from the whole gas cooling 

 down, owing to the heat of combustion no longer being added to it. 



Experiments undertaken to determine which of these three suppositions 

 was true, showed that this contraction could be almost completely accounted 

 for, from the decrease of temperature of the gas, proving therefore that the 

 first two assumptions were groundless. This contraction is so small that it 

 does not in the least degree interfere with the accuracy of the observation. 



In order still more fully to test our apparatus, an arrangement was made 

 by means of which a small jet of coal-gas could be brought within different 

 measured distances of the sensitive mixture, and the amount of the decom- 

 position effected measured. The results thus obtained showed most exactly 

 that the chemical action varied inversely as the square of the distance from 

 the source of light, proving that the chemical rays obey the same general 

 law as the visible rays, and affording another evidence of the accuracy of 

 the results obtained by this instrument. Observations made with this ar- 

 rangement also showed that exactly the same action was effected by the 

 flame, placed at the same distance, at different times extending over a period 

 of one month. The amounts of action effected by the same flame on various 

 days from the 12th to the 26th of June, were 13-99, 13-8.S, 13-76, 13-84. 



Photochemical Induction. 



Chemical affinity, or the force which causes different bodies to unite and 

 form chemical compounds, is in every particular case a certain definitive, 

 unalterable quantity, which like all other forces (and matter itself) can 

 neither be created nor destroyed. Hence it is incorrect to say that, under 

 certain circumstances, a body attains an affinity which under other circum- 

 stances it loses. All that can be said in such a case is, that the body at one 

 time follows the chemical attraction, and at another time is retarded by 

 forces acting in an opposite direction. This opposite action may be con- 

 ceived to be a resistance similar to that occurring in friction, or in the 

 passage of electricity through conductors. This resistance is overcome 

 when we facilitate the formation of a precipitate by agitation, or when che- 

 mical action is brought about by increase of temperature, catalytic action, 

 or insolation. The existence of such a resistance presupposes a certain com- 

 bining power, which may be measured by the amount of combination caused 

 by the unit of force in the unit space of time. 



The act by which this resistance is overcome, and the state reached in 

 which combination take* place, we have called Chemical Induction. The 

 laws Avhich regulate the action of chemical affinity, when this resistance is 

 fully eliminated, are as yet entirely unknown to us ; and although the solution 

 of tliis, the most important problem in our science, appears at present so far 

 removed, it is at least desirable that facts should be found which may form 

 starting-points in this new field of research. The interesting relations in 

 ■which the phsenomena of photochemical induction stand to these questions, 

 have induced us to examine this part of the subject with particular attention. 



1856. F 



