248 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 



will be seen to exert a very powerful effect, the move- 

 ment taking place and ceasing in an instant. 



This remarkable experiment not only serves to prove 

 sensitiveness, but also brings before us new views of the 

 powers of that extraordinary agent electricity. That 

 energetic chemical effects can thus be produced at a 

 distance by an electric spark in its momentary passage, 

 effects which are of a totally different kind from the 

 common manifestations of electricity, is thus proved ; 

 these phenomena being distinct from those of induction 

 or molecular movements taking place in the line of dis- 

 charge, they are of a radiant character; and we are led at 

 once to infer that the well-known changes brought about 

 by passing an electric spark through gaseous mixtures, 

 as when oxygen and hydrogen are combined into w r ater, 

 or chlorine and hydrogen into hydrochloric acid, arise 

 from a very different cause than those condensations and 

 percussions by which they are often explained a cause 

 far more purely chemical in its kind. If chlorine and 

 hydrogen can be made to unite silently by an electric 

 spark passing outside the vessel which contains them, at 

 a distance of several inches, there is no difficulty in un- 

 derstanding why a similar effect should take place with 

 a violent explosion when the discharge is made through 

 their midst, nor how a great many mixtures may be 

 made to unite under the same treatment. A flash of 

 lightning cannot take place, nor an electric spark be dis- 

 charged, without chemical changes being brought about 

 by the radiations emitted. 



Proofs of the exactness of the indications of this pho- 

 tometer. The foregoing examples may serve to illustrate 

 the extreme sensitiveness of this instrument. I shall 

 next furnish proofs that its indications are exactly pro- 

 portional to the quantities of light incident on it. 



