Prof, Draper on the Chemical Action of Light. 381 



the cause of the extinction of the latter^, or, as we term it, ab- 

 sorption. That species of motion which, as respects frequency, 

 appertains to it, cannot be propagated through the medium, 

 because the group cannot take it on. Its atoms are dislocated 

 in the attempt ; and as that kind of motion cannot pass through 

 the medium, the ray in question is absent when we examine the 

 issuing beam. 



Decomposition, which under these circumstances is but an- 

 other term for re-arrangement, is only the incidental, and not 

 necessary result of absorption. Many causes may intervene to 

 prevent such re-arrangement taking place ; for example, cohesion, 

 as determining the solid state. 



If we compare the mode of action of a ray as respects its heat- 

 ing effect and chemical action, a very striking difference is de- 

 tected. The calorific effect upon a ponderable mass passes from 

 point to point, one scries or group of molecules being affected 

 after another by conduction, as it is termed. But in the chemical 

 action of light' there is nothing equi\'alent to conduction, there 

 is no propagation of effect. It is this principle that gives per- 

 fection to the photographic art ; the Daguerreotype plate pre- 

 sents with mar\-ellous accuracy every dot or line ; there is no 

 lateral spreading, nothing analogous to conduction ; the change 

 which one atom of iodide of silver undergoes has no influence on 

 the adjacent ones. The cause of this difference is explained by 

 the facts now before us ; a rise of temperature is due to an in- 

 creased rapidity or intensity of the oscillations of the groups of 

 vibrating molecules, but chemical decomposition is due to the 

 dislocation of their parts. A system of vibrating molecules will 

 solicit an adjacent one to execute similar motions, through the 

 medium of the intervening sether ; but it of course by no means 

 follows, that when a compound molecule is undergoing entire dis- 

 ruption, those that are in the neighbourhood should be compelled 

 to ])ass into a similar state. For, the veiy reason that chemical 

 decomposition takes place is, because the group which receives 

 the provoking ray cannot vibrate consentaneously with it ; and 

 if that group cannot assume the motion in question, how is it 

 possible it should transmit it to any other ? 



Upf)n the whole, we may therefore conclude, that it is not the 

 intensitij of a beam which determines its decomposing powei*, 

 and that we cannot produce greater effects by the action of con- 

 verging mirrors and lenses than we can by the application of the 

 simple sunbeam, continued for an equivalent period of time. 

 Nor can such optical contrivances effect the decomposition of 

 substances on which a feeble beam has no action. It is not on 

 the quality of intensity that tlu; chemical action of a ray depends ; 

 for a feel)le ray, if applied sufficiently long, will at last equal the 



