432 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



It might be supposed that this instantaneous expansion 

 is due to a heat disturbance arising from the absorption 

 of rays that are not engaged in producing the chemical 

 effect. But this interpretation seems to be incompatible 

 with the instantaneously following contraction. Though 

 it is admissible that heat should be instantaneously dis- 

 engaged by the preliminary actinization, it is difficult to 

 conceive how it can so instantaneously disappear. 



When the radiation is withdrawn and the hydrochloric 

 acid absorbed, there is no after-combining. The action is 

 perfectly definite. For a given amount of chemical ac- 

 tion, an equivalent quantity of the radiation is absorbed. 



The instances I have cited in this discussion of the 

 mode of action of radiations are, one of decomposition, in 

 the case of the silver iodide ; and one of combination, in 

 the case of hydrochloric acid. I might have introduced 

 another the dissociation of ferric oxalate, which I have 

 closely studied but it would have made the Memoir of 

 undue length. From the facts herein considered the fol- 

 lowing deductions may be drawn : 



When a radiation impinges on a material substance, it 

 imparts to that substance more or less of its vis viva, and 

 therefore undergoes a change itself. The substance also 

 is disturbed. Its physical and chemical properties de- 

 termine the resulting phenomena. 



(1st.) If the substance be black and undecomposable, 

 the radiation establishes vibrations among the molecules 

 it encounters. We interpret these vibrations as radiant 

 heat. The molecules of the medium do not lose the vis 

 viva they have acquired at once, since they are of greater 

 density than the ether. Each becomes a centre of agita- 

 tion, and heat-radiation and conduction in all directions 

 are the result. The undulations thus set up are com- 

 monly of longer waves ; and as the movements gradually 



