Chemical Force in the Spectrum. 441 



that are not engaged in producing the chemical effect. But 

 this interpretation seems to be incompatible with the instan- 

 taneously following contraction. Though it is admissible that 

 heat should be instantaneously disengaged by the preliminary 

 actinization, it is difficult to conceive how it can so instanta- 

 neously 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When the radiation is withdrawn and the hydrochloric acid 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 action 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 

 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 following deductions may be drawn : — 



When a radiation impinges on a material substance, it im- 

 parts to that substance more or less of its vis viva, and there- 

 fore undergoes a change itself. 



The substance also is disturbed. Its physical and chemical 

 properties determine the resulting phenomena. 



1st. If the substance is black and undecomposable, the ra- 

 diation 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 agitation ; and heat-radiation and conduc- 

 tion in all directions are the result. The undulations thus set 

 up are commonly of longer waves ; and as the movements gra- 

 dually decline the shorter waves are the first to be extinguished, 

 the longer ones the last. This, therefore, is in accordance with 

 what I found to be the case in the gradual warming of a solid 

 body, in which the long waves pertain to a low temperature, 

 the short ones arising as the temperature ascends (Phil. Mag. 

 May 1847). 



In some cases, however, instead of the disturbing undulation 

 giving rise to longer waves, it produces shorter ones, as is shown 

 when a platinum wire is put into a hydrogen flame, or by Tyn- 

 dalPs experiment, in which invisible undulations below the red 

 give rise to the ignition of platinum. 



2nd. If the substance is coloured and undecomposable, it 

 will extinguish rays complementary to its own tint. Its tem- 

 perature will rise correspondingly. 



