460 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



In this Memoir I have endeavored to examine how 

 far the decomposing action of a radiation is dependent 

 on the amplitude, the frequency, or the direction of its 

 vibrations. The result arrived at is that decompositions 

 are not determined by amplitude that is, brilliancy 

 since a faint light continued long enough can produce 

 precisely the same effect as the more concentrated ray of 

 a burning-lens applied for a shorter time. Nor does the 

 direction of motion, as involved in the idea of polariza- 

 tion, whether plane or circular, exert any effect, but it is 

 the frequency of the periodic impulses that is the sole 

 determining cause. And the phenomena of interference 

 from the superposition of such small motions occur ex- 

 actly as might have been predicted. 



The immediate cause assigned for such decompositions 

 is that a ray forcing the material particles on which it 

 falls into a state of rapid vibration, it comes to pass in 

 many compound molecules that their constituent atoms 

 can no longer exist together as the same group, because 

 of the impossibility of their being animated by consenta- 

 neous or conspiring motions, and dislocation, rearrange- 

 ment, or decomposition is the result. 



In this Memoir I have spoken of heat and light as 

 though they were distinct agencies, and considered such 

 facts as conductibility, etc., displayed by the one and not 

 by the other. But if we recall what has been said in 

 preceding Memoirs to the effect that these are only modes 

 of motion, and that the difference of the effects they dis- 

 play turns on the character of the receiving surface or 

 substance, there will be no difficulty in translating this 

 commoner language into terms that are more exact, and 

 in presenting the phenomena in question under a more 

 rigidly scientific point of view. Familiar expressions 

 very frequently convey to the mind clearer ideas than 

 others which, perhaps, may be more strictly correct. 



