370 Prof. Draper on the Chemical Action of Light. 



The atoms of the chemical elements differ in weight. We 

 should not therefore expect that the fethereal vibrations would 

 throw them into movement witli equal facility, but some would 

 yield more readily than others. Is not this what we express in 

 chemistry by the temi specific heat ? a body the capacity of 

 which is great, requiruig a prolonged application of fethereal 

 pulses before a consentaneous motion is reached, and in its turn 

 impressing on the fether during cooling a correspondingly pro- 

 longed series of motions. And is not this the cause of that 

 remarkable relation between the atomic weights of elementary 

 bodies and their speeiiic heats, discovered by Didong and Petit ? 



These considerations may lead us to inquire whether the ge- 

 neral cause of the decomposition of compound bodies by radia- 

 tion is due to the circumstance, that all the atoms of which 

 their molecules are composed take on the vibratoiy motion with 

 unequal facility. Thus, if a certain compound molecule be sub- 

 mitted to the influence; of an intense radiation, some of its con- 

 stituent particles may vibrate consentaneously at once, and the 

 others more tardily. Under these circumstances, the continued 

 existence of the group may become impossible, and decomposition 

 ensue in the necessity of the case. 



In entering upon the experimental analysis of the action of a 

 ,ray upon a decoraposaljle body, there are three dift'erent points 

 to be considered, so far as the ray itself is concerned : — 1st, To 

 what extent, and in what manner, is the result affected by the 

 intensity of the ray, or by the amplitude of the vibrating excur- 

 sions ? 2nd, How is it affected by the frequency of the pulsatoiy 

 impressions ? and 3rd, How by the direction in which the vibra- 

 tions are made, as involved in the idea of polarization ? I shall 

 now examine these in succession. 



1st. To what extent, and in zvhat manner, is the decomposition 

 of a composite body affected by the intensity of a ray, or the am- 

 plitude of the vibrating excursions ? 



If the different facility with which atoms receive the impres- 

 sion of aethereal vibrations be the true cause of decomposition by 

 light, we should expect that many such changes would become 

 possible under the influence of a burning lens which are not so 

 in the direct rays of the sun. 



This idea is favoured by what we find in the case of heat. 

 The burniug-lens has long had celebrity in that respect, and in 

 former times was the most powerful means of reaching a high 

 temperature. 



The action of the lens is due to the rapidity with which it can 

 supply caloric, contrasted with the loss by conduction, radiation, 

 &e. Thus an object of any kind exposed to the sun receives heat 

 at a certain rate. But it is simultaneously experiencing a loss 

 by conduction, radiation, and cun-ents in the air. Exposed to 



