112 Prof. Draper on the Production of Light 



monochromatic yellow light. That the yellow light comes 

 from this flame is proved by the greater length of its image. 



VII. Physical cause of the production of light by chemical action. 



Do not the various facts here brought forward prove that 

 all chemical combinations are attended by a rapid vibratory 

 motion of the parts of the combining bodies, which vibrations 

 become more frequent as the chemical action is more intense? 



The burning particles which constitute the inner shell of a 

 flame are executing about four hundred billions of vibrations 

 in one second ; those in the middle about six hundred billions, 

 and those on the exterior, in contact with the air, about eight 

 hundred billions in the same time. The quality of the emitted 

 light, as respects its colour, depending on the frequency with 

 which those vibrations are accomplished, increases in refran- 

 gibility as the violence of the chemical action becomes greater. 



The parts of all material bodies are in a state of incessant 

 vibration : that which we call temperature depends on the fre- 

 quency and amplitude of those vibrations conjointly. If by 

 any process, as by chemical agencies, we increase that fre- 

 quency to between four and eight hundred billions of vibra- 

 tions in one second, ignition or combustion results. In the 

 case of the former of these numbers the temperature is 977° F. 

 At this temperature or epoch the waves propagated in the 

 aether impress the organ of vision with a retl light. This also 

 is the temperature of the innermost shell of a flame. If the 

 frequency of vibration still increases, the temperature corre- 

 spondingly rises, and the light successively becomes orange, 

 yellow, green, blue, &c,, and this condition obtains in the suc- 

 cessive strata of a flame as we pass from its interior to its ex- 

 terior superficies. 



The general principle at which I thus arrive, as the final 

 result of this experimental investigation, viz. that there is a 

 connexion between the vehemence with which chemical affinity 

 is satisfied and the refrangibility of the resulting light, assumes 

 the position of a simple consequence of the undulatory theory. 

 Is it not very natural, if all chemical changes are attended by 

 vibratory motions in the particles of the bodies engaged, that 

 those vibrations should increase in frequency as the action 

 becomes more violent? But an increased frequency of vibra- 

 tion is the same thing as an increased refrangibility. 



I think that in this manner the theory of ethereal undula- 

 tions is on the point of including many of those fundamental 

 facts in chemistry which until now have been believed to be 

 adverse to it, or at all events as standing apart from it. I re- 

 call the admirable remark which Mr. Whewell has made, in 

 his History of the Inductive Sciences, how this theory, like 



