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Article XVIII. 



Observations and Experhnoits on the Theory of the Identity 

 of the Agents tvhich jjrodnce Light and Radiant Heat ; by 

 M. Mellonx. 



(Communicated to the Academy of Sciences of Paris, Dec. 21, 1835.) 

 From the Annales de Chimie et de Physique, vol. l. p. 418. 



.A-MONG the hypotheses on which it has been proposed to explain the 

 radiation of heat, there is a remarkably simple one which M. Ampere 

 has lately modified and developed with great ingenuity. It consists in 

 regarding radiant heat as a series of undulations produced in the aether 

 by the vibrations of bodies possessing heat. Those undulations should be 

 longer than the undulations which constitute light, if the calorific source 

 were dark ; but when the source is at the same time calorific and lu- 

 minous, there must always be a group of waves simultaneously possess- 

 ing the property of heating and that of illuminating. Viewed in this 

 way, there would be no essential dilFerence between radiant heat and 

 light. A very extensive series of asthereal undulations coming into con- 

 tact with the different parts of our body would produce the sensation 

 of heat : a more limited number of these would also possess the power 

 of exciting in the retina of the eye a vibratorj- movement calculated to 

 produce the sensation of light. 



No cause had been yet assigned for the quick transition of the purely 

 calorific to the shorter waves, which are at the same time calorific and 

 luminous. M. Ampere has found a very plausible explanation in the 

 phaenomena presented by the immediate transmission of terrestrial heat 

 through water. 



If an iron ball be heated at different times to different temperatures, 

 and brought each time to act on a thermoscope placed behind a layer of 

 water (either pure or charged with salt) measuring from three to four 

 millimetres in thickness, the thermoscope exhibits no indication of heat 

 so long as the metallic mass remains obscure, but as soon as the ball 

 becomes decidedly red, it indicates a slight calorific transmission. Now, 

 as the eye contains a certain cjuantity of watery humour, an absorption 

 and transmission similar to that exhibited by the layer of water will 

 take place in the interior of this organ also, which will therefore suffer 

 none but the undulations producing luminous heat to arrive at the retina. 



On the supposition that both agents are identical, it is needless to 

 show that the calorific rays are propagated in a straight line, and that 

 their angle of reflection is ctjual to their angle of incidence. 



