388 
Articte XVIII. 
Observations and Experiments on the Theory of the Identity 
of the Agents which produce Light and Radiant Heat ; hy 
M. MEL ont. 
(Communicated to the Academy of Sciences of Paris, Dec. 21, 1835.) 
From the Annales de Chimie et de Physique, vol. u. p. 418. 
Aone the hypotheses on which it has been proposed to explain the 
radiation of heat, there is a remarkably simple one which M. Ampére 
has lately modified and developed with great ingenuity. It consists in 
regarding radiant heat as a series of undulations produced in the ether 
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 difference between radiant heat and 
light. A very extensive series of zthereal 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 vibratory 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. Ampére has found a very plausible explanation in the 
phznomena 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 quantity 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 equal to their angle of incidence. 
