RADIANT HEAT. 341 



sion, gives a comparative statement of the reflections from rock crystal 

 and copper. 



Analogies of Light and Heat : Melloni and Forbes. 



M. Melloni' s "Observations and Experiments on the Theory of the 

 Identity of the Agents which Produce Light and Heat" were read to 

 the Academy of Sciences, December 21, 1835, published in the Ann. 

 de Chimie, No. 50, p. 418, and translated in Taylor's Scientific Me- 

 moirs, Part III, p. 388. 



In this paper the author combats the views of M. Ampere, who had 

 proposed some ingenious speculations for explaining, on the theory of 

 undulations, the identity of light and heat, the difference of effect being- 

 dependent solely on the different wave-lengths, those producing heat 

 being supposed longer than those giving rise to light. Athermanous 

 media, such as water, intercept the longer waves, but not the shorter. 

 Thus the aqueous humor of the eye prevents the retina from being 

 affected by heat as well as light. 



The author admits that many phenomena may be sufficiently ac- 

 counted for by the mere supposition of the difference of wave-lengths; 

 but he mentions some experiments in which he thinks decisively that 

 this will not hold good. 



The spectrum formed by a rock-salt prism gives the maximum of 

 heat considerably beyond the red end. On interposing water of in- 

 creasing thickness, the maximum successively occurs in the red, and 

 thence upwards to the green. A similar effect is produced by color- 

 less glasses; but with colored glasses, whilst the luminous spectrum is 

 variously absorbed and altered, the place of the maximum of heat re- 

 mains unaltered, and the decrease from it quite regular. 



Another experiment consists in interposing a diaphanous body, which 

 absorbs all the calorific, but only a part of the luminous rays. On 

 using in this way a peculiar species of green glass colored by oxide 

 of copper, the greenish light transmitted " exhibits no calorific action 

 capable of being rendered perceptible by the most delicate thermoscopes, 

 even when it is so concentrated by lenses as to rival the direct rays of the 

 sun in brilliancy." 



On these points Professor Forbes has made some remarks in the 

 London and Edinburgh Journal of Science, March, 1836. 



Such experiments as these, he justly observes, and indeed many 

 more simple, clearly show that heat is not light, but nothing more. 

 It is a question, then, what is the point really aimed at in these spec- 

 ulations. The author agrees with Melloni in the result, "that one 

 and the same undulation does not invariably impress the senses of 

 sight and feeling at once. The great difficulty is this: to account for 

 the equal refrangibility of two waves having different properties." 



New Phenomena of Transmission : Melloni and Forbes. 



It appears by the Comptes Rendus that on September 2, 1839, M. 

 Arago communicated to the Academy of Sciences a letter by M. Mel- 

 loni, containing some new and highly interesting experiments on the 



