FOURTH SERIES. LAMINATED AND SMOKED SURFACES. 5 



8. Oil-lamp heat transmitted by Common Mica. 



9. _-.__ _- s _-.---- -, Glass (Argaud lamp). 



10. Citric Acid. 



11. Alum. 



]2. Ice. 



A clear appreciation of the scale of refrangibility as the important test for the 

 qualities of heat cannot be too clearly apprehended and admitted. Heat from 

 any source, if it admit of transmission at all through glass, alum, or water, will 

 ultimately have the character of glass-heat, alum-heat, or water-heat, just as 

 light from the sun, or from a candle, becomes red, blue, or green, by transmission 

 through glasses of these colours. 



5. Now, when M. MELLONI had shewn (and this experiment I believe was 

 original to him), that substances which stop every ray of even intense light (as 

 opaque glass and some kinds of dark mica), yet transmit a sensible quantity of 

 heat, it was not unnatural to inquire whether the invisible heat thus obtained 

 from a luminous source, might not possess the qualities of heat from a dark source, 

 in other words, whether bodies, like black glass and mica, instead of stopping the 

 less refrangible rays like glass, alum, &c., would not suffer these to escape, and 

 absorb the most refrangible rays, acting upon heat as a body does upon light, 

 which stops the yellow, blue, and violet rays, that is, as Red glass does. 



6. Experiment partly fulfils this expectation, and partly not. The careful 

 and complete series of experiments made by M. MELLONI upon the qualities of 

 the invisible heat thus obtained,* shews, that although it resembles low-tempera- 

 ture-heat, in so far as it is very feebly transmitted by alum or citric acid, yet low- 

 temperature-heat (that from boiling- water for instance), is but very faintly trans- 

 mitted through the black glass or mica, which ought not to be the case if these 

 bodies acted like a sieve, which arrested the more refrangible rays, and suffered 

 the others to escape. 



7. The direct test, however, of examining the refrangibility of the heat-rays 

 issuing from opaque screens yet remained ; and in applying this, I proved that 

 opaque glass and mica act as clear glass and mica do in elevating the mean re- 

 frangibility of the transmitted heat. Hence I concluded that the effect of such 



media upon heat is to absorb the rays of greatest and least refrangibility, in 

 short, to act as homogeneous yellow glass would do upon light, the mean refran- 

 gibility being on the whole, however, increased by transmission. I also pointed 

 out that heat from luminous sources is probably far more compound in its nature 

 than dark heat ; that the darkness of heat is no test of its refrangibility ; and that 

 even the most refrangible rays may contain heat separable from the light which 

 accompanies it.f 



8. In all this, then, there appears nothing exactly equivalent to the action 



* Annales de Chimie, Avril 1834. f Researches on Heat, Third Series, art. 73, 81, &c. 



VOL. XV. PART I. B 



