74 Professor Forbes's Researches on Heat. 



in applying this, I proved that opake glass and mica act as 

 clear glass and mica do in elevating the mean ref tangibility 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 refrangibility being on 

 the whole, however, increased by transmission. I also pointed 

 out that heat from luminous sources is probably far more com- 

 pound in its nature than dark heat ; that the darkness of heat 

 is no test of its refrangibility ; and that even the most refran- 

 gible rays may contain heat separable from the light which 

 accompanies it*. 



8. In all this, then, there appears nothing exactly equiva- 

 lent to the action of red glass upon light, — no substance which 

 transmits most easily heat of low refrangibility and tempe- 

 rature, and which separates heat of that description from the 

 compound emanation from luminous sources. Reasoning 

 probably upon the conclusions just stated, M. Melloni con- 

 ceived the happy idea of combining an opake substance, such 

 as smoke, with a solid, which itself should effect no specific 

 change upon the incident heat. He therefore smoked rock-salt, 

 and found that it presented a complete analogy to red glass, 

 transmitting most easily heat of low temperature and refran- 

 gibility. 



9. Whilst I give full credit to M. Melloni for the ingenuity 

 and importance of his experiment, I must be permitted to state, 

 that I conceive that I preceded him by eighteen months in 

 the discovery of a substance possessing similar properties, 

 although I very readily admit, that, having been led to that 

 observation incidentally, I first pursued the remark into con- 

 sequences which I considered important, after M. Melloni had 

 called particular attention to the experiment with smoked sur- 

 faces. On the 27th February, 19th and 20th March 1838 

 (as appears by my Journal of Experiments), I proved that 

 mica, split into very thin films by the action of heat, such as 

 I employ for polarizing, possesses the property of transmitting 

 in larger proportion several of the less refrangible kinds of 

 heat, and in particular, that it transmits heat from a source 

 perfectly obscure, in almost exactly the same proportion with 

 the highly refrangible heat of a lamp transmitted through glass. 

 I have no hesitation in saying, that no other substance known 

 previously to M. Melloni's experiments with smoked salt, gave 

 any approximation to the following results, which are taken 

 from the Third Series of my Researches, art. 24-. 



* Researches on Heat, Third Series, art. 73, 81, &c. 



