(i 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



of red glass upon light, no substance which transmits most easily heat of low 

 Refrangibility and Temperature, and which separates heat of that description 

 from the compound emanation from luminous sources. Reasoning probably upon 

 the conclusions just stated, M. MELLONI conceived the happy idea of combining 

 an opaque 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 refrangibility. 



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 pro- 

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

 incidentally, I first pursued the remark into consequences which I considered im- 

 portant, after M. MELLONI had called particular attention to the experiment with 

 smoked surfaces. 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 ex- 

 actly 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 approxima- 

 tion to the following results, which are taken from the Third Series of my Re- 

 searches, art. 24. 



TABLE of the proportion of Heat from different Sources transmitted by the Po- 

 larizing Mica Plates I and K, contrasted with the transmissions by Mica in 

 its usual state, and with Black Glass. 



10. This singular result of the mechanical condition of the mica did not fail 

 to strike me greatly at the time, and was not published until after careful repe- 

 tition. It afforded a triumphant reply to an objection against my experiments 

 which I was then combating, that the quantity of heat absorbed by the polarizing 



