238 Scientific Intelligence. [March 



posterior surface of the glass plate would radiate the heat conducted 

 from its interior towards the pile if the hypothesis that the heat is 

 communicated by conduction were correct. But this does not occur, 

 and hence, there is no alternative left but the conclusion that heat 

 permeates bodies directly. Heat and light agree, therefore, in this 

 property, that both possess the power of passing through bodies. It 

 is proper that each should have such a capacity distinguished by 

 appropriate names, until their identity be proved. Melloni terms 

 the permeating power of heat through bodies, diathermal power, 

 just as we indicate capacity of bodies to transmit light by the names, 

 transparency, opalesence, &c. The diathermal power is subject to 

 similar modifications. Heat, however, differs from light in this 

 respect, that the facility with which it is transmitted by different 

 bodies has no relation to their transparency. 



Thus if we suppose the rays of a constant heat to be represented by 

 100, the only body which appears but slightly to diminish this when 

 interposed as a screen is rock salt, whose diathermal capacity is 92, 

 but the quantity of heat transmitted through a crystal of smoke-co- 

 loured quartz will be denoted by 57, and through a crystal of alum 

 by 12 where the difference is so very great as to excite astonishment. 

 This and similar facts have induced Melloni to conclude that heat 

 and light are distinct ; but in this opinion Dr. Faraday does not 

 coincide. 



Melloni has also examined the diathermal relation of colours, and 

 has found that their powers are in the following order : violet 53, 

 yellowish red 53, purple red 51, bright red 47, pale violet 45, orange 

 red 44, clear blue 42, deep yellow 40, bright yellow 34, golden yel- 

 low 33, dark blue 33, apple green 26, mineral green 23, very deep 

 blue 1 9. Hence, we see that the mineral relations of the colours to 

 their heating power is so completely altered that the violet ray which 

 in the spectrum possesses temperature 25 or 30 times below that of the 

 red ray, observes here a higher temperature, but the result seems mo- 

 dified as occurs with light by the nature of the power employed, to 

 illustrate the comparative experiments. 



Dr. Faraday exhibited many of the experiments which Melloni 

 has described in his papers, especially in reference to the diathermal 

 properties of rock salt, glass, alum, with screens of which substances 

 he had been supplied. The absorptive power of different colours, in 

 relation to the solar spectrum was well illustrated by means of the oxy- 

 hydrogen blowpipe. The contrivance of passing the decomposed ray 

 through a volume of disengaged ammonia had a happy effect, the co- 

 lours of the spectrum being as it were made to float in the air. 



He likewise exhibited the method of polarizing light by means 

 of tourmaline, by which fanciful figures are formed, and light 

 transmitted or withheld by merely altering the relative position 

 of the screens properly adapted. 



Dr. Faraday directed the attention of the meeting to a fine bust 

 of Mr. Fuller by Chantry, and observed that the title engraved on it 

 was a sufficient eulogium : " John Fuller, who gave ,£10,000 for the 

 encouragement of Science in the Royal Institution." A fine painting 

 of Earl Spencer was exposed for the first time in the library, where 

 also several ingenious models and casts, and several specimens of litho- 

 graphic printing from zinc plates attracted notice. 



