534 Prof. TyndalPs Contributions to Molecular Physics. 



In the earlier memoirs of MM. De la Provostaye and Desains*, 

 and in that of MM. Wiedemann and Franz, I find the following- 

 facts : — The radiative power of platinum is five times that of 

 silver ; its conductive power is one-tenth that of silver. Plati- 

 num has more than twice the radiative power of gold ; it has 

 only one-seventh of the conducting power. Zinc and tin are 

 almost equal as conductors, and they are also nearly equal as 

 radiators. Silver has about six times the conductive power of 

 zinc and tin ; it has only one-fourth of their radiative powers. 

 Brass possesses but one-half the radiative energy of platinum ; it 

 possesses more than twice its conductivity. Other experiments 

 of MM. De la Provostaye and Desainsf confirm those hitherto 

 referred to. Taking the absorbent power, as determined by these 

 excellent experimenters, to express the radiating power, and 

 multiplying their results by a common factor to facilitate com- 

 parison with those of MM. Wiedemann and Franz on conduc- 

 tion, we obtain the following Table : — 



Table XLII. — Comparison of Conduction and Radiation. 

 Name of metal. Conduction. Radiation. 



Silver . 



. . 100 



11 



Gold . 



. . 53 



27 



Brass . 



. . 24 



42 



Tin . 



. . 15 



90 



Platinum 



. . 8 



100 



We here find that, as the power of conduction diminishes, 

 the power of radiation augments — a result, I think, completely 

 in harmony with that to which a consideration of the molecular 



of a cube was coated with lampblack, and the other half with cinnabar. 

 The cube being filled with oil at a temperature of 173° C, it was found 

 that the emission from the cinnabar was more copiously absorbed by a 

 plate of glass than that from the lampblack. In the second experiment, 

 they found that, while 39 per cent, of the radiation from a bright surface 

 of platinum was transmitted by a plate of glass,, only 29 per cent, of the 

 radiation from the opposite surface of the same plate, which was coated 

 with borate of lead, was transmitted. These results are quite in harmony 

 with the views which I have ventured to enunciate. We may infer from 

 them that the heat emitted by the respective compounds — the cinnabar and 

 the borate of lead— is of slower period than that emitted by the elements ; 

 for experiment proves that as the periods are quickened the glass becomes 

 more transparent. At a temperature of 100° C, moreover, the emission 

 from borate of lead was found equal to that from lampblack ( Comptes 

 Rendus, vol. xxxviii. p. 442), while at a temperature of 550° C. it had only 

 three-fourths of the emissive power of the lampblack. With reference to 

 the theoretic views which these researches are intended to foreshadow, the 

 results of MM. De la Provostaye and Desains are of the highest interest. 



* Comptes Rendus, 1846, vol. xxii. p. 1139. 



f Annates de Chimie, 1850, vol. xxx. p. 442. 



