532 Prof. TyndalPs Contributions to Molecular Physics. 



salt. I made a few experiments to test the diathermancy of the 

 salt, with the following results : — 



For the heat of a hydrogen-flame, the transmission through a 

 perfectly transparent plate of rock-salt was 



82-3 per cent. 



For a spiral of platinum wire heated to whiteness by an electric 

 current, the transmission was 



87 per cent. 



For the same spiral lowered to bright redness, the transmission 

 was 



84*4 per cent. 



For the same spiral lowered to moderate redness, the transmis- 

 sion was 



83*6 per cent. 



Nothing was changed in these experiments but the heat of the 

 spiral ; the direction of the rays, and the size of the radiating 

 body, remained throughout the same; still we find a gradually 

 augmenting opacity on the part of the rock-salt as the tempera- 

 ture of the source is lowered. There cannot, I think, be a 

 doubt that MM. De la Provostaye and Desains are right in their 

 conclusion that rock-salt acts differently on different calorific 

 rays, and is not, as Melloni supposed, equally transparent to all. 

 For the heat of the hydrogen-flame it is more opake than for 

 that of the moderately red spiral. 



§ XIV. Connexion between radiation and conduction. 



This memoir ought perhaps to end here ; I would, however, 

 ask permission to make a few additional remarks on a subject 

 which was briefly touched upon towards the conclusion of the 

 first of this series of memoirs. I make these remarks with diffi- 

 dence, for I have reason to know that authorities for whom I 

 entertain the highest respect do not share my views regarding 

 the connexion which subsists between the radiation and conduc- 

 tion of heat. 



Let us suppose heat to be communicated to the superficial 

 stratum of the molecules of any body ; say, the molecules at the 

 extremity of a metal bar. They vibrate, and the motion com- 

 municated by them to the external aether is dispatched in waves 

 through space. The vibrating superficial molecules must also 

 set in motion the aether within the body, and a portion of this 

 motion will be transferred to the stratum of molecules next 

 adjacent to the superficial ones, heat as a consequence appearing 

 to penetrate the mass. But irrespective of the aether, the mole- 



