Radiant Heat, and its Conversion thereby into Sound. 439 



The first point of importance established in 1859, and 

 developed in the. memoir just mentioned, was that already 

 referred to, namely the fact of absorption and large differences 

 of absorption. The second point — destined, I think, to throw 

 light on the deeper problems of molecular physics, was the 

 proof, that while elementary gases offered a scarcely sensible 

 impediment to radiant heat, equally transparent compound 

 gases exhibited, in many cases, an energy of absorption com- 

 parable to that of the most athermanous solids and liquids. 

 Determining, for example, the action of a mechanical mixture 

 of two elementary gases, it was proved that without altering 

 either the quantity of matter, or its perfect transparency to 

 light, the absoi*ption of invisible heat might be increased 

 many hundredfold by the passage of the constituents of the 

 mixture into a state of chemical combination. 



A similar deportment may be detected in liquids and solids. 

 The quantity of iodine vapour generated at ordinary tempe- 

 ratures is so small that its action on radiant heat is, as might 

 be expected, insensible. But iodine itself, when liquefied by 

 a powerful solvent, behaves as an almost perfectly transparent 

 body to the obscure calorific rays, even when it is able to 

 extinguish totally the light of the sun. Liquid bromine is 

 also highly diathermanous. The same may be said of phos- 

 phorus. In Melloni's table Sicilian sulphur comes next to 

 rock-salt in transmissive power. A concentrated solution of 

 sulphur in bisulphide of carbon exerts no sensible action on 

 radiant heat. By fusing together iodine and sulphur Pro- 

 fessor Dewar has produced a " ray-filter " which separates 

 Math extreme sharpness the visible from the invisible rays. 

 The remarkable diathermancy of certain specimens of vul- 

 canite, brought to light in the experiments of Mr. Graham 

 Bell and Mr. Preece, is probably due to the sulphur they 

 contain. Melloni showed that lampblack is to some extent 

 diathermanous. But when a suitable source of heat is chosen, 

 lampblack proves far more pervious to radiant heat than 

 Melloni found it to be. An opaque layer of this substance 

 transmits 41 per cent, of the radiation from a hydrogen-flame. 

 Were the lampblack optically continuous, the transmission 

 would, doubtless, be still greater. An opaque solution of 

 iodine transmits 99 per cent, of the radiation from the same 

 source; while a layer of pure water, 0-07 of an inch in thickness, 

 transmits only 2 per cent, of the radiation from a hydrogen- 

 flame. Such results indicate that a profound change in the 

 relation of ponderable matter to the luminiferous ether ac- 

 companies the act of chemical combination. 



One of my principal aims in the Bakerian Lecture of 1861 



