Mr Ritchie on Radiant Heat. 139 



surface. This attraction will evidently act as an antagonist 

 force to the tendency which the molecules of heat have to ra- 

 diate from the surface. The flow of caloric will therefore be 

 less than if no such counteracting force existed. By the addi- 

 tion of several coats the exterior film of heat is removed beyond 

 the sphere of attraction of the metal, and hence an increase of 

 radiant heat will be the necessary consequence. There is also 

 another circumstance which must materially aid the rapid dis- 

 persion of heat, by the application of additional coats to the 

 metallic surface. It is obvious that in soft or porous bodies ra- 

 diation goes on not only from the surface, but also from some 

 depth below it. In this observation I am supported by the 

 high authority of M. Biot, who seldom risks, an hypothesis that 

 is not well founded. That accurate and profound philosopher 

 concludes : " Car si, comme tout Findique, le rayonnement 

 n'emane pas seulement de la surface, mais aussi d'une petite pro- 

 fondeur dans Pexterieur des corps, cette profondeur devra aug- 

 menter a mesure que la temperature s'elevera, puisque la ma- 

 tiere qui forme le corps deviendra plus permeable aux rayons 

 calorifiques, et cette double circonstance devra produire un 

 rayonnement plus abondant."* Now, if radiation takes place 

 at a greater depth below the surface than the thickness of a 

 single pellicle of gold-beater's skin, a much greater quantity of 

 caloric will radiate from a metallic body having two or three 

 such coatings than only one. The same thing will obviously 

 take place by using coatings of oil or jelly of different degrees 

 of thickness. 



But the most formidable objection to the theory of aerial 

 pulsation is derived from my experiments with thin liquid 

 sci*eens, published in the last volume of the Philosophical 

 Transactions. "^ If heat be carried along on the wings of the 

 ambient air, the aerial wave will be as completely stopped by 

 the thinnest film of a viscous fluid as by an opaque screen of 

 considerable thickness. It can only communicate its heat to 

 the liquid film, which in its turn will act as a new source of 

 radiant heat. But the effect on the thermometer was proved 

 to be instantaneous, and such as could not possibly arise from 



• Traite de Physique, tome iv. 642. 

 t See this Journal, No. xiv. p. 348. 



