Prof. Magnus on Thermal Radiation. 59 



and of other like substances, does not arise from the particles de- 

 tached at a glowing heat. 



The radiating power of these particles is at most very small, 

 in fact much less important than is stated in the " Note on the 

 Constitution of the Sun " above referred to. New experiments 

 have shown that when all precautions are taken, so that none of 

 the rays proceeding from solid bodies which do not belong to 

 the flame can reach the pile, the luminous soda-flame scarcely 

 radiates more than the non-luminous flame. The particles of soda 

 in the flame suffice, it is true, to increase its luminosity ; but 

 their mass is too small to augment to any great extent the radia- 

 tion of heat. 



Although these particles are entirely absent at a temperature 

 of 100°, the radiation of sodium relative to that of platinum is 

 just as great w r hen both bodies have this temperature as it is 

 when they are both raised to a glowing heat. 



Moreover the radiation of platinum itself varies greatly. 

 When a smooth platinum plate is covered with spongy platinum, 

 either by applying strong heat after pouring a solution of am- 

 mouio-platiuum on the plate, or by precipitating platinum 

 thereon by a galvanic current, its radiating power becomes so 

 great as to equal, and sometimes even to exceed that of sodium. 

 In the experiments with a Bunsen's flame we might attribute 

 this augmented radiation to the fact that the spongy platinum, 

 owing to its porosity, assumes a higher temperature than does 

 the smooth plate with which the former is compared ; the fact, 

 however, is, that even at a temperature of 100° a plate covered 

 with spongy platiuum radiates more heat than a smooth one, and 

 that in the same ratio as when the two are heated in a flame. 

 At 100°, however, when the posterior surface of the plate is 

 warmed, the spongy platinum can in no case be warmer than 

 the plate from which it receives its heat. 



When the spongy platinum is submitted to the pressure of a 

 polishing-iron its radiating power diminishes, and this radiation 

 continues to diminish according as the sponginess is caused more 

 and more to disappear by pressing, hammering, or in any other 

 manner increasing the density. This circumstance, however, 

 cannot be regarded as a proof that the radiation depends upon 

 the density rather than upon the roughness ; for as the density 

 of the spongy platinum increases, the roughness of its surface 

 diminishes. 



The radiation of so-called platinum-black appeared on this 

 account worthy of examination. This substance, as is well 

 known, consists of platinum solely ; its molecular constitution, 

 however, differs from that of spongy platinum, its particles 

 being far more finely distributed. This substance, which can 



