of Heat by Roughness of the Surface. 449 



When a platinum plate was covered with a thin layer of spongy 

 platinum, by spreading a thin layer of ammoniochloride of plati- 

 num upon it and then strongly heating, without treatment with 

 nitric acid, it indicated seven times as much radiation as before 

 being treated with spongy platinum. Spongy platinum is looser 

 than the plate upon which it adheres ; but each individual par- 

 ticle is undoubtedly as hard as a particle of the heated plate. 

 Hence the action of the spongy platinum is due to its presenting 

 more points and corners. This is more probable from the fact 

 that the radiation of a plate covered with spongy platinum dimi- 

 nishes when it is frequently and continuously heated. It is 

 possible that at each fresh heating some of the sponge is de- 

 tached ; but in any case the extreme points and corners are 

 rounded off. Harder they cannot become. 



The author is of opinion that the increase in the radiation with 

 a roughened surface depends essentially on the refraction which 

 the heat experiences on its emergence from the surface of the 

 radiating body. He explains this influence for various forms of 

 the surface, and arrives thereby at the following conclusion. 

 The greater the refractive index of heat between the radiating 

 substance and air, the smaller is the radiation from the plane 

 surface, and then the quantity of heat reflected inwards increases. 

 The metals have doubtless a very high refractive index. Hence 

 they reflect the rays from without, and allow but few to penetrate ; 

 and hence they reflect internally those coming from the interior, 

 and allow but few to emerge. Great inequalities of the radiating 

 surface do not occasion any important alteration in the radia- 

 tion. Such a one only occurs when the radii of curvature are 

 very small, and change greatly, and when the radiating surface 

 has but little diathermancy. In general, the roughness of the 

 surface may effect both an increase and a diminution in the ra- 

 diation ; but if the inequalities are very fine and very deep, 

 there is almost always an increase in the case of substances 

 which are but slightly diathermanous, as the metals. When 

 there is a very fine powder of the same substance on the radiating 

 surface, the radiation is considerably increased, not merely from 

 substances which, like the metals, are but little diathermanous, 

 but also from very diathermanous ones, such as rock-salt. 



Phil. Mag. S. 4. Vol. 39. No. 263. June 1869. 2 G 



