384 KNOBLAUCH ON RADIANT HEAT. 



With this is connected the fact, that a number of dissimilar 

 rays of fieat are not altered by it in their properties, e. g. in their 

 capability of passing through certain diathermanous bodies, as 

 Melloni has shown in his experiments; he used well-polished 

 metallic mirrors for this purpose. 



Observations on the diffuse reflexion occurring upon rough 

 surfaces were first made by Herschel and Leslie; they could 

 not however lead to accurate results, because in them the heat 

 emitted by these surfaces themselves was not separated from the 

 reflected heat. The diffusion was first accurately proved by 

 Melloni, M'ho protected the thermoscope from the rays of heat 

 emitted by the reflecting body itself by a glass screen, whilst 

 e. g. the heat of a flame diffusely reflected by a white plate, and 

 which passed through the glass, exerted a perceptible action upon 

 the instrument. 



As on reQexion at a definite angle, so also on diffuse reflexion, 

 the intensity of the reflected heat of course varies according to 

 the properties of the reflecting body and the structure of its sur- 

 face, a result which is evident from the phaenomena of absorption 

 which have been already detailed, to which the phaenomena of 

 reflexion are complementary. The magnitude of the angle of in- 

 cidence of the rays which reach the diffusely-reflecting surface, in 

 this case exerts but very slight influence upon the intensity of the 

 reflected rays. An important distinction from simple I'eflexion 

 consists in various kinds of heat being reflected in a different man- 

 ner by one and the same body. Melloni, to whom science is in- 

 debted for the great advances made in all these departments, dis- 

 covered this phsenomenon also. He observed that a white surface 

 reflected the heat of a Locatelli's lamp, according to whether it was 

 used with or without a glass chimney, as also that of red-hot plati- 

 num and copper heated to 752° F., with different degrees of in- 

 tensity. 



Metallic plates only, the surfaces of which are rough, reflect 

 heat from all soarces in an equal degree, whilst lamp-black ex- 

 hibits a scai'cely perceptible amount of diffusion in any. 



It has not hitherto been determined whether heat, on diffuse re- 

 Jlexion, ea-periences changes in its properties which distinguish it 

 from that tvhich is not reflected. 



I therefore instituted a series of experiments on this point, 



was found to diminish in proportion to the thickness of tlic layer applied ; a phre- 

 nomeiion which simply depends upon the fact, that heat is absorbed to a greater 

 extent by a diathermanous substance of greater thickness than by one of less. 



