374: RADIANT HEAT. 



Thus, to determine whether ivory, e. g., is really diathermanous; 

 the source of heat is a known adiathermanous substance kept heated 

 by a lamp; the effect is observed; a known diathermanous screen is 

 then interposed and the effect again observed; the ivory is then sub- 

 stituted for the diathermanous body, and the direct effect equalized to 

 the former; the same screen is then interposed, but now a great 

 effect is transmitted. It follows that part of the original heat is 

 transmitted directly by the ivory, along with that radiated from it; or 

 the ivory is diathermanous. 



Section V, on "the Comparison of the amount of Heat diffusely 

 reflected by different bodies," refers to that kind of irregular reflec- 

 tion, or dispersion (as it has been sometimes called) of the rays from 

 the roughened, or at least unpolished surfaces of bodies, and which. 

 is distinguished from regular reflection, which is governed by the law 

 of equal angles of reflection and incidence by occurring equally at all 

 angles. And the object is stated by the author to be the determina- 

 tion "whether heat, or diffuse reflection, experiences changes in its 

 properties which distinguish it from that which is not reflected.'' — 

 (384.) 



The heat being incident on a rough surface is, of course, partly 

 absorbed and radiated again; to guard against error from confounding 

 this with the proper reflected heat, various and careful precautions 

 were adopted. 



The author then proceeds to detail the observations, which are of 

 voluminous extent, and the results recorded in a long series of tables. — 

 (Tables XXII to XXXII, inclusive.) 



In all this first series the source employed was an Argand lamp 

 without its chimney, and in all cases the mode of operating was 

 similar. 



The unpolished surface under examination was exposed to the rays 

 of the lamp at different distances and at different inclinations, and the 

 direct effect noted; the experiment was then repeated with the inser- 

 tion of a series of variously diathermanous screens. 



The substances used as reflectors were extremely varied; such as 

 bodies agreeing in one property and differing in another, or totally 

 homogeneous, or totally heterogeneous. 



Thus an immense range of substances of animal, vegetable, and 

 mineral origin, opaque or transparent, of all colors and textures, were 

 examined, and the agreements or discordances, according to their 

 various properties, were found extremely varied and curious. 



By these results, the author observes, "it is placed beyond all 

 doubt that heat, on diffuse reflection, is very differently modified by 

 some bodies to a great extent, while by others it is unchanged." — 

 (400.) 



And these results completely confirm the position already advanced, 

 "that the transmission of heat through diathermanous media depends 

 solely upon the nature of these bodies, by virtue of which they trans- 

 mit some rays more easily than others." — (402.) 



In a second series the same subject is continued with reference to 

 different sources of heat, which were, besides the lamp, platinum at a 



