37G RADIANT HEAT. 



reflections of luminiferous and calorific rays, are not perhaps expressed 

 with that clearness which might be wished. 



The subject of Section VI is " On the Sources of Heat;" which is 

 further explained to relate to the differences in the nature of the rays, 

 or in general the heterogeneity of the rays, emitted from one and the 

 same source at the same time; and the differences in this respect of 

 different sources. 



From the previous experiments the author concludes, in general, 

 that " the variety of the rays of heat emitted is greatest with the 

 Argand lamp, less with red-hot platinum, still less when the flame of 

 alcohol is used, and has entirely disappeared with the cylinder heated 

 to 212° Fahrenheit.' 7 — (426.) 



But he now proceeds to test and extend such conclusions in another 

 way, viz: by the differences exhibited by the rays in the different 

 action of diathermanous bodies upon them, according as they have in 

 the first instance passed through certain diathermanous bodies, or pro- 

 ceeded direct from the source. The differences thus exhibited give 

 increasing proofs of heterogeneity. 



And one more important point belonging to this inquiry he investi- 

 gated, by platinum heated to successive stages, (1) below 234°, (2) at 

 a red, (3) at a yellow, and (4) at a white heat; while in each case the 

 heat was reflected diffusely by various surfaces, and in every instance 

 intercepted by the same series of screens. — (Tables LII to LV.) 



From these he draws the conclusion that the differences which the 

 rays evolved at the successive stages exhibit after diffuse reflection, 

 on transmission through diathermanous media, are in every instance 

 greater at the stage (4) than at (3;) these greater than at (2,) and 

 these than at (1.) Or, in general, that the heat emitted by platinum 

 at these successive stages is successively more heterogeneous as we 

 advance from the lower to the higher. 



Again, with the same body it is not in the mere proportion of the in- 

 crease of temperature, as such; nor in different bodies does it follow 

 any proportion to the temperature (as before observed.) 



The author hints generally at the relation between these degrees of 

 heterogeneity and the differences in the nature of the luminous rays 

 emitted; but without laying down any very precise or clearly drawn 

 distinctions as to their characteristic properties. 



Transmission of Heat through Crystals. 



Melloni had raised the question whether in one and the same body, 

 in a crystal, for instance, the quantity of radiant heat transmitted was 

 different along the different axes. In experimenting on this subject, 

 in connexion with M. Knoblauch, he found that with transparent 

 rock crystal and with calcspar no difference of this kind was detected. 



In a memoir (in Poggendorff's Annalen, 1852, No. 2, translated in 

 Taylor's Foreign Memoirs, November, 1852, and February, 1853,) en- 

 titled "On the Dependence of Radiant Heat in its Passage through 

 Crystals on the Direction of Transmission," M. Knoblauch has pur- 

 sued the subject commenced by Melloni. 



