Intelligence and Miscellaneous Articles. 397 



Hence, for one and the same source of heat, the proportion of rays 

 which pass (eliminating a constant quantity of heat which falls 

 directly on the plate) diminishes as the distance from the source 

 increases, — and the more rapidly the greater the diffusive power 

 of the screen. 



7. By conveniently arranging the experiment, it is possible to 

 cause this transmission in greater abundance of the rays from a source 

 at 100°, compared with those of a lamp (mentioned in No. 1), to dis- 

 appear, and even, conversely, to bring about a greater transmission 

 of the heat of the lamp. 



III. 



1 . In the passage of calorific rays through rock-salt covered by 

 lampblack, an elective absorption (suspected by Melloni) is pro- 

 duced. A diffusive effect (supposed by Forbes) is never produced 

 by the rough surface of the layer of lampblack, but sometimes in 

 consequence of a tarnishing of the plate of salt during its being 

 blackened. 



2. In the transmission of rays through thin metallic layers depo- 

 sited upon glass, an elective absorption is produced without diffusion. 



3. The most certain manner of recognizing the existence of an 

 elective absorption during the transmission, is to see if the heat before 

 and after its passage through the substance in question retains un- 

 altered its faculty of traversing other diathermanous substances 

 (limpid and polished), or if this faculty is modified. 



4. The best means of recognizing an action of diffusion are the 

 following : — 



(a) If solar heat is made to pass through the screen examined, 

 and the rays transmitted are compared with direct rays, either the 

 two pencils pass through colourless rock-salt in the same proportion, 

 or the first pencil preponderates over the second. In the latter case 

 the plate operated upon is diffusive. 



(b) If, of two identical thermal pencils, one of which consists of 

 parallel and the other of diffused rays, the latter passes most readily 

 through the substance examined, this substance is diffusive. This 

 method gives also a means of comparing the different degrees of 

 diffusion (within wide limits). 



IV. 



1 . (a) In general, when the angle formed by the calorific rays 

 with a rough or turbid plate diminishes, the diffusion produced 

 increases. This increase with the change in inclination, in the 

 first place becomes larger with the generally diffusive property of 

 the screen, but then again grows less to such a degree that in very 

 rough and sufficiently cloudy plates, just as in the case of clear ones, 

 no difference can be detected in the behaviour of rays which are 

 transmitted at different angles of inclination. 



(b) On the contrary, the diffusion produced by reflexion from an 



