426 KNOBLAUCH ON RADIANT HEAT. 



less than those observed with red-hot platinum (see p. 404 and 

 405), it is evident that a still less number of different rays of heat 

 emanate from the flame of alcohol than from heated platinum. 

 Lastly, when we see how the heat of the cylinder heated to 212° F. 

 fails to exhibit the slightest differences from whatever surface it 

 may be reflected (see p. 406-409), which in the instances pre- 

 viously considered produced very considerable alterations, we 

 must admit that the metallic cylinder at the above temperature 

 emits a single kind of radiant heat only. 



Thus if the sources of heat used in the previous investigation 

 be compared in respect to this point, 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 en- 

 tirely disappeared with the cylinder heated to 212° F. 



There is a means of testing this in a different manner. 



Thus if a number of heterogeneous rays of heat, as e. g. are 

 emitted by an Argand or a Locatelli's lamp, be made incident 

 upon different diathermanous bodies, different kinds of rays pass 

 through them according to the nature of the substances. There- 

 fore these, according as they appear in the one or the other of 

 them, permeate a second diathermanous plate in a different 

 manner. 



The differences thus found will evidently be so much greater 

 the more varied the original source of heat is. But if only one 

 kind of rays of heat were allowed to enter these substances, so 

 that only one could pass through them, on a second transmission 

 they would not yield any differences from whatever diatherma- 

 nous bodies the heat issued. 



Now if it is found that the heat radiating from a cylinder 

 heated below 234° F. constantly permeates red glass, blue glass, 

 alum, rock salt, calcareous spar and gypsum in the same manner 

 whether it issues immediately from the source of heat, or whether 

 it has previously passed through ivory, post paper, a thin layer of 

 carmine, lac, black glass, white glass, or any other diathermanous 

 substance (see Table XVIII.), this is a new proof that one kind 

 of rays of heat only is emitted by the cylinder*. 



* As in the previous experiments the heat diffusely reflected by various 

 bodies exhibited differences when the tempei-atnre exceeded 234° F. (see the 

 note, p. 407), in this instance also, on surpassing this limit, differences were 

 apparent. 



For whilst the heat emitted below 234° F., on inserting the red glass, con- 



