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LXIII. Intelligence and Miscellaneous Articles. 



ON THE PASSAGE OF RADIANT HEAT THROUGH ROCK-SALT, EITHER 

 POLISHED, ROUGH, OR COVERED WITH LAMPBLACK, AND ON 

 THE DIFFUSION OF CALORIFIC RAYS. BY H. KNOBLAUCH. 



Hj^HE following are the results at which Prof. Knoblauch has 

 A arrived after a complete investigation of the subject. 



I. 



1. Limpid and chemically pure rock-salt allows all kinds of calo- 

 rific rays to traverse it in equal proportions, whether the difference 

 in the rays arises from — 



(a) Having been diffusively reflected from different bodies ; 



(b) Having traversed different diathermanous bodies ; 



(c) Having emanated from different sources of heat. 



2. It has been confirmed that after this passage, which exerts the 

 same influence on all elementary rays, the maximum heat in the 

 solar spectrum of a rock-salt prism is in the obscure space beyond 

 the red ; within the visible part of the spectrum the calorific condi- 

 tions are the same for a prism of rock-salt and of flint glass. 



II. 



1. If rock-salt be used, whether unpolished or turbid, the calorific 

 rays from the sun pass in less quantity than those from an Argand 

 lamp, and the latter, in general, in less quantity than those from a 

 calorific source at 100° C. 



Increase of roughness enfeebles the passage of all kinds of rays ; 

 but it acts most strongly upon solar heat, less upon that of the lamp, 

 and still less upon that from a source of obscure heat. 



2. Eliminating the fact of elective absorption (thermocrose) of the 

 substance, the rough surface of unpolished glasses, and the internal 

 cloudiness of milky glasses exert corresponding effects. 



3. These phenomena are not to be attributed (as by Mr. Forbes) 

 either to an absorption unequally exerted on calorific rays of 

 different quality, or (as M. Melloni has done) to a dispersion by 

 the rough or turbid media, unequal in extent and dependent on the 

 calorific colour, the effect of which dispersion would be to deflect 

 more or less the rays of the thermoscope. Nor is the roughness of 

 the surface itself, or the direction of the rays emanating from a single 

 point, the determining condition. 



4. Heat diffused either by passing through rough or turbid screens, 

 or by reflexion from an unpolished surface, traverses diffusing screens 

 in proportions greater (a) as the rays are more diffuse, (6) as the 

 screens are more diffusive for parallel rays. 



5. In fact the really determining condition of the passage through 

 these screens is whether the incident rays are parallel, or more or less 

 variously radiated from a greater or less number of points. 



