370 RADIANT HEAT. 



heating rays emanating' at the same time from the source, some lumi- 

 niferous, and some not so, it is in no way a matter of surprise, or an ex- 

 ceptional case, that the transmissive power of rock salt, or of any 

 other substance, should bear no proportion to the mere temperature of 

 the source. 



M. Knoblauch, however, instituted an elaborate set of experiments 

 to ascertain whether any such relation could be maintained. 



The experiments were all conducted by means of the thermo-mul- 

 tiplier, which in this instance was constructed with especial precau- 

 tions to insure extreme accuracy and sensibility. 



M. Knoblauch's first series of experiments included, as sources, al- 

 cohol flame, incandescent platinum, hydrogen flame, Argand lamp, of 

 which the temperatures were in the order of enumeration, the first 

 being the highest. The effect of each on the thermo-multiplier was 

 observed with the intervention of a series of screens, colored glass, 

 alum, mica, colorless glass, calcspar, gypsum, <fec. The transmitted 

 effects varied, of course, with the different screens; but in every in- 

 stance they were smallest with the first source, and increased in the 

 order of enumeration, or in the inverse order of the temperatures. — 

 (Table I.) 



In this series the real nature of the results is, in fact, evident from 

 the distinctions above drawn. The effect is simply dependent on the 

 light, or heating power of Species II, mixed, no doubt, to a certain 

 extent in some cases with that of Species I; which last probably doc* 

 bear a close proportion to the temperature of the source, but is in 

 these instances overruled by the effect of Species II being very feebly, 

 or not at all, transmitted by the screens. 



In this series also M. Knoblauch found that the transmission through 

 rock salt was not exactly equal for all the sources, contrary to the as- 

 sertion of Melloni; the difference, however, is very small. 



In the second series the sources were a vessel of hot water of dif- 

 ferent temperatures, from 93° to 212°, the radiating side being in 

 each instance covered successively with lamp-black, glass, wool, and 

 in each instance the thermo-multiplier being placed at a greater dis- 

 tance, in the ratio of the increased temperature, so that the effects of 

 direct radiation were equalized. In each instance, then, a series of 

 screens (the same as before) were interposed, each screen transmitting 

 t different amount of heat, but the results with each temperature be- 

 ing found equal. — (Table II.) 



In tho third series the sources were hollow cylinders of iron and of 

 copper closely surrounding the flame of a lamp, and heated by it to 

 several temperatures, from 234° Fahrenheit up to a little below red- 

 ness; in each instance the same equalization was effected as in the 

 last series, and the same series of screens gave varied effects for each 

 screen, but equal effects for each temperature, as in the last series. — 

 (Table III.) 



The fourth series included platinum in successive stages of heat : 

 1st, dark; 2d, just red; 3d, yellow; 4th, partly white. In each in- 

 stance the series of screens was applied. In the 1st, 2d and 4th stages 

 of heat these gave uniform results, increasing with the heat; but in 



