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Article III. 



On the Dependence of Radiant Heat in its passage throughCry- 

 stals upon the direction of transmission. By H. Knoblauch, 

 Professor of Natural Philosophy in the University of Marburg, 



[From PoggendorfTs Annalen, 1852, No. 2.] 



1 HE quantity of radiant heat which passes through diather- 

 manous bodies, depends, as is known, upon the nature of the 

 latter. Melloni was the first to raise the question, whether, in 

 one and the same body — in a crystal for instance — the quantity 

 transmitted was different along the different axes. In the results 

 arrived at by Melloni and myself while experimenting with 

 transparent rock-crystal, and with calcareous spar, no difference 

 of the kind was detected. 



Notwithstanding this, I have been induced to extend the 

 inquiry to other crystals. A brown specimen of quartz was first 

 subjected to experiment. It was a complete cube, having four 

 of its sides parallel to the axis of the crystal, and the remaining 

 two perpendicular to the same. The sun^s rays appeared to be 

 peculiarly well adapted to experiments of this nature, inasmuch 

 as they offer the double advantage of being parallel among them- 

 selves and of possessing the highest possible intensity. 



By means of the reflector of a heliostat, a sheaf of rays was 

 thrown into a dark room ; the direction of the sheaf, notwith- 

 standing the changing position of the sun, did not materially 

 alter during the time of observation. The rock-crystal was 

 placed in the path of the rays, being fixed upon a disc which 

 permitted of a rotation of 90° in a horizontal plane. The heat, 

 after having passed through the cube, was permitted to fall upon 

 the square forward surface of a thermo-electric pile, and the 

 consequent excitation was measured by a multiplier connected 

 with the pile. It is scarcely necessary to remark, that all inci- 

 dental heat was carefully shut out, and those rays only were 

 allowed to fall upon the pile which had first traversed the mass 

 of the crystal. 



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