and Diathermancy of Alt and llydroyen. 



417 



diathermanous ; and it was perfectly justifiable to trust such an 

 authority. But Melloni was not aware that air absorbs a con- 

 siderable proportion of heat-rajs : he, indeed, regarded it as 

 diathermanous, because he had observed that rays, after once 

 entering air, lose nothing after traversing a distance of several 

 metres. He never compared the diathermancy of air with 

 that of a vacuum, nor did he try to send rays from a vacuum 

 into rock-salt. Thus it might easily occur that rock-salt, 

 though it allowed all rays coming from air to pass, neverthe- 

 theless absorbed a portion of rays issuing from their source 

 directly through the vacuum. 



To solve this question experimentally, my apparatus required 

 only a slight alteration. The soldered junction of the thermo- 

 pile, which was at 45 millims. distance below the bottom of 

 the brass vessel, was surrounded with a jacket of thin sheet 

 brass ; this jacket, open at both ends, rested on the plate of 

 the air-pump, and was provided at its upper end, which was 

 about 20 millims. above the level of the soldered junction, 

 with a circular opening about 44 millims. wide, in which a 

 plate of clear rock-salt could be inserted. This plate, 3 mil- 

 lims. thick, had been cut and carefully polished by M. Steeg 

 in Homburg v. d. H. 



The following experiments were made with air at 4*2 mil- 

 lims. tension : — 



Table VIII. 



Opening. 



T. T. 



t. 



T, 



z. 



7 

 7 



7 

 7 

 7 



100 r 



t 



500 

 526 



34-9 

 26-3 

 300 



Free 



/700 



167-0 



r89-0 

 <! 70-0 

 ' 32() 



470 

 450 



580 

 47-5 

 230 



40-7 

 38-4 



51-3 

 415 

 170 



20-3 

 20-2 



17-8 



10 9 



5 1 



Covered with rock- 

 salt 





These experiments confirm the hypothesis previously stated. 

 They show that less than 60 per cent, of the rays from the hot- 

 water vessel pass through a plate of clear rock-salt only 3 mil- 

 lims. in thickness. A very considerable proportion of these 

 rays is thus retained by the rock-salt. 



The question still remained, how far air and rock-salt agree 

 in their thermal colours. The following series of experiments 

 give us a solution of this problem. In every case the circular 

 opening was covered with the rock-salt plate, and the interior 

 of the glass cylinder was lined with pasteboard, 



Phil. Mag, S. 5. Vol L No. 27. Dec, 1877. 2 E 



