1879.] B u ff' s Experiments on the Diathermancy of Air. 17 



-which makes the transmission of a plate of rock-salt 2'6 millims. thick 

 92" 3 per cent, of the total radiation is, in my opinion, much nearer the 

 truth than that of Professor Buff. Be that as it may, I can affirm 

 that in my case the transmission through plates of rock-salt of five 

 times the thickness of that employed by Professor Buff amounts to 

 80 per cent, of the entire radiation. Subtracting from the 20 per cent, 

 intercepted, 8 per cent, due to reflection at the two surfaces of the salt, 

 we have 12 per cent, remaining for absorption * 



Were the thermal colours of air wholly, instead of partially, as 

 affirmed by Professor Buff, identical with those of rock-salt, the 

 coincidence would fail to account for the high absorption which he 

 ascribes to air. But let us examine whether the alleged coincidence 

 exists at all. To treat this question exhaustively, we must be able 

 to compare the transmission of rock-salt in vacuo with its transmission 

 in air. The apparatus employed for this purpose is rendered in- 

 telligible by the rough section, fig. 2. From the cube C the experi- 



R 



mental tube extends to the plate of rock-salt t, which closes the tube. 

 Midway, a rectangular chamber B is introduced, large enough to 

 permit of one of my plates of rock-salt s to be raised and lowered 

 within it. It is soldered on to the experimental tube, being ex- 

 hausted when the tube is exhausted. The plate of salt s is clasped 

 by a ring attached to the rod R, which moves air-tight through a 

 stuffing-box in the bottom of the chamber B. The plate of salt s is 

 2} inches in diameter ; while across the experimental tube, right and 

 left of B, are diaphragms with apertures 1£ inches in diameter. When 

 the salt is pushed up, it comes between these apertures, the rays of 

 heat passing through the central portion of the salt. The thermo-pile, 

 with its two cones, is placed within the chamber P, while the water- 



* In the case of the second plate of salt we have to deal with surface reflection 

 merely, the heat having been sifted by the first plate. 



VOL. XXX. c 



