Radiant Heat) and its Concersion thereby into Sound. 483 



from which wires pass to the battery. R shows the position 

 of the rock-salt lens, while one end of the experimental tube is 

 shown at T. The greatest care is here necessary to protect 

 the spiral from any agitation of the air. Such care, however, 



it is always in the experimenter's power to bestow. To secure 

 constancy in the radiation, the battery was charged three 

 times a day, the strength of the current being regulated by a 

 rheostat, and checked by a tangent-galvanometer. 



An experimental tube of the description shown in fig. 3 was 

 first employed by me about twelve years ago; and with it I 

 then verified all the experiments I had previously made on 

 the absorption of radiant heat by gases and vapours. The 

 experiments were not published in any scientific journal; but 

 they are thus referred to at page 394 of my volume of col- 

 lected memoirs, entitled i Contributions to Molecular Physics 

 in the domain of Radiant Heat :' — " The two ends of an ex- 

 perimental tube, 38 inches long and 6 inches in diameter, were 

 each provided with an aperture 2"6 inches in diameter. These 

 apertures were closed with plates of rock-salt. The source of 

 heat was a platinum spiral, well defended from air-currents, 

 and heated to redness by an electric current. In front of the 

 spiral was a rock-salt lens, which sent a slightly convergent 

 beam through the tube. Behind the most distant plate was 

 formed a sharply-defined image of the spiral, its size being- 

 such that it was wholly embraced by the plate of salt. Here, 

 then, was a beam of heat passing through an experimental 



