Prof. Tyndall on Recent Researches on Radiant Heat. 261 



through a vacuum into the experimental tube. To obtain that 

 chamber gave me great trouble : I had to unite its anterior wall 

 with silver solder to its sides ; and this, moreover, had to be done 

 for every special source of heat employed. I had to cause this 

 chamber to pass through a copper vessel, soldering it water-tight 

 at its place of entrance and of exit. This vessel I had to connect 

 by a tube 20 feet long with the water-pipes of the Institution, so 

 as to get a supply ; and to carry off the water, I had the stone floor 

 of the laboratory perforated, and one of our drains connected by 

 a second tube with the vessel. As already known, this vessel was 

 intended to prevent the heat of the source from reaching my first 

 plate of rock-salt. The introducing of this plate air-tight be- 

 tween the front chamber and the experimental tube was also a 

 difficult matter, which required special means to meet it. Now 

 let me ask what could have induced me to go to all this trouble ? 

 The obtaining of suitable plates of rock-salt has been one of my 

 greatest difficulties ; why then did I expend my time in seeking 

 for a pair of them ? Why did I not content myself with a single 

 plate to stop the remote end of my tube, and allow the latter to 

 form a continuous whole from the radiating surface to the re- 

 mote end ? Nay, why did I not abandon both plates, and simply 

 cement my pile air-tight into the remote end of my tube ? All 

 these devices passed through my mind and formed subjects of 

 experiment at an early stage of this inquiry. These experiments 

 taught me that by bringing the gas whose deportment I wished 

 to examine into direct contact with my source of heat, or into 

 direct contact with the face of my pile, I entirely vitiated my 

 results. And this arrangement, which in my case would have 

 been perfectly fatal as far as accuracy is concerned, is that which 

 Prof. Magnus has adopted, and is, I believe, the sole source of 

 the differences which have shown themselves between his results 

 and mine. 



His chief apparatus may be thus described* : — A glass vessel fits 

 like a receiver with its ground edge on the plate of an air-pump. 

 To the top of this receiver a second glass vessel is fused, and par- 

 tially filled with water. Into this water steam is conducted, which 

 causes the water to boil — a temperature of 100° C. being thus im- 

 parted to the bottom of the vessel, which is at the same time the 

 top of the receiver. On the plate of the air-pump a thermo-electric 

 plate is fixed with its face turned upwards, so as to receive the 

 radiation from the heated top of the receiver. The face of the 

 pile can be screened off at pleasure from the radiation from above. 

 From the pile, wires proceed through the plate of the air-pump 



* The apparatus itself is drawn, and a translation of the paper to which 

 it refers is published, in the Philosophical Magazine, vol. xxii. pp. 1, 81, 

 PL I. fig. 2. 



