1859.] on the Transmission of Heat through Gases. 157 



whether the removal of the air had any influence on the transmission 

 of the heat. No such influence showed itself — the needle remained 

 perfectly steady. A similar result was obtained when hydrogen gas 

 was used instead of air. 



Thus foiled, the speaker put his questions to Nature in the follow- 

 ing way : a source of heat, having a temperature of about 300^ O., was 

 placed at one end of the tube, and a thermo-electric pile at the other — 

 a large deflection was the consequence. Round the astatic needle, 

 however, a second wire was coiled, thus forming a so-called diflerential 

 galvanometer ; a second pile was connected with this second wire, so 

 that the current from it circulated round the needle in a direction 

 opposed to that of the current from the first pile. The second pile 

 was caused to approach the source of heat until both currents exactly 

 neutralised each other, and the needle stood at zero. Here then we 

 had two powerful forces in equilibrium, and the question now was 

 whether the removal of the air from the tube would disturb this balance. 

 A few strokes of the air-pump decided the question, and on the entire 

 removal of the air the current from the pile at the end of the tube 

 predominated over its antagonist from 40° to 50°. On readmitting the 

 air the needle again fell to zero ; thus proving beyond a doubt that the 

 air within the tube intercepted a portion of the radiant heat. 



The same method was applied with other gases, and with most 

 remarkable results. Gases differ probably as much among themselves 

 with regard to their action upon radiant heat as liquids and solids do. 

 Some gases bear the same relation to others that alum does to rock- 

 salt. The speaker compared the action of perfectly transparent coal- 

 gas with perfectly transparent atmospheric air. To render the effect 

 visible to the audience, a large plano-convex lens was fixed between two 

 upright stands at a certain height above a delicate galvanometer. 

 The dial of the instrument was illuminated by a sheaf of rays from an 

 electric lamp, the sheaf being sent through a solution of alum to sift it 

 of its heat, and thus avoid the formation of air-currents within the glass 

 shade of the instrument. Above the lens was placed a looking-glass, 

 so inclined that the magnified image of the dial was thrown upon a 

 screen, where the movements of the needle could be distinctly observed 

 by the whole audience. Air was first examined, the currents from the 

 two piles being equilibrated in the manner described, the tube was 

 exhausted, and a small but perfectly sensible deflection was the result. 

 It was next arranged that the current from the pile at the end of the 

 tube predominated greatly over its antagonist. Dry coal-gas was now 

 admitted into the tube, and its action upon the radiant heat was so 

 energetic, the quantity of heat which it cut off" was so great, that the 

 needle of the galvanometer was seen to move from about 80° on one 

 side of zero to SO*' on the other. On exhausting the tube the radiant 

 heat passed copiously through it, and the needle returned to its first 

 position. 



Similar differences have also been established in the case of vapours. 

 As representatives of this diverse action, the vapour of ether and of 

 bisulphide of carbon may be taken. For equal volumes, the quantity 



