Radiation of Heat by Gases and Vapours. 173 



action. In my instrument, for example, the quantity of heat 

 necessary to move the needle from 60° to 61° is about twenty 

 times that required to move it from 11° to 12°. Now in the 

 case of the small deflections above referred to, the needle was, it 

 is true, in a sensitive position; but then the total amount of 

 heat passing through the tube was so inconsiderable that a small 

 per-centage of it, even if absorbed, might well escape detection. 

 In the case of the large deflections, on the other hand, though 

 the total amount of heat was large, and though the quantity 

 absorbed might be proportionate, the needle was in such a posi- 

 tion as to require a very considerable abstraction of heat to pro- 

 duce any sensible change in its position. Hence arose the 

 thought of operating, if possible, with large quantities of heat, 

 while the needle intended to reveal its absorption should con- 

 tinue to occupy its position of maximum delicacy. 



The first attempt at solving this problem was as follows : — My 

 galvanometer is a differential one — the coil being composed of 

 two wires wound side by side, so that a current could be sent 

 through either of them independent of the other. The thermo- 

 electric pile was placed at one end of the tin tube, and the ends 

 of one of the galvanometer wires connected with it. A copper 

 ball heated to low redness being placed at the other end of the 

 tube, the needle of the galvanometer was propelled to its stops 

 near 90°. The ends of the second wire were now so attached 

 to a second pile that when the latter was caused to approach the 

 copper ball, the current thus excited passed through the coil in 

 a direction opposed to the first one. Gradually, as the second 

 pile was brought nearer to the source of heat, the needle de- 

 scended from the stops, and when the two currents were nearly 

 equal the position of the needle was close to zero. 



Here then we had a powerful flux of heat through the tube ; 

 and if a column of gas four feet long exercised any sensible 

 absorption, the needle was in the position best calculated to 

 reveal it. In the first experiment made in this way, the neutral- 

 ization of one current by the other occurred when the tube was 

 filled with air ; and after the exhaustion of the tube had com- 

 menced, the needle started suddenly off in a direction which 

 indicated that a less amount of heat passed through the partially 

 exhausted tube, than through the tube filled with air. The 

 needle, however, soon stopped, turned, descended quickly to zero, 

 and passed on to the other side, where its deflection became per- 

 manent. The air made use of in this experiment came direct 

 from the laboratory, and the first impulsion of the needle was 

 probably due to the aqueous vapour precipitated as a cloud 

 by the sudden exhaustion of the tube. When, previous to its 

 admission, the air was passed over chloride of calcium, or 



