176 FRAGMENTS OF SCIENCE. 



experiments is a plate of copper, against the back of which 

 a steady sheet of flame is permitted to play. On emerging 

 from the copper, the waves, in the first instance, pass 

 through a space devoid of air, and then enter a hollow 

 glass cylinder, three feet long and three inches wide. The 

 two ends of this cylinder are stopped by two plates of rock- 

 salt, this being the only solid substance which offers a 

 scarcely sensible obstacle to the passage of the calorific 

 waves. After passing through the tube, the radiant heat 

 falls upon the anterior face of a thermo-electric pile, 1 which 

 instantly applies the heat to the generation of an electric 

 current. This current conducted round a magnetic needle 

 deflects it, and the magnitude of the deflection is a measure 

 of the heat falling upon the pile. This famous instrument, 

 and not an ordinary thermometer, is what we shall use in 

 these inquiries, but we shall use it in a somewhat novel 

 way. As long as the two opposite faces of the thermo- 

 electric pile are kept at the same temperature, no matter 

 how high that may be, there is no current generated. The 

 current is a consequence of the difference of temperature 

 between the two opposite faces of the pile. Hence, if after 

 the anterior face has received the heat from our radiating 

 source, a second source, which we may call the compensat- 

 ing source, be permitted to radiate against the posterior 

 face, this latter radiation will tend to neutralize the former. 

 When the neutralization is perfect, the magnetic needle 

 connected with the pile is no longer deflected, but points to 

 the zero of the graduated circle over which it hangs. 



And now let us suppose the glass tube, through which 

 pass the waves from the heated plate of copper, to be ex- 

 hausted by an air-pump, the two sources of heat acting at 

 the same time on the two opposite faces of the pile. Per- 

 fectly equal quantities of heat being imparted to the two 



1 In the Appendix to the first chapter of " Heat as a Mode of Motion," 

 the construction of the thermo-electric pile is fully explained. 



