38 FRAGMENTS OF SCIENCE. 



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

 instantly converts the heat into an electric current. 

 This current conducted round a magnetic needle de- 

 flects 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 enquiries, 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 

 a 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 compensating source, be 

 permitted to radiate against the posterior face, this 

 latter radiation will tend to neutralise the former. 

 When the neutralisation 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 the waves from the heated plate of copper are 

 passing, to be exhausted by an air-pump, the two 

 sources of heat acting at the same time on the two 

 opposite faces of the pile. When by means of an ad- 

 justing screen, perfectly equal quantities of heat are 

 imparted to the two faces, the needle pointstto zero. 

 Let any gas be now permitted to enter the exhausted 

 tube ; if its molecules possess any power of intercepting 

 the calorific waves, the equilibrium previously existing 

 will be destroyed, the compensating source will triumph, 



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

 Motion,' the construction of the thermo-electric pile is fully ex- 

 plained. 



