RADIATION. 



183 



which could pass through glass and the other which could 

 only pass with difficulty : the effect of introducing the first 

 piece of glass was to cut off one kind of heat. This subject 

 was fully investigated in a splendid series of researches by 

 Melloni ; he was led to take up these researches by asso- 

 ciating himself with Nobili, an Italian, who has the merit 

 of inventing this beautiful instrument called the thermo- 

 pile, an instrument which renders experiments in radiant 



FIG. 3. 



heat practicable in many cases where they would not be so 

 without it. I will not waste time in describing the prin- 

 ciple of the thermopile, except to say briefly that there are 

 a number of bars of antimony and bismuth joined together 

 in pairs alternating at the two ends. It had been found 



