176 LECTURES TO SCIENCE TEACHERS. 



by hand or by other mechanism, and force can be applied to 

 the disc without affecting the vacuum at all. There is also 

 an instrument which I will describe presently, called the 

 thermopile, and by an ingenious arrangement of a baro- 

 meter-tube a motion can be given to the thermopile without 

 affecting the vacuum These gentlemen experimenting with 

 this apparatus exhausted the receiver of air* as completely 

 as it was possible to do, and then put the disc into very 

 rapid rotation for a considerable time, and testing it with a 

 thermopile, it was found to be heated. I cannot go into 

 the precautions which they took to make sure that this 

 heat was not due to friction or to the resistance of the air, 

 and so forth ; but they took careful account of these things, 

 measured the amount of these sources of heat as far as 

 possible, and allowed for it, and still there was outstanding 

 an amount of heat which could not be accounted for except 

 on the assumption that it was produced by the friction of 

 the ether. 



Now the series of researches which we illustrate to-day 

 is one which connects the undulatory theory of light with 

 radiations generally, and with heat radiations in particular. 

 The first experiments which proved that there was some 

 analogy between heat and light were probably made by 

 Porta in the sixteenth century. He employed mirrors to 

 reflect heat, arranged as you see them here. He was able 

 to converge the heat to a focus, just as you can converge 

 light to a focus, and this experiment was repeated by Pictet 

 at the beginning of the present century. Pictet employed 

 the very mirrors which you see before you. He so arranged 

 them that the light placed in the focus of one mirror was 

 seen to throw an image of itself by the double reflection 

 from these two mirrors upon a certain definite point. 

 Having found that these were in the luminous focus, all that 

 remained was to show that they were in the heat focus 

 also ; and in order to do this Pictet employed a freezing 

 mixture, and showed not only that the heat was reflected, 

 but apparently that the cold was reflected too. If we 

 place a freezing mixture in the focus of one mirror we shall 

 probably be able to see that the air thermometer in the 

 focus of the other shows a sensible production of cold. But 

 that does not mean really that cold is radiated from the 

 freezing mixture, but that heat is being radiated from the 





