5 8 (22) 



examining the methods employed in these experiments 

 somewhat in detail. Inasmuch as our methods for 

 measuring minute amounts of electricity are very perfect, 

 and the methods for the conversion of heat into elec- 

 tricity are equally delicate, it has been found that smaller 

 differences of temperature may be recognized by con- 

 verting the heat into electricity, than can be detected 

 thermometrically. The apparatus, first used by Melloni 

 in i832,3 2 is very simple, consisting first, of a pair of 

 metallic bars like those described in the early part of 

 the lecture, for effecting the conversion of the heat ; and 

 second, of a delicate galvanometer, for measuring the 

 electricity produced. In the experiments in question 

 one of the bars used was made of bismuth, the other 

 of an alloy of antimony and zinc.33 Preliminary trials 

 having shown that any change of temperature within 

 the skull was soonest manifested externally in that de- 

 pression which exists just above the occipital protuber- 

 ance, a pair of these little bars was fastened to the head 

 at this point ; and to neutralize the results of a general 

 rise of temperature over the whole body, a second pair, 

 reversed in direction, was attached to the leg or arm, so 

 that if a like increase of heat came to both, the elec- 

 tricity developed by one would be neutralized by the 

 other, and no effect be produced upon the needle unless 

 only one was affected. By long practice it was ascer- 

 tained that a state of mental torpor could be induced, 

 lasting for hours, in which the needle remained sta- 

 tionary. But let a person knock on the door outside 

 the room, or speak a single word, even thc-ugh the ex- 

 perimenter remained absolutely passive, and the recep- 

 tion of the intelligence caused the needle to swing 



