and Conducting Power of Solid Bodies. ■ 163 



tween the temperature of the rod and the surrounding air as 

 would come with accuracy within the scope of the Newtonian 

 law of cooling, and this must be done with rods of indefinitely 

 small diameter, having their continuity unimpaired by any 

 holes bored into their substance. The thermo-electrical battery 

 appeared to me to be an instrument admirably adapted for 

 this purpose; and arranged in the manner which I shall pre- 

 sently describe, I hope that it will be found a much more ac- 

 curate measure for observing uncombined heat in solid bodies 

 than any other means that have yet been applied to that 

 purpose. 



The experiments were performed in the laboratory of Pro- 

 fessor Magnus, who was kind enough to lend me the necessary 

 apparatus, and to whose friendly guidance the successful re- 

 sult of the experiments is chiefly to be attributed. 



I found by several preliminary experiments, that the same 

 divergence in the needle of the multiplier was always attained 

 when the end of a thermo-electrical battery, consisting of but 

 few alternations, was connected in a similar manner with a 

 body of constant temperature, and pressed against it with the 

 same amount of force. It was always 2 to 2\ minutes before 

 the needle of the multiplier became stationary; the connec- 

 tion might then be continued for an indefinite time without 

 perceptibly affecting the position of the needle. In order to 

 secure perfectly uniform contact, which is hardly possible 

 with a battery composed of many alternations, I had one con- 

 structed of only two elements, bismuth and antimony, having 

 therefore but one soldered point of contact at each end. The 

 ends were filed off presenting facets^ so that each presented a 

 rectangular surface of 1*7 millimetre in length and 0*7 milli- 

 metre in breadth. The whole length of the bars was 36*3 

 millimetres; each bar was very thin, being 1*7 millimetre in 

 width and 1*0 millimetre thick. 



On a strong horizontal board, on which divisions had been 

 marked, three uprights were erected, bearing each a forked arm 

 in which were fixed two perpendicular glass rods drawn out 

 to a point and placed opposite to each other, between which 

 the metallic bars to be examined were clamped parallel to the 

 horizontal divided board, and at about 24 centimetres above it ; 

 a fourth upright at the end of the board served to fix securely, 

 with the help of a screw, the cool end of the rod during the 

 experiment. To obtain for a length of time a uniform source 

 of heat, hot water was used : the end of the rod to be heated, 

 passed through a cork into the water by an opening made in 

 the boiling vessel below the surface of the water. 



By means of two double polished brass screens, through 



N 2 



