PEIRCE. — THERMAL CONDUCTIVITY OP VULCANITE. 77 



the second column of which is given the number of calories required to 

 raise one gram of a certain kind of thin sheet rubber from 25° C. to 

 the temperatures given in the first column. The third column gives the 

 average specific heat between 25° C. and these tempei'atures. 



The average specific heat of another specimen of thin hard rubber be- 

 tween 25° C. and 100° C. was 0.370. 



Since hard rubber is an extremely poor conductor of heat, a long time 

 was required in the case of any prism for the final state to be reached 

 approximately. After about seven or eight hours, if the temperatures of 

 the hot and cold boxes were kept quite constant, the temperatures at all 

 points on the axis could generally be assumed to be within one twentieth 

 of a degree of their final values ; and, in view of the variable composi- 

 tion of what is called "hard rubber," "ebonite," or "vulcanite" in the 

 market, and of the extreme difficulty of keeping the temperature of the 

 hot box constant within less than one tenth of a degree for any great 

 number of hours, it did not seem desirable to extend the experiments 

 further. In what follows, the conductivity is given in every case to 

 three decimal figures, but it is evident that the third figure is not quite 

 determined. 



Experiment (a). — A compound slab, made up of two plates (A and B), 

 of a certain kind of hard rubber which I chose as a standard, with their 

 thermal elements, was placed between two other pieces of hard rubber 

 to form a prism. Each plate was about 60 era. long by 50 cm. broad. 

 The average thickness of A was 1.270 cm., and that of B 1.260 cm. In 

 the final state the thermal elements on the warmer side of A, between 

 A and B, and on the cooler side of B, indicated 74°. 9, 45°. 2, and 15°. 7 

 respectively. The rate of melting of the ice in the box was 102 

 grams in almost exactly 8,760 seconds. Assuming the area of the bot- 

 tom of the ice pot to be 126.7 square centimeters, and the latent heat 



