Thermal Conductivity of Vulcanite. 29 



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

 composition 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 jL 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, was placed between two other pieces of hard rubber 

 to form a prism. Each plate was about 60 centim. long by 

 50 centim. broad. The average thickness of A was 1*270 centim. 

 and that of B 1*260 centim. 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 re- 

 spectively. The rate of melting of the ice in the box was 

 102 grams in almost exactly 8760 seconds. Assuming the 

 area of the bottom of the ice-pot to be 126*7 square centim. 

 and the latent heat of melting ice to be 79'25, this corresponds 

 to the conductivity for each slab of 0*000311. 



Experiment (6). — Plate A with two thermal elements en- 

 closed by two other " guard-sheets " of hard rubber was made 

 into a prism with three plates of glass. In the final state the 

 temperatures of the elements on the faces of A were 60 o, l and 

 24°*9 respectively. In 11,220 seconds, 154*8 grams of ice 

 were melted. This again corresponds to a conductivity 

 between 60° and 25° of 0*000311. 



Two other experiments, in which the final gradients on the 

 axes of prisms built up of disks about 20 centim. in diameter, 

 made of this rubber, were determined, failed to show any 

 sensible variation of the conductivity with the temperature 

 between 65° and 16°. 



Experiment (c). — A compound slab made of A and a plate 

 C (of the same dimensions as A but purporting to come from 

 another maker), with their thermal elements and "guard- 

 plates " of rubber, were used to form a prism for the large 

 apparatus. In the final state the indications of the elements 

 on the warm side of A, between A and C, and on the cool 

 side of G were, respectively, 69°*8, 41°*1, and 13°*1, so that the 

 conductivity of C between 41° and 13° appears to be 00003 19. 



After this a number of disks 20 centim. in diameter, which 

 seemed alike in their physical properties, were cut from the 

 standard rubber and used with other disks to form prisms for 

 the smaller apparatus. 



