28 



Prof. B. Osgood Peirce on the 



were not regular ; and for this reason 1 have felt it unwise, as 

 I have already said, to trust the indications of thermal 

 elements which lie between substances of widely different 

 conductivities. Ofcourseitis of great importance that the 

 faces of the slabs should be practically plane. 



I had at my disposal fourteen different specimens of hard 

 rubber, thick enough to be used conveniently in my apparatus, 

 and much of this was in sheets about 120 centim. by 50 centim. 

 So much of it as was bought for the purposes of this investi- 

 gation came from three well-known makers. Besides this 

 new rubber, however, there were a number of pieces from 

 unknown sources, which had been lying in the laboratory for 

 indefinite periods. The specific gravities of the different 

 specimens examined differed slightly from each other, but by 

 not nearly so much as 1 per cent, in the extreme cases. The 

 specific gravity of the rubber used as a standard is about 1*202. 

 The specific heat of the rubber increases with the temperature, 

 as may be inferred from the subjoined Table, in the second 

 column of which is given the number of calories required to 

 raise one gram of a certain specimen 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 temperatures. 



Temperatures. 



Calories. 



Average 

 Specific Heat. 



o 



35 



3-17 



0-317 



45 



6-46 



0-323 



55 



9-91 



0-330 



65 



13-47 



0-337 



75 



17-20 



0-345 



85 



21-22 



0-354 



95 



25-48 



0-364 



100 



27-90 



0-372 



The average specific heat of another specimen of thin 

 rubber between 25° C. and 100° C. was 0*370. That of the 

 thicker standard rubber was 0*339. 



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 2 L of a 



