14 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



TABLE I. 



After the absolute conductivity of a particular specimen has been care- 

 fully determined, between various pairs of temperature limits, the conduc- 

 tivity of any other specimens can be easily obtained by determining the 

 temperature-gradient, in the final state, on the axis of a prism built up of 

 the slab already tested, and the slab to be examined, with their attendant 

 thermopiles and such other thin slabs as may be conveniently used. By 

 varying the order of the slabs on different occasions, the temperatures at 

 the faces of the slab to be examined can be altered, it being always 

 understood that the thermal elements must be placed between slabs 

 of approximately the same conductivities. The relative conductivities 

 of different materials, if these conductivities are not widely different, can 

 be determined with great accuracy by this method ; and it is possible to 



