20 Prof. B. Osgood Peirce on the 



a distance from the axis equal to the thickness of the prism. 

 This inference was justified by the results of our experiments. 



In some of our work we used, instead of the ribbon thermal 

 elements with tinfoil u wings " described above, fine wire 

 thermal junctions inserted in shallow grooves accurately cut 

 to a given depth in the faces of the slabs to be tested. These 

 grooves were made in a Brown and Sharpe Universal Milling- 

 Machine by extremely thin hard steel saws (No. 34 B. & S. 

 Gauge) held between flat disks of somewhat smaller diameters 

 than the saws to prevent buckling. The wire that we used 

 fitted the grooves very closely, and we hoped that the indi- 

 cations of the thermal couples would enable us to determine the 

 mean temperature of the walls of the groove when the grooved 

 slab was placed against a flat one. We found, however, that 

 the results were often most irregular ; and although we spent 

 some time in attempts to make observations obtained in this 

 way trustworthy, we met w T ith little success. Sometimes our 

 results were good, and sometimes considerably in error. 



The ribbon thermal elements gave consistent results at all 

 times, provided that the junctions themselves were in contact 

 with the slabs between which they lay. If the tinfoil wings were 

 considerably thicker than the junction-ribbon, or if the junction 

 itself were scraped thin, the reading might be in error by an 

 amount not easy to be accounted for by the mere resistance 

 of the thin air-film on each side of the junction. We shall 

 wish to discuss this matter at some length in the near future 

 in the light of some of our observations. 



It is obvious that in the case of such material as glass, the 

 use of fine wire thermal elements in the manner just described 

 is practically impossible, and that the specific resistance of 

 such metal leaves as we use was in every case hundreds of 

 times less than that of the slabs between which they were 

 placed. The question of contact-resistance (Ueber gangs- 

 wider stand) will be discussed later on. In some experiments 

 we used wire thermopiles inserted in a thin metal disk : we 

 had also at command a prism-holder so constructed that the 

 fine wire elements could be immersed in very thin layers of 

 mercury. 



The present paper gives an account of a series of measure- 

 ments of the thermal conductivities of different specimens of 

 "hard rubber " or " vulcanite." These determinations w r ere 

 made, for the most part, with the help of apparatus previously 

 used by Dr. Willson and myself. That the method may be 

 understood, it will be necessary to describe this apparatus 

 somewhat at length. 



