PEIRCE. THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 653 



live safety valve and prevented any appreciable rise in temperature vpitliin. 

 Z^ could be refilled vphen necessary with boiling water from B', without 

 stopping the constant flow of steam through A by means of the siphon f 

 which was provided with a valve. The steam, after passing through the 

 hot chamber, was led to the outer air by a jacketed pipe h descending 

 from the bottom of A. 



Figure 2. 



The connections of the thermal elements were led out of the sides of 

 the prism shut in by Z and win-e held between slabs of wood, which 

 acted as a sort of guard-ring jacket to the prism, for about 40 cm. before 

 they emerged. The platinoid or german-sHver leads of these thermal 



