PEIRCE. THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 655 



bolted securely to A with 

 the prism to be examined 

 betweeu the two. While Z 

 was in the lathe, a small 

 hole, H, about 3 mm. in 

 diameter and 4 mm. deep, 

 was drilled exactly in the 

 centre of its upper face. 

 Subsequently a piece of solid 

 drawn brass tube 12.3 cm. in 

 outside diameter and 13.5 

 cm. high, with carefully 

 squared ends, was held cen- 

 trally in Z by means of a 

 disc turned to fit it, and a 

 central pin inserted in N, 

 and was then soldered firmly 

 to Z with the help of white 

 pitch as a fiux. The walls 

 of the pot thus formed were 

 jacketed on the outside ex- 

 cept for a height of about 2 

 ram. at the bottom (Figure 4) 

 by an inch-thick casting of 

 hard rubber made for the 

 purpose in the form of a 

 cylindrical shell. This cast- 

 ing, which was cut off square 

 at the top of the pot, tapered 

 to nothing near the bottom, 

 but did not rest upon the 

 floor. Upon the top of this 

 jacket was fastened a hard- 

 rubber cover shaped some 

 what like a cylindrical hat. 

 This had an opening at the 

 top which could be closed 



by an accurately fitting ebonite plug. In the box, P, thus made, was 

 placed a tliin-walled copper ice-holder, Q, open at top and bottom, of the 

 same outside diameter below as the inside of the brass pot, but some- 



