PEIRCE AND WILLSON. — THERMAL CONDUCTIVITIES. 



35 



tures betvveeu 0° C. and 100° C. of relatively poor conductors like plates 

 of stone or glass ; the otiier designed merely for comparing the con- 

 ductivities of slabs which form a prism or " wall," through which there 

 is a steady flow of heat. 



Of this second form of apparatus, which is much simpler than the other, 

 we have three of different sizes for plates G5 cm., 35 cm., and 20 cm. 

 in diameter respectively. Rough diagrams which show the essential 

 parts of two of these, without their elaborate stands and jackets, are 

 given in Figures 6 and 7. In each, the prism to be tested is enclosed 



Figure 6. 



between the horizontal planed plates of two castings, which are fastened 

 firmly together by bolts around their edges to insure close contact with 

 the body under experiment. Both castings are hollow ; one forms a 

 jacketed chamber through which steam or mercury vapor may be passed 

 for an indefinite period. The upper casting, which is provided with a 

 system of stirrers or scrapers operated by an electric motor, may be 

 kept at a low temperature by filling it with ice or by sending through 

 it a steady stream of water from a very large tank within the tower of 

 the laboratory. 



In Figure G, A represents the hot chamber, weighing about two hun- 

 dred kilograms, which rests in a thick jacket on a heavy table or stand 

 made to hold it. A is connected directly with one (^B) of two stout- 

 walled copper boilers, B and i?', each of which holds about 40 litres of 

 water. A light cup-shaped weight, inverted and laid on a large tube 

 with squared end which projects above the top of the boiler, acts as a 

 sensitive safety valve and prevents any appreciable rise in temperature 

 within the boiler. B can be refilled when necessary with boiling water 



