PEIRCE AND WILLSON. — TIIKRMAL CONDUCTIVITIES. 



37 



and flows back into the retort through the trap h. This apparatus takes 

 slabs 35 centimeters square. Although we found it possible to maintain 

 with this arrangement a temperature above 350° C, for many hours at a 

 time, it was difficult to avoid superheating by conduction through the 

 massive iron of the hot box, and we intend to discard mercury in future 

 and use some less troublesome source of heat. If a substance of greater 

 heat of vaporization than mercury is employed, the retort can be removed 

 to such a distance that all danger of superheating is removed. We have 

 not yet been al)le to test au electrical stove which we hope may prove 

 to be a convenient and a sufficiently constant source of heat for many 

 purposes. 



Figure 7. 



The apparatus just described has been furnished with trunnions so that 

 the axis of the prism can be made horizontal or vertical at pleasure. 

 This renders it possible to use a layer of mercury on each side of the 

 slab to be tested, when this is desirable. 



Our third apparatus of this kind is made entirely of brass. It is in- 

 tended only for small thin plates about 20 cm. in diameter, but is in 

 essentials like the apparatus just described. 



Figure 8 represents the apparatus which we have used to determine 

 the absolute conductivities at temperatures between 0° C. and 100° C. of 

 various materials. The boilers and the hot chamber are those of the 

 apparatus shown in Figure 6; the ice box, which is the outcome of 



