Thermal Conductivity of Vulcanite. 27 



mately the same conductivities. The relative conductivities 

 of different materials, if these conductivities are not widely 

 different, can be determined with great accuracy by this 

 method ; and it is possible to use much smaller slabs than are 

 necessary if one is to determine absolute conductivities. I 

 have two smaller pieces of apparatus very like the large one 

 already described, except that neither cold box has a device 

 for determining the amount of heat coming through the prism. 

 One of these is intended for slabs 35 centim. square; the 

 other, made wholly from brass, for disks 20 centim. in diameter. 

 Since it is not easy to get thin plane-faced slabs of some 

 materials 60 centim. square, it is usually best to compare in 

 the small apparatus small disks of such materials with a disk 

 of some standard material cut from a plate previously tested 

 in the larger apparatus. 



When one considers the passage of heat through a prism or 

 wall built up of separate slabs, one is led to ask whether the 

 cracks between the slabs do not introduce great irregularities. 

 We know that it is practically impossible so to press together 

 flat slabs of any material that there shall not be a relatively 

 large contact-resistance to the passage of an electrical current 

 across the common surface. {Should we not expect the thermal 

 conductivity of a prism made up of two or more slabs of a 

 given material to be different from that of a prism of the 

 same thickness made of a single slab of this material ? It 

 was necessary to appeal to experiments for an answer to this 

 question. Fortunately 1 had considerable quantities of a 

 special kind of plate-glass and of a special brand of hard 

 rubber in which 1 have been unable to detect any change 

 of thermal conductivity with the temperature between 

 20° C. and 80° C. All the glass was nearly of the same 

 thickness, but, by fastening a disk of the rubber to the 

 face-plate of a large massive engine-lathe, and using a broad, 

 very sharp tool (kept cool by water) with slow speed, and a 

 very fine cut, it was possible to reduce the thickness of the 

 disk and to keep the faces almost exactly plane. Of these 

 materials 1 have built up at different times a number of prisms 

 in different ways, and have tested these prisms in my apparatus. 

 In every case the conductivity seemed to be entirely inde- 

 pendent of the thickness of the prism and of the number of 

 slabs out of which it was built. It has also appeared that the 

 conductivity of a prism made up of a number of disks was 

 not changed to any measurable amount by introducing thin 

 metallic leaves to separate every slab from its next neighbours. 

 In the cases of some heterogeneous prisms built up of slabs of 

 different substances separated by thermal elements, the results 



