62 Mr. H. H. Poole on Thermal Conductivity and Specific 



removed if breakages occurred. The latter cement was found 

 best for cementing the rocks together. Both cements stood 

 the high temperature well. As it seemed possible that the 

 cements might attack the wires at high temperatures and alter 

 their thermo-electric properties, one couple was tested after 

 removal from the rock and found to be absolutely unchanged. 



First Basalt Experiment. 

 It seemed to be desirable to measure the P.D. between two 

 points on the central wire actually inside the rock cylinder, 

 so as to eliminate errors due to want of uniformity of 

 temperature and hence of resistance of the wire near the 

 ends. Accordingly the cylinder was made in four pieces 

 cemented together end to end. The length of each end piece 

 was 8*9 cm. and of each centre piece 5*1 cm., so that the 

 total length was 28'0 cm. As the diameter was only 

 3'67 cm., so that the cylinder fitted loosely inside the 

 copper tube of the furnace, it would seem as if end effects 

 must be negligible. The thermo- junctions measuring the 

 mean temperature and the temperature gradient were sunk 

 in grooves cut in the faces between the two central cylinders, 

 which were ground to fit accurately together and joined by the 

 thinnest possible layer of cement. The wires for measuring 

 the potential gradient in the central wire were led in through 



Fig. 2. 



grooves cut in the inner faces of the end cylinders, and were 

 fused to the central wire, these junctions being just at the 

 ends of the central cylinders. 



The arrangement of the couple for measuring the tem- 

 perature gradient was very similar to that previously 

 described and is shown in fig. 2, which is approximately to 



