Thermal Conductivity of Rocks at High Temperatures. 45 



of the ratio compared with those found by Stuhlmann for the 

 cathode rays produced by ultra-violet light. The value he 

 gives for silver is 1*07, and for platinum, which seems to 

 behave, for X-rays at least, about the same as gold, 1*17. 

 This is about the same as the mean of all the values which I 

 have obtained for gold. His value for silver is somewhat 

 less than the mean of my determinations. If the mechanism 

 of production of cathode rays is the same with ultra-violet 

 light as it is with X-rays, the ratio of emergence to incidence 

 effect does not seem to vary much over a range of absorba- 

 bility corresponding to the X-rays from tin up to that 

 corresponding to ultra-violet light. 



Summary. 



The ratio of emergence to incidence cathode rays produced 

 in gold and silver by beams of fluorescent secondary X-rays 

 has been measured with the object of finding the dependence 

 of the ratio on the penetrating power of the exciting rays. 



The fluorescent secondary X-rays from tin, zinc, iron, and 

 chromium were used as exciting rays, representing an 

 increase in absorbability as measured in aluminium of about 

 8000 per cent, between tin and chromium. 



After allowing for the absorption of the exciting rays in 

 the layer of the metal from which the cathode rays come and 

 in the air of the ionization -chamber it was found that there 

 was no definite variation in the ratio of emergence to incidence 

 effect. 



In closing I wish to express my thanks to Professor H. A. 

 Bumstead for the interest he has taken in the work. 



Sheffield Scientific School, 



Yale University, New Haven, Conn. 



March 1912. 





IY. On the Thermal Conductivity of some Rocks at High 

 Temperatures. By Horace H. Poole, M.A., Sc.B* 



A KNOWLEDGE of the thermal conductivity of rocks 

 at high temperatures being of considerable importance 

 in some geophysical problems, an attempt has been made to 

 ascertain how the conductivity varies as the temperature is 

 raised in the case of some typical rocks. The method 

 employed is the well-known one of generating heat elec- 

 trically at a known rate uniformly along the axis of a long 

 cylinder of rock, and measuring the difference of temperature 

 between two points at different distances from the axis. 

 * Communicated by the Author. 



