760 Prof. J. C. McLennan on the 



p. 694) o, 5 0. must be subtracted from sea-level tempera- 

 ture for every 100 metres general elevation of the land 

 surface or about 18° for an elevation of 3500 metres, and 

 this fall may be ascribed to radiation in some such way as 

 that here set forth. 



If the atmosphere of Mars is comparable with our own 

 atmosphere at high levels, and if the effect is of the same 

 general character in the two cases, it appears pro bable that 

 the surface temperature of Mars is actually lower by many 

 degrees than that which the surface of the Earth would have 

 at the same distance from the Sun. 



LXXY. On the Radioactivity of Lead and other Metals. By 

 J. C. McLennan, Ph.D., Professor of Physics, University 

 of Toronto*. 



I. The Relative Activities of Different Metals. 



IN a paper in the Phil. Mag. of September 1906, Eve states 

 that while investigating the natural ionization of air 

 confined in vessels made of different metals, he found that 

 24 ions per c.c. were generated per sec. when the receivers 

 were made of copper, zinc, iron, and tinned iron, while 96 

 ions per c.c. were regularly produced in air per second 

 when the confining vessels were made of lead. 



The high conductivity of air contained in lead vessels has 

 been frequently noted by other observers ; and from Eve's 

 results it would appear that lead either contains some active 

 impurity from which other metals are entirely free or else 

 it possesses an intrinsic radiation very much stronger than 

 that exhibited by other metals. 



The view that lead contains an active impurity is supported 

 by a description in the Phys. Zeit. of November 1906, of 

 some experiments by Elster and Geitel, in which they suc- 

 ceeded in extracting from lead oxide small quantities of an 

 active substance which from ifs characteristics they were 

 inclined to think was Radium F. In this paper they state 

 that they were unable to obtain any active emanation from 

 the materials treated, and on this account they suggest that 

 possibly the source of the Radium F can be traced to the 

 presence of Radium D in the lead. 



Since the decay period for Radium D is forty years it 

 would follow, if the high activity of lead is due to the pre- 

 sence of this radium product, that very old lead should 



* Communicated by Prof. J. J. Thomson, F.R.S. 



