534 Mr. W. Baker on the Metallurgy of Lend. 



assume that it is shorter for light gases than for heavy gases ; for 

 it is inversely proportional to the radius of the sphere of action, 

 and it would be difficult to assign a greater sphere of action to 

 the lighter than to the heavier molecules. If, accordingly, e is 



not smaller for the lighter gases, the fraction — ==, and therefore 



s/ a 



the conducting-power for heat, must be greater in this case than 



in the case of the heavier gases. 



This result accords perfectly with the results of observation 

 hitherto obtained, and especially with those of the beautiful 

 investigations by Magnus, in which he avoided the currents of 

 gas which, in the experiments of Dulong and Petit, existed at 

 the same time as the conduction of heat properly so called. From 

 these experiments we see very plainly that the lightest gas 

 (hydrogen) conducts considerably better than the other gases. 



§ 30. We may sum up as follows the conclusions at which 

 we have arrived. 



1 . Gases conduct heat considerably worse than the metals. An 

 approximate calculation based upon the mean length of excur- 

 sion of the molecules, as deduced by Maxwell, gives, for the con- 

 ducting-power of atmospheric air near the freezing-point, a num- 

 ber which is 1400 times smaller than that which represents the 

 conducting-power of lead. 



2. The conducting-power for heat depends on the tempera- 

 ture of the gas, and increases in the same ratio as the velocity 

 of sound. 



3. The conducting-power for heat is, within certain limits, 

 independent of the pressure to which the gas is subjected. 



4. Light gases conduct heat better than heavy gases. Hydro- 

 gen must therefore conduct heat considerably better than any 

 other gas. 



Zurich, October 1861. 



LXXIL Contributions to the Metallurgy of Lead. By William 

 Baker, Associate of the Government School of Mines, F.C.S.* 



IT was shown, in a paper published in the ' Chemical Gazette/ 

 October 1, 1856, that Pattinson's process of concentrating 

 silver in lead by crystallization accomplished at the same time a 

 separation of copper, the latter being found in the larger pro- 

 portion in the fluid lead which had been drained from the cry- 

 stals. It was supposed therefore that this method offered a 

 valuable means of preparing an inferior quality of lead for those 

 purposes in the arts for which a metal of the' greatest purity 

 attainable is required. 



* Communicated bv the Author. 



