Emission and Transmission of Heat 53 



following figures for the quantity of heat in British thermal units, 

 which would be transmitted in an hour through plates with a 

 superficial area of one square foot, a thickness of one inch, and 

 of which the surfaces are maintained at constant temperatures dif- 

 fering by one degree Fahrenheit. 



Silver .... 1330 Iron .... 158 

 Copper .... 980 Steel . . . . 154 

 Gold .... 706 Lead . . . . 113 

 Tin 193 Platinum . . 112 



But these conductivities have been obtained under special 

 circumstances which are never met with in practice. 



According to some old experiments of Clement, a sheet of 

 copper 10.8 square feet in area, and one-tenth of an inch in thick- 

 ness, bathed by steam on one side and water at 82 on the other, 

 condensed 220.5 pounds per hour for a temperature difference of 

 130 degrees, which is equivalent to 1.7 pounds for a difference of 

 one degree Fahrenheit. 



According to the more recent experiments of Thomas & Lau- 

 rens, with a single copper tube of small diameter, there was 

 evaporated 81.8 pounds of water per hour, per square foot, for a 

 temperature difference of 81, which makes one pound for a dif- 

 ference of i F. 



The figure obtained by Thomas & L,aurens is much greater 

 then Clements, for the surface of transmission being a tube of 

 small diameter, the air was completely expelled, a circumstance 

 which greatly augments the amount of steam condensed. 



It is evident that even under the most favorable circumstan- 

 ces, the figure obtained for the transmission of heat through cop- 

 per, when the liquid wetting the surfaces is not renewed, is much 

 less than that resulting from the experiments we have been con- 

 sidering. This is on account of the sensibly motionless layer of 

 water which covers at least one of the surfaces. 



834. Thus, although the laws admitted by physicists for the 

 transmission of heat through plates may be exact, these laws are 

 not applicable to the transmission of heat from one liquid to an- 

 ductivities of the metals found by M. Despretzf we obtain the 



tSee foot note to paragraph 829. The figures given here are based on Pdclet's experi- 

 ment with lead and the relative values of Franz and Wiedermann. 



