68 Emission and Transmission of Heat 



relative radiating powers, that which radiates the least being on 

 the vessel heated by steam. For glass I used the arrangement 

 shown in figure 178, the opening in the vessel being closed by a 

 thin sheet of glass held in the same way as already described for 

 thick plates of the same material. 



858. As in the former methods, we may write an equation 

 between the quantity of heat traversing the plate and that emitted 

 from the exposed surface, and we thus arrive at the very simple 

 formula ; 



e Q (?-?>) 

 /-/' 



In which Cis the conductivity of the material, e the thickness of 

 the plate ; Q the value of K-\-K' modified by the coefficients cor- 

 responding to the temperatures (figures i , 2 and 3), /, / and /," 

 the temperatures of the steam, of the outer surface of the plate 

 and of the surrounding air. 



859. I have deduced from these experiments the following 

 tables of values of C. These numbers give the quantity of heat 

 in B. T. U. which would pass in one hour through a plate of the 

 given material one inch thick, one square foot in area, and of 

 which the two surfaces were at temperatures differing by one de- 

 gree Fahrenheit. 



TABLE OF VALUES OF C. 



B. T. U. per hour, per square foot, per inch, per one degree. 

 SOLID MATERIALS. 



Density. C. 



Marble, gray, fine grained .... 2.68 '28.1 



Marble, white, coarse grained . . . 2.77 22.4 



Limestone fine grained . . . . 2.34 16.8 



Limestone do do . . . . .2.27 13.6 



Limestone do do . .... 2.17 13.7 



Limestone coarse grained . . . .2.24 10.6 



Limestone do do .... 2.22 10.2 



Plaster, ordinary . ..... 2.22 2.67 



Plaster do very fine . . . . 1.25 4.20 



Brick 1.98 5.56 



Brick 1.85 4.11 



Fir, (wood) transmission perpendicular to fibres .48 .75 



