Emission and Transmission of Heat 63 



849. For solid bodies such as stone and marble I used hol- 

 low cylinders painted inside with oil, and covered on the outside 

 with paper; more often to be the better assured that water could 

 not penetrate the material the outer surface was covered with 

 tin foil. The cylinders were closed at each end by a sheet 

 of rubber and then by a disc of wood. In the case of the wooden 

 cylinders, the steam entered at top, and escaped at bottom as 

 with the tin ones. They were supported from the lower end, 

 and insulated at both ends by means of paper cylinders filled 

 with cotton. (Fig. 173). For wood I used the same arrange- 

 ment when determining the conductivity perpen- 

 dicular to the fibres ; but to measure it parallel 



with them I employed portions of cylinders hav- 

 ing surfaces perpendicular to the fibres, these were 

 strongly compressed against a tin cylinder covered 

 with paper, having an outer radius equal to the 

 inner radius of the wooden cylinder. 



850. Papers and cloths were rolled around a 



tin cylinder, always covered with paper, and they Flg " 173 Flg- 1?4 

 were held in position by a cylinder of strong paper having a height 

 double that of the tin cylinder. This paper cylinder was held by 

 the metallic band (Fig. 174). 



85 1 . The formula which gives the conductivity is as fol- 

 lows: 



Q R'm(t"-t'") (log. JT-lQg. R) 



t'-t" 

 / 



In this formula C is the conductivity of the material, R, R' 

 the radii of the inner and outer cylinders; #2 = 2.3025; /,' /," /,'" 

 the temperature of the steam, of the outer surface of the envelope 

 and of the air; Q represents the coefficient of emission of the heat 

 from the surface, and equals K-\-K' (791), the values of A' and 

 K' having been corrected by the coefficients taken from the 

 curves of figure i, 2 and 3. 



852. Finally in the last method I employed vertical rectang- 

 ular plates, insulated around their perimeter; one of the surfaces 

 was heated by steam, and the other exposed to the free air of a 

 constant temperature chamber. A formula even more simple 

 than that of the cylinders permitted the calculation of the con- 



