40 Emission and Transmission of Heat 



and K a number depending on the nature of the surface of the 

 radiating body. 



Values of K for different surfaces. In B. T. U. per hour per 

 square foot per i F. excess of temperature. 



Polished silver . . . .027 Zinc 049 



Silvered paper . . . .085 Polished tin 044 



Polished brass . . . .053 Tin plate 086 



Gilded paper 047 Sheet iron polished . . .092 



Polished copper . . . .033 Sheet iron leaded . . .133 

 Cast iron new . . . .649 Sheet iron (ordinary ) . .567 

 Cast iron rusted . . . .688 Sheet iron rusted . . .688 



Glass 596 Building stone . . . .737 



Powdered chalk . . . .680 Plaster and brick . . .737 



Saw dust 723 Wood 737 



Powdered charcoal . . .700 Woolen cloth . . . .753 



Fine sand 741 Calico or canvas . . .747 



Oil paint 759 Silk 759 



Paper 772 Water 1.087 



Lampblack 820 Oil ....... 1.482 



795. For paper and cloth, color has no influence. It 

 appears from this table that powdered materials have very nearly 

 the same emissive power. Masson has already recognized that 

 all substances in a very finely powdered state, obtained by pre- 

 cipitation and not crystallized, have the same emissive power. 



796.* 



797. Heat Transmitted by Air Contact. The loss of heat 

 arising from air contact is independent of the nature of the surface 

 of the body, and of the absolute temperature of the surrounding 

 air ; it depends solely on the excess of the temperature of the 

 body over that of the surrounding air, and on the form and dimen- 

 sions of the body. 



* The tables given by P6clet in this paragraph are here replaced by the curves of 

 figures 2 and 3, page 21. 



It is more convenient, as will be apparent later on, to use Newton's I,aw for compu- 

 ting the loss of heat from a given surface and to modify the values of K given in the table 

 above by coefficients obtained from formula ( 2 ) shown graphically in figures 2 and 3. 



Figure 2 gives the coefficients by which K must be multiplied for differeut excesses 

 Of temperature. The product thus obtained is multiplied by a second coefficient obtained 

 from figure 3 which corrects for different temperatures of the body radiated to. 



