7 Emission and Transmission of Heat 



their conductivity is the same as that of motionless air. The 

 conductivity of starch-paste may also be regarded as that of mo- 

 tionless water. I have also noticed, in the case of poor conduc- 

 tors of heat, that dampness greatly increases their conductivity.* 



TRANSLATOR'S NOTE. Although the experiments described above were made more 

 than fifty years ago there is but little to add due to more recent research. Pficlets figures 

 for the conductivity of the metals have been shown to be too low, due probably to the 

 fact the liquids in contact with the surfaces of the metal plate were not frequently enough 

 renewed. This defect would not disturb the accuracy of the experiments on the materials 

 of low conductivity, "and it is only these that we use in practical applications of this sub- 

 ject. Peclets figures for these substances have been repeatedly confirmed by more recent 

 experimenters. Peclet evidently made no experiment on the conductivity of motionless 

 air, simply reasoning that it must be the same as that of the textile materials. It has quite 

 recently been proven by direct experiment to be 0.152 which is about one-half of Peclet's 

 value, but in practice we never obtain perfectly motionless air, the process of convection 

 aiding to at least some extent that of conduction. In 879 we find experimental evidence 

 of the value of Peclets figure for the kind of practical application in which we most want 

 to use it. 



For further notes on the reliability of this portion of Peclets work the reader is refer- 

 red to a very interesting paper by Mr. A. B. Reck of Copenhagen, presented in December, 

 1901, before the American Society of Heating and Ventilating Engineers. 

 The following table is from Jude and Gossln, Physics. 1899. 



VALUES OF c 



Silver 4440 Lead 334 



Copper 3192 Ice 17.42 



Gold 2100 Snow 2.03 



Zinc 888 Water 4.41 



Tin 572 Air 0.16 



Iron 476 



The ratios between these conductivities for the metals is almost the same as Frantz 

 & Wiedermann's and the value for lead is about three times as large as that found by 

 Peclet. 



