Thermal Conductivities. 

 Flint glass v// cm. thick. 



27 



X 



V 



dx 







•177 







18-98 



•211 



•234 



From this I find k for flint glass = -00208. 



Taking the index of refraction of crown glass as about 

 1*5, and of flint glass as about i'6, it is seen that the 

 thermal conductivity of glass increases as its index of 

 refraction diminishes, as Kundt found to be the case in 

 metals. 



The numbers '00235 and '00208 may be relied on to 

 a greater degree relatively to each other than absolutely, 

 owing to change of Electromotive Force of the thermo- 

 junctions, produced by an unavoidable straining of the 

 wires in packing the saw-dust between the bars and the 

 outer cylinder. On this account the apparatus has been 

 modified, and I am at present engaged in re-determining 

 the conductivities of the glass discs used in the above 

 experiments, and in determining the conductivities of 

 quartz and Iceland spar in different directions. The results 

 of these experiments I hope shortly to communicate to the 

 Society. 



