Conductivity of some Solid Insulators. 503 



the thermal conductivity of which is given by Landolt and 

 Bornstein, was of the same composition as the " flint glass " 



whose density is recorded by Kaye and Lab v. The same 

 applies to the other substances mentioned. It is desirable, 

 therefore, to ascertain if the relation observed holds gdod if 

 the constants concerned are measured using the same glass. 



The composition of an ordinary glass is liable to be 

 different with each melt, and it may even vary throughout 

 the same melt. In the case of optical glass, however, 

 homogeneity throughout the melt is essential, and in order 

 that the glass may possess definite optical properties, pre- 

 cautions are taken to ensure that the composition is always 

 the same. Optical glasses are thus very suitable for testing 

 the validity of the relationship k = ~Ep. 



The physical constants of a large number of these glasses 

 made in the Jena Optical Works have been determined by 

 various observers, and these results have been used 1o 

 compile the following table. The figures are taken from 

 Hovestadt's " Jena Glass/' but the original papers are re- 

 ferred to : — 



No. of 



Glass. 



Density 



Elas 



X 

 Bit. 



sticity 

 10 5 . ' 

 B^ 



Thermal 



Conductivity 



/.xio 3 §. ' 



E lP 



xio 9 . 



E,p 

 X 10". 



/,'/E,o 

 XIO' 12 . 



/•/E.,0 

 X lo 1 - 



19 



2-370 



7296 



7563 



2-712 



1-729 



1-793 



1-569 



1-513 



20 



.3-944 



5088 



— 



1-595 



3-025 



— 



0527 



— 



21 



2-758 



5474 



5468 



2-103 



1-510 



1-508 



1-393 



1-394 



90 



2243 



4699 



4906 



1-927 



1054 



1-100 



1-828 



1-752 



23 



3-532 



7952 



7972 



2041 



2-808 



2-822 



0-727 



0-701 



24 



3 578 



5389 



— 



2-044 



1-929 



— 



1059 



— 



25 



2572 



6498 



6766 



2-458 



1-670 



1-7-10 



1-471 



1-413 



26 



3-879 



5467 



5461 



2003 



2-120 



2118 



0944 



0-945 



07 



2-588 



6780 



— 



1-974 



1-755 



— 



1-126 



— 



28 



2-580 



6626 



6599 



2-269 



1-710 



1-702 



1-327 



1-332 



29 



2-629 



6514 



6638 



2-407 



1-713 



1-745 



1-405 



1-379 



30 



2-518 



— 



6014 



2-128 



— 



1-515 



— 



1-405 



31 



3070 



6296 



§873 



1-819 



1-933 



1-956 



0-920 



0-909 



32 



2 668 



5862 



5843 



2016 



1-564 



1-550 



1-289 



1-293 



33 



4-731 



5512 



5477 



1-715 



2608 



2-592 



0-657 



0-662 



38 



2-585 



— 



7465 



2-442 



— 



1-930 





1-267 



* Winkelmann & Sehott, Aim. d. Phys. 1894, li. p. 73'). 



+ Log. cit. p. 697. 



: Winkelmatin, loc. cit. 1897, Ixi. p. 119. 



§ Focke, loc. cit. 1899. Ixvii. p. 132. 



The two values of the elasticity given were obtained by 

 Winkelmann at different times. He notes, in explanation 

 of the differences, that glasses 1 ( .) and 22 were from different 

 meltings, and glass 38 contained bubbles. The error in the 



