OF CRYSTALS AND OTHER BAD CONDUCTORS. 
507 
References :— 
Lorenz, ‘ Wiedemann’s Annalen,’ vol. 13 (188L). 
F. Weber, ‘ Monatsber. cler Berliner Akad.,’ 1880, p. 457. 
F. Neumann, ‘ Annales de Cliim. et de Phys.,’ Ill ser., vol. 66 (1862). 
Angstrom, ‘ Poggendorff’s Annalen,’ vol. 123, p. 640 (1861). 
PI. Meyer, ‘ Wiedemann’s Annalen,’ vol. 34, p. 600 (1888). 
Peclet, ‘Annales de Cliim. et de Phys.,’ Ill ser., vol. 2. 
G. Forbes, ‘ Roy. Soc. Proc. Edin.,’ vol. 9, p. 64 (1873). 
Tuschmidt, ‘ Beibliitter,’ 1884, p. 490. 
Yamagawa, ‘Journal, College of Tokio,’ vol. 2. p. 263 (1888^. 
Winkelmann, ‘ Poggendorff’s Annalen,’ vol. 153, p. 481 (1874). 
In the foregoing table the conductivities of a few metals and liquids are given in 
order to show the position of the bodies experimented on amongst other conductors. 
Where experiments have been previously made by other experimenters their results 
are given for comparison. 
Such comparison shows a fair degree of concordance between the present results 
and those obtained by totally different methods, with the exception of the results of 
G. Forbes, which were obtained by the fob owing met hod. A can about 9 cms. 
diameter was filled with a freezing mixture, placed on a sheet of the material the 
conductivity of which was to be determined, and the lower surface of the sheet 
brought down on to a surface of water at 0° C. Ice is formed on the surface of the 
sheet, and Forbes works out an expression for the thickness of this ice in terms of 
the temperature of the freezing mixture (which was observed by a thermometer placed 
in it), the thickness of the material, the time, and the conductivities of ice and the 
material. In working out this expression, however, he assumes the conductivity of 
the materials between the thermometer in the freezing mixture and the upper surface 
of the sheet to be infinitely great, which, when one considers the layers of liquid, 
metal, and air present, is scarcely justifiable. On this account almost all Forbes’s 
results are low. 
Peclet’s results for metals were all found to be low, and this led to the assumption 
that all his results were the same. It is, however, evident from the present experi¬ 
ments that his results for bad conductors are correct. This is probably owfing to the 
fact that the layers of still water which, in his experiments, adhered to the surfaces 
of the sheet experimented on, are of much less importance when the sheet is a bad 
conductor. 
Tuschmidt’s results are obtained by a method which Weber used for liquids. 
The sheet of liquid is simply replaced by one of the crystal, the contacts being made 
by means of glycerine. As glycerine has a conductivity which is only about -doth of 
those of the crystal plates, the glycerine layers have a great effect on the flow of heat 
through the plates. This is a defect of the method, and, in addition, the plates them¬ 
selves must be large, and are therefore expensive. Tuschmidt’s results and those of 
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