636 Proceedings of the Royal Society of Edinburgh. [Sess. 
Electrical Resistance. 
The passage of electricity through the glass is always accompanied by 
polarisation, showing that decomposition occurs. 
To measure the resistivity a thin bulb about 2 cms. in diameter was 
blown upon the end of a tube of ordinary soda glass. This was then boiled 
in water, carefully cleaned before filling with pure dry mercury, and con- 
nected by means of a platinum-tipped wire to a Wheatstone’s bridge. The 
bulb stood in a cup of clean dry mercury which was similarly connected to 
the bridge, and a key was provided whereby the mercury within and without 
Fig. 3. 
the bulb could be joined direct to a sensitive mirror galvanometer. In spite 
of every care to dry and clean all parts of the arrangement a deflection was, 
under these conditions, always obtained — the outer surface being invariably 
positive to the other. A trace of metallic sodium added to the mercury in 
the cup increased the deflection. It appears, therefore, that the composition 
of the inner and outer layers of a glass bulb may be different, and that being 
in a state of thrust the latter is tending always to squeeze out sodium. 
The current * set up in the glass, due to its initial difference of richness in 
sodium, gradually carried more of this material to the inner surface. 
All the usual methods of measurement where polarisation is concerned 
* It should be noticed that this current appears to be due to the oxidation of the different 
amounts of sodium upon the two surfaces and is not to be regarded as a voltaic action between 
sodium and glass . — July 20, 1908. 
