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Proceedings of the Koyal Society of Edinburgh. [Sess. 
beads, and the whole arrangement coated with paraffin wax. Placed in 
series with a sensitive reflecting galvanometer and twelve accumulators, a 
well-marked deflection (50 mms.) was obtained, which varied greatly as 
the temperature was slightly altered. It was seen that the resistivity 
diminished with rise of temperature, so that the substance was behaving in 
one sense as a (so-called) non-conducting solid, while at the same time 
conducting electricity comparatively well. But sodium silicate when fused 
to a glass is soluble in boiling water, and in addition the material has 
a high fusing point, is very brittle and difficult to work. Attempts were 
therefore made to combine some substance with it which would increase its 
stability and improve it in other ways without interfering with its ability 
to conduct electricity. 
In a paper read before the Koyal Society of London in 1898, Professors 
Andrew Gray and Dobbie * pointed out that “ a glass which approaches in 
composition to a definite chemical compound has a high resistance.” It 
therefore appeared necessary, in the first place, to test the silicate in order 
to find whether its conductivity was due to the presence of impurities. 
The sample used was found to be free from iron, and a quantity of 
chemically pure silicate when fused conducted as well as the commercial 
product. It does not appear, therefore, that the foregoing generalisation 
can be upheld. 
The addition of a small quantity of lead, bismuth, or lime seriously 
affected the resistivity of the glass, although these substances helped to 
prevent the attack of hot water. Of all the numerous materials tried borax 
gave the best result ; it can be added to the extent of 25 per cent, without 
appreciably diminishing the conductivity, while at the same time rendering 
the glass far less soluble in hot water. 
The addition of a small quantity of good lead glass was also found to 
still further improve matters, provided that the amount did not exceed one 
part in thirty-two of the silicate. Otherwise the conductivity was seriously 
affected. In order to ensure uniformity of this added substance the flint 
glass made by Messrs Powell of London was chosen. 
The composition was therefore as follows : — 
Sodium silicate ...... 32 parts 
Borax . . . . . . . 8 „ 
Powell’s flint glass 1 part 
It is certainly surprising that a glass made of these things and with such 
a preponderance of alkali can remain sufficiently permanent, when it is 
* Proc. Roy. Soc ., vol. lxiii., Feb. 1898. 
