1907-8.] Preparation of a Glass to Conduct Electricity. 629 
remembered that borax is very hydroscopic and that the sodium silicate 
itself is readily attacked by water. But I will exhibit plates of this glass 
which have been lying exposed in my laboratory for three years without 
deterioration. Their durability is largely dependent, however, upon a 
process of maturing to be described later. 
The addition of borax so far lowers the fusion point of the mixture 
that both in the preparation and the working special methods have had 
to be developed. 
And again, on account of the instabihty of the sodium silicate compound 
at high temperatures, no ordinary crucible is of use. 
Messrs Powell very kindly made several attempts to prepare plates of 
this glass, but complained that the mixture, when molten, ate its way 
through their crucibles. I am also indebted to the Morgan Crucible Com- 
pany for their efforts to supply a clay sufficiently refractory to resist the 
action of the mixture, but in the end the only possible crucible was made 
for me of platinum by Messrs Johnson & Mathey. It stands 7 5 cms. high 
and has a capacity of 156 c.c. This crucible shows no deterioration after 
innumerable firings in contact with the mixture at a white heat. 
It was desirable to produce a glass which, while conducting electricity 
to a sufficient extent, should be transparent, capable of being re-heated with- 
out devitrification so as to be workable in the blow-pipe, able to sufficiently 
resist the action of water or acid, of a suitable hardness without excessive 
brittleness, to enable it to withstand ordinary usage, and finally, so prepared 
that fine annealing was possible in a reasonably short time. 
In the manufacture of ordinary glass the molten mass is kept white hot 
for twenty-four hours or so in order to drive off the myriads of air-bubbles. 
Owing to the low fusion point of the conducting glass it is found practicable 
to make use of a vacuum for this purpose. It is evident that in the pre- 
paration of special glasses prolonged heating may produce serious loss of 
some of the ingredients through evaporation. By means of a vacuum it is 
possible to reduce the time required to remove the air-bubbles to about two 
and a half hours. The evaporation of the borax after three hours generally 
results in the glass being left too brittle to work. Plates cast in brass 
moulds fly to pieces before they can be placed in the annealing muffle, and 
with such explosive force as to be exceedingly dangerous. A suitable 
method for the rapid production of special glasses on a small scale consists 
in placing the crucible with its white-hot contents into a vacuum vessel 
until all the air-bubbles disappear. In this way a block of conducting 
glass measuring 10x3x1 cms. can be made perfectly clear and cast in a 
time not exceeding two and a half hours. 
