1907-8.] Preparation of a Glass to Conduct Electricity. 
641 
Pressing Plates. 
Great difficulty was met with at first in making thin plates of the glass. 
The cast slabs were far too thick for most purposes. It was ultimately 
found possible, however, to produce good results by pouring a quantity of 
the molten glass upon a hot brass plate and lowering a heavy flat disc of 
the same metal (also heated) slowly down upon the plastic mass. 
Plates may, in this way, be readily obtained as thin as two mms. after 
polishing. The action of the press sets up considerable stress in the plates, 
especially at their edge, so that they require to be annealed for a few hours 
before grinding. 
Conclusion. 
Although there is no intention to do more in this paper than describe 
the preparations and some of the chief properties of the glass, it may, 
however, be pointed out that this substance is not without practical utility. 
Plates of it are already in use for the windows of electrometers and 
electroscopes, and fibres have been found to be sufficiently conducting over 
their surface to justify their replacing the gold leaves in the latter 
instrument. A length of glass fibre fixed in position with a small hinge of 
gold leaf thus affords an extremely sensitive index whose position may be 
accurately read by means of a telescope ; instruments of this kind are of 
especial use in the study of radio-activity. 
The conducting property enables the contact potential difference 
between a metal and glass to be further studied — the glass behaving as a 
solid electrolyte. The effect of a current of electricity through the glass 
upon its optical properties or optical behaviour in a magnetic field can 
now, I hope, be ascertained. Incidentally, since the material is, relatively 
to ordinary glass, somewhat easily attacked by strong HC1, the process of 
disintegration may be fully examined. 
The actual structure of the substance when cold which permits con- 
duction of electricity, and the rapid change of that condition at about 
90° C., are matters of interest. That the atomic aggregates existing in glass 
are capable of re-arrangement under the influence of heat is shown by the 
absorption of certain wave-lengths of light as the temperature changes. 
A trace of iron oxide, for instance, turns the conducting glass blue-green 
while plastic and the colour disappears when cold, whereas a small amount 
of copper oxide gives a clear molten glass which becomes a dark blue-green 
on cooling. Although the actual resistivity of the glass is not likely to be 
VOL. XXVIII. 41 
