630 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The apparatus for performing this operation is shown in fig. 1. 
A Fletcher blast gas furnace is used for fusing the mixture, the wind 
pressure being maintained by a motor-driven blower running at 500 
revolutions per minute. The platinum crucible is wedged into a clay one 
by means of small pieces of platinum foil, and a hole bored near the bottom 
of the outer crucible prevents distortion of the inner one, due to pressure 
differences when in the vacuum pot. There is considerable efflorescence 
when the mixture is first heated, so that to prevent foaming over the edge 
of the platinum crucible it was found best to make a supply of roughly 
fused glass in a clay pot and to break up the slab formed by pouring this 
out upon a brass plate, and to use it in the platinum crucible. 
The heat is then increased till the whole mass fuses and becomes very 
fluid. At first the contents appear cloudy with air-bubbles, which soon 
begin to rise and burst. After twenty minutes at a white heat, and 
before the liquid glass has lost its cloudy appearance, the crucible is 
removed with tongs from the furnace and placed in the vacuum pot V. 
This is a gunmetal casting 7 inches high and 6 inches in diameter, jacketed 
as shown, and kept cool by a flow of water. The gunmetal lid L rests upon 
a flat ring of rubber R, and is provided with a window, made preferably of 
quartz, and cemented in position with any suitable heat-resisting material. 
The platinum crucible retains its heat well on account of its clay cover- 
ing, and the time taken to place it within the vacuum pot and close the lid 
need not exceed three seconds. The tap T 1 being closed, T 2 is opened 
carefully, thus placing the pot in communication with the reservoir S 
(previously pumped out by means of a mechanical air pump). The 
mercury gauge M shows the pressure in the pot Y when T 2 is wide open. 
The capacity of S should be large enough to maintain the vacuum practi- 
cally constant with the pump working, in spite of the gas given off by the 
molten glass. At first the bubbles rush out so rapidly that some practice is 
required to prevent the contents of the crucible rising over the lip. For 
fully two minutes they continue to stream off, and then the tap T 2 is closed 
and r L\ opened, so that clean air may enter through the cotton- wool in C. 
The lid can now be raised and the crucible replaced in the furnace for a 
second heating. 
After four times in the vacuum vessel the fluid glass is so clear that 
the bottom of the platinum crucible may be easily seen through it. 
The process of stirring is then begun. A rod of fused silica or clay was 
rapidly attacked, and a platinum stirrer became necessary. In order to 
minimise the cost of this, advantage was taken of the recent improvements 
in the manufacture of fused silica tubing. 
