28 
MR. FARADAY ON THE 
earthenware which will not fly in the fire, contains but little iron, and is free 
from glaze, are to be prepared, of such size that they will fit in loosely round 
the guage, covering the rest of that 
half of the chamber bottom, and serv- 
ing to support the sides of the tray 
when in its place. This support to 
the tray is highly needful; for, other- 
wise, the weight of the glass, and the 
action of stirring, would be more than 
the thin and heated platina could sup- 
port. The thickness of the pieces 
should be, for the plate in question, about 1 inch, and they should be all 
uniform in that respect. They should never rise so high as the edge of the 
platina, lest glass should accidentally pass from the tray to them, or im- 
purities from them to the glass. An excellent guide to their thickness is, to 
make it similar to that of the intended plate. When they have been roughly 
arranged around the guage, the latter should be withdrawn, and the tray 
itself introduced, the pieces being now finally adjusted about it. They should 
not be so arranged as to press against its sides ; but the latter should be at 
liberty, though only so much, that upon the least tendency of the sides out- 
wards, they should be supported by the pieces. The assistance thus given 
should be directed rather to the sides than the corners, and it is better that 
the latter should not be in contact with these adjuncts, but be allowed to sus- 
tain themselves, for they are strong enough for the purpose, and the corners 
are always those places at which, from one circumstance or another, the glass 
is most likely to pass outwards. 
58. The piece of earthenware which is fitted nearest the mouth of the air tube 
should have its angle taken off, or some other provision made, as by making 
the orifice of the tube oblique, that the passage of air may be uninterrupted; 
and on that side the tube itself may frequently form the support to the tray. 
If it does, and is glazed on the exterior, a piece of loose platina foil should be 
wrapped round it at the part where it touehes the tray, to prevent adhesion 
by the glaze when cold. The general disposition of the tray, the tube, and 
the packings, may be seen in the sketch above. 
