260 On the Manufacture of Optical Glass. [1829. 



should be placed on the middle of the half next the fire, and 

 then a piece of connecting air-tube taken, which being laid on 

 the bottom of the chamber, may extend from the fixed air-tube 

 by the side of the gauge as far as the middle, or even towards 

 the further side of the chamber. After this, pieces of Cornish 

 tile (53), or other clean earthenware which will not fly in the 

 fire, contain but little iron, and are free from glaze, are to be 

 prepared, of such size that they will fit in loosely round the 

 gauge, covering the rest of that half of the chamber bottom, 

 and serving to support the sides of the tray when in its place. 

 This support to the tray is highly needful ; for, otherwise, the 

 weight of the glass, and the action of stirring, would be more 

 than the thin and heated platinum could support. The thick- 

 ness 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 platinum, lest glass 

 should accidentally pass from the tray to them, or impurities 

 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 gauge, 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 should be withdrawn, and the tray itself introduced, the 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 outwards, 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 sustain 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 



