MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
27 
55. Hence it became necessary to use some certain means of maintaining 
an oxygenating atmosphere about the glass ; to obtain which, and also to pre- 
vent any other injurious vapours from the fire entering the space beneath and 
within the earthenware covers (50), the expedient was adopted of allowing a 
current of fresh air to pass continually into that space and circulate about the 
glass. To effect this, a clean earthenware tube, glazed within, was let horizon- 
tally into the side of the furnace, in such a manner that one extremity was 
flush with the inside of the chamber, and of such height, that its lower edge 
corresponded with the level of the bottom upon which the glass in its tray was 
to be placed, whilst the other end of the tube reached to and was flush with the 
outside of the furnace. A loose piece of tube, similar in kind but smaller in dia- 
meter, being laid upon the bottom of the chamber, and applied at its end to the 
orifice of the larger one, served as a continuation of it until the inner extremity 
reached to and was under the cover of the glass experiment. When the furnace 
was hot, there was always a draught inwards through this tube ; but the quan- 
tity of air admitted was regulated by a valve (70). The air, by first passing 
through the hot sides of the furnace, then through the shorter ignited tube ser- 
ving for connection, was transmitted in a thoroughly heated state to the place 
where its presence was required, without producing any serious cooling effect ; 
it there maintained a continually oxygenating atmosphere, and, judging from 
the effects, prevented the draught inwards of any vapours from the fire to the 
space beneath the glass covers. 
56. The next point of importance, in the preparation of the glass, is the ar- 
rangement of the tray in the furnace, whose powers have just been described. 
To understand this, it will be necessary to say that the glass chamber is 25 
inches long, 13 inches wide, and 8 inches deep, and that the fire being at one end, 
the flue is at the other. Plates of glass 7 inches square have been made in it ; 
but it would probably require a larger furnace to make much larger pieces. 
5 7. The bottom of the chamber being perfectly level and clean, the guage 
board, on which the tray was formed (35), 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 guage as far as the middle, or even towards the other 
side of the chamber. After this, pieces of Cornish tile (53), or other clean 
e 2 
