MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
23 
at the same time the substances are fused and access for stirring 1 allowed, the 
essential condition of excluding impurity or reducing matter is also fulfilled. 
45. The fire-place itself is of the ordinary construction, and fed with fuel by 
an aperture in front in the usual way. I have found abundant reason to be 
satisfied that the passage of steam beneath the bars of the grate is of con- 
siderable use; for which reason an iron trough charged with water occupies the 
lower part of the ash-pit. The bars are by this arrangement preserved very 
cool and do not burn away ; they are easily kept open and clear of clinkers ; 
the free passage of air to the fire is permitted ; and the action of the furnace 
retained at a high point for any number of hours together. 
46. That part of the furnace beneath the chamber requires peculiar and 
careful arrangement ; for at the same time that such a heat as will soften the 
neighbouring materials is produced there, the bottom of the chamber in its 
softened state and charged with several pounds of materials, has to be firmly 
supported for many hours together without change of position. 
47- The coke necessary in this part is introduced by two or more holes in 
the side of the furnace, which, when necessary, are stopped by bricks. The 
bottom of the chamber is supported on ledges at the sides, and upon the ends 
of fire bricks in the middle, firmly placed at intervals so as neither to stop 
the passage for smoke and flame, nor the cross passages for the introduction of 
coke. 
48. The value of the coke arrangement in this as in the other furnace is 
very great. The heat obtained by the united action of the coke and the flame 
from the fire-place, is abundantly sufficient ; and whilst obtained at the 
necessary point does not involve that degree of mechanical action required 
for stoking and stirring, which is necessary with coals, and would risk the 
destruction of the soft thin bottom of the glass chamber. It further occa- 
sions the perfect combustion of the smoke produced in the coal fire, which at 
first was so considerable in quantity that, had it continued unaltered, the ex- 
periments must have been removed from the Royal Institution ; in which case 
they would probably have been discontinued altogether. 
The flue is the same as that connected with the former furnace, and has 
a damper for regulating the heat, especially useful during the annealing 
operation. 
