30 
MR. FARADAY ON THE 
the tray and its charge is made. The air tube is carefully wiped, and its ex- 
ternal aperture closed by a clean loose plug of dry sponge. The tray is for 
the last time freed from dust by inversion and blowing upon it, and is put into 
its place. The quantity of rough glass necessary for the required plate, about 
8lbs in the present instance (29), is carefully weighed out, and then introduced 
by an evaporating basin, or some other means which shall not allow of the admis- 
sion of any reducing or colouring matter, or permit any portion of glass to pass 
beyond the edges of the tray. The glass covers are then to be arranged in their 
places ; the iron covers of the chamber likewise adjusted, and over all are to 
be placed a set of thick earthenware tiles, which have been fitted together so 
as to constitute a general covering to the whole, well calculated to retain heat. 
63. The ensuing part of the process is one in which the precise order of, and 
most advantageous proceedings have not yet been ascertained. Variations 
have been made up to the very last experiment, and it is only by still more ex- 
tensive experience that the arrangement wall ultimately be settled. 
64. A fire being lighted in the furnace, and some coke put beneath the glass 
chamber, the temperature is gradually raised. In about an hour the bottom 
of the chamber begins to appear ignited, and in four hours the top iron covers 
are usually dull red hot. These appearances are useful, as indications of the 
progress of the operation. When the furnace has been heated for the first half 
hour, then every care is taken that the temperature may be fully sustained to 
the end of the experiment ; and besides the ordinary kind of attention to the 
fire, particular care is taken that coke be supplied, by the lateral holes, to the 
part beneath the chamber ; for, if the fuel there be allowed to burn out, the 
heat soon falls, notwithstanding the flame from the coals. Although the fire 
may seem quickly to have attained its best condition, yet the temperature con- 
tinues to rise in the chamber long afterwards ; for, from the quantity of lateral 
brick-work to be heated, it is usually many hours before the sides of the cham- 
ber are so hot, that the tray and its contents can attain their highest tempera- 
ture. At the same time it must be understood that the heat of the glass is very 
much governed, especially at the early part of an experiment, by the number 
of covers over it, and rises far more rapidly, and much higher, with two or 
three covers than with one. 
65. Perhaps the glass may with propriety be examined once, early in the 
