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peratures must be used. But besides this, furnaces are only 
heated on the inside, and thej must be made sufficiently 
thick to bear the wear and tear to which they are subjected. 
The bricks are bad conductors of heat, and being heated 
only to a little depth, the consequence is, that many fire- 
bricks, even those considered of good quality, become worn 
out, not by gradual fusion, but by breaking off to the depth 
penetrated by the heat. The pieces that snip off* sometimes 
fall into the furnace at once, and at others remain semi- 
detached ; when the heat is again increased, it gets behind 
the detached piece and fuses it into the form of drops ; 
should the fused brick fall on the iron it will cause a bad 
forging. It has been attempted to prevent this, in the case 
of fire-bricks used for some of the Durham coke ovens, 
(which have to be heated frequently and then suddenly 
cooled to a certain extent, by injecting water, in order to cool 
the coke before it is drawn out), by making them consider- 
ably less in section (about 2-3- in. square), but of the same 
length as ordinary bricks. It is also sometimes attempted, 
instead of building furnaces with the ends of the bricks 
presented to the heat, to build them a double thickness, with 
the side of the brick presented to the heat, so as to allow 
the inner row of bricks to expand ; but it is clear that if a 
single brick gives way, the flame will get behind that thick- 
ness of bricks, and will soon bring down the whole side. 
Fire-bricks, therefore, will be better and last longer, if they 
contain a substance which will conduct the heat through 
the brick, and make the expansion more regular, even though 
this substance does not improve its infusibility. This 
quality appears to be present in bricks containing a greater 
proportion of peroxide of iron than the ordinary fire-brick. 
The minerals to meet these wants are found in the Ganister 
which lies in the Lower Coal Measures, just above the MiU- 
stone Grit, both at the north and west boundaries of the 
