138 
not contiguous to the Durham Coal-field. The supplies of 
coal and fire-clay obtained from this district in large quanti- 
ties, make the Cleveland Iron the cheapest in the world. 
The district known as the Yorkshire Coal-field (from 
Leeds southward) contains in its ganisterbeds and various fire- 
clays, its carbonaceous iron ores, and its excellent coals, all 
the materials necessary for making the best iron that can be 
produced ; the reason of this superiority may perhaps be 
best explained if we first consider what is required in these 
substances. 1st — 
FIRE-RESISTING MATERIALS. 
The requisite materials for withstanding heat are various, 
and have to be regulated by the kind and quantity of heat 
used ; they must be of such a nature that they may not be 
decomposed by, nor combine chemically with, the substances 
heated in the furnaces or crucibles made of them. 
Thus, graphite crucibles are perhaps the best for resisting 
absolute heat, but when used for melting steel cause a 
deterioration in quality, by its chemical combination with 
the carbon. 
It is practically impossible to melt silica, but at the same 
time, a furnace made of silica would be of no use, because 
repeated heating and cooling would crumble it into sand, 
though it would not melt it. Materials must therefore be 
found containing silica, with such a proportion of alumina 
and an alkaline earth, such as lime, potash, or soda, as wiQ 
combine chemically and flux with it to such an extent as to 
prevent its crumbling down, and at the same time, not so 
much as to allow it to flow into a liquid. 
It wiU be seen that these proportions will vary with 
almost every use to which these materials can be put. It 
would be useless to provide the same substances for moderate 
temperatures, which are absolutely necessary when high tem- 
