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COMPOSITION OF CAST IRON. 119 
Carbonate of iron . . - . + - 29°03 
Carbonate of lime . . . - « - 52 
Carbonate of magnesia. « » - «+ 3°59 
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If we abstract the coal, the carbonate of iron will amount to 
37 per cent. of the ore. In another specimen of the same band 
containing 54 per cent. of coal, I found 62 per cent. of carbonate 
of iron. 
It is remarkable that the proportion between the silica and 
alumina in the two specimens was quite different. In the first 
there were 121 atoms silica to 9 atoms of alumina, and in the 
second 124 atoms silica to 2 of alumina. This seems to show 
that the clay in the clay iron stone does not owe its existence 
to the decomposition of any mineral consisting of a definite 
compound of silica and alumina. ; 
The existence of these foreign bodies in the clay iron stone, 
from which the cast iron subjected to analysis was derived, will 
enable us to understand the source of certain substances from 
which no cast iron hitherto examined is free. The ore, before 
it is put into the furnace, is always roasted, which drives off the 
carbonic acid from the carbonate of iron, and thus reduces the 
weight of the ore, at an average, about 31 per cent. 
It is also mixed with carbonate of lime, which has the well- 
known property of fusing with clay into a liquid glass when 
sufficiently heated. This removes the clay from the ore, and 
enables the oxide of iron to come in contact with the ignited 
coals, which reduce it to the metallic state. I subjected the 
limestones used at most of the smelting houses round Glasgow 
to a chemical analysis. I need not observe that none of them 
was a pure carbonate of lime; for even the most transparent 
and colourless calcareous spar always contains a sensible quan- 
tity of foreign matter. The purest limestone I met with con- 
tained 94°6 per cent. of carbonate of lime. The foreign matter 
in all was silica, alumina, and peroxide of iron. In one only I 
found carbonate of magnesia, to the amount of 2 per cent., and 
in none could I detect the presence of manganese. 
The coal used for fuel leaves, when incinerated, from 1 to 10 per 
cent of ashes. They are composed chiefly of silica, alumina, 
and oxide of iron. Coal is seldom quite free from iron pyrites. 
This enables us to account for the presence of minute quantities 
of sulphur in some of the specimens of cast iron analyzed. 
When the Clyde iron works were established, about 50 years 
ago, to obtain 1 ton of cast iron ten tons of coals were required. 
This coal was previously converted into coke, by which process 
