COMPOSITION OF CAST IRON. 121 
with the fuel as soon as it enters into the furnace, and that the 
oxygen of the air of the cold blast is not all consumed immedi- 
ately, but makes its way upwards, and is gradually consumed in 
its ascent, producing a scattered heat which is of no use in 
smelting the iron, but merely serves to consume the fuel. When 
the hot blast is used the combustion is concentrated towards the 
bottom of the furnace ; with the cold blast it is much more dif- 
fused. Hence the reason of the saving of the coals in the former 
case, which constitutes the great advantage attending the new 
method. 
This greater concentration of the combustion must subject 
the iron to a greater heat than when the combustion is more 
scattered. This accounts for the smaller quantity of limestone 
necessary for separating the clay; for the higher the tempera- 
ture the smaller is the quantity of limestone necessary for the 
fusion of the clay. Hence also the greater rapidity of the pro- 
cess, and consequently the additional quantity of cast iron ob- 
tained from a furnace in a given time. 
I think then we may conclude, that when the hot blast is 
usedthe heat is more concentrated, and consequently higher than 
when the cold blast is employed. 
I shall now state the result of various analyses of cast iron, 
No. 1, smelted by means of cold, and also by means of hot air 
in the different iron works round Glasgow. These analyses 
were made in my laboratory, partly by myself and partly by Mr. 
John Tennent, upon whose accuracy and skill I could completely 
rely. All the iron works round Glasgow employ ut present 
nothing but heated air, except the Carron Company, who are 
in the habit of making cast iron, No. 1, both by the hot and cold 
blast. -I applied to the manager of the Carron works, and he 
very kindly supplied me with specimens of cast iron, No. 1, 
made by both processes. These specimens I carefully analyzed, 
and considered the comparison of the two specimens as very 
satisfactory, because the nature of the ore and the process was 
exactly the same in both cases, and because the Carron Com- 
pany have the reputation of making cast iron of the very best 
quality. I had specimens of cast iron, No. 1, from the Clyde 
Iron Works which I had obtained before the new process was 
known, and consequently when nothing but the cold blast was 
employed; and I had also specimens of cold blast iron from 
Muirkirk, and some which had been given me as Swedish cast 
iron. 
I shall now point out the differences which were observed 
between cast iron, No. 1, made by the cold and the hot blast. 
1. The specific gravity of cast iron smelted by the cold blast 
is less than that of cast iron by the hot blast. 
