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110 Observations on Iron and Steel. 
Mr. Nicholson is equally unfortunate in the ac- 
count given in his Chemical Dictionary. He says, 
that the usual time required for the cementation of 
iron is from six to ten hours, and cautions us against 
continuing the cementation too long; whereas the 
operation, from the beginning to the end, requires 
sixteen days at least. In other parts of the opera- 
tion he is equally defective, confounding the making 
of bar with that of cast steel, and not fully describ- 
ing either. In speaking of the uses of steel, or ra- 
ther of what constitutes its superiority, Mr. Nicholson 
is also deficient. He observes, that “ its most use- 
ful and advantageous property is that of becoming 
extremely hard when plunged into water.” He has 
here forgotten every thing respecting the temper, 
and tempering of steel instruments, of which how- 
ever he takes some notice in the same page. 
«s Plunging into water” requires a little explanation: 
for if very hot steel be immersed in cold water with- 
out great caution, it will crack, nay, sometimes break 
to pieces. It is however necessary to be done, in 
order to prevent the steel from growing soft, and 
returning to the state of malleable iron; for, were it 
permitted to cool in the open air, the carbon which 
it holds in combination would be dissipated.* 
- * It is the opinion of some metallurgists, that a partial 
abstraction of oxygen takes place, by plunging hot metal 
into cold water, 
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