MINUTES OF PROCEEDINGS OF 
04 
The proportion in each is as follows : — 
Wrought-iron. # 1 to *3 per cent by weight. 
Steel . ’3 to 2 „ _ „ 
Cast-iron . 2 to 5 „ „ 
Carbon imparts hardness and fusibility, but it has the baneful effect of 
making the material brittle. 
If we apply a weight of 4 tons to a bar of cast-iron one inch in section, 
the bar will stretch to a certain extent, and on the weight being removed 
will return to its original length. This weight is called “ the limit of 
elasticity," for did we apply a greater weight the bar would be permanently 
elongated; and when we reached about 10 tons, fracture would take place. 
Hence the latter is called “ the breaking weight." 
The limits of elasticity and the breaking weights differ with the quality 
of the iron, but the following table gives the average of good specimens 
tested by the machine in the Royal Gun Factories. 
Material. 
Limits of elasticity 
per sq. inch. 
Breaking weight 
per sq. inch. 
Wrought-iron (in the direction of its fibre) 
25 tons. 
Cast-iron. 
. 4 „ 
10 „ 
Steel . 
. 13 „ 
31 „ 
Steel toughened in oil . 
45 „ 
(Wrought-iron across its fibre is only about as strong as cast-iron). 
It will be seen from this table that steel, especially when toughened in oil, is 
the toughest, most elastic, and strongest of the iron family, but though it 
bears so well a gradual pressure such as the machine exerts on it, it has the 
glass-like defect of brittleness when exposed to a violent and sudden blow, 
such as the explosion of gun-cotton or even that of gunpowder. Hence a 
large steel gun, unless very carefully manufactured, may burst at any time, 
and when it does so, the fragments will be dangerously numerous; whereas 
wrought-iron, especially when coiled, will not burst explosively under 
ordinary circumstances; on the other hand, steel is much harder than 
wrought-iron, and thus a steel barrel is better able to resist abrasion from 
the studs of the projectile than is a wrought-iron barrel, and steel can also be 
made free from flaws and defects which is more than can be predicted of 
wrought-iron. 
Improvements are taking place every day in the manufacture of steel, and 
it is now adopted altogether for the barrels of our muzzle-loading rifled 
guns, whilst wrought-iron coiled is used for the exterior parts, as its fibrous 
and pliant nature counteracts the explosive tendency of the steel. 
By heating steel to a high temperature and plunging it into water (even 
into boiling water) we harden it very much by the rapidity with which it is 
cooled, but the sudden action makes it brittle and snappish. Oil is however, 
as a liquid, a very bad conductor of heat. It does not boil under 600 F. 
Hot steel, therefore, when plunged into oil parts with its heat slowly and is 
toughened as well as hardened. 
