359 
and strength of hard and soft steel. 
i8,ooolbs. for the tension with a straw yellow temper ; and 
133,000 — 85,000 = 48,ooolbs. for the tension in hard steel. 
And if this view of the subject be correct, the phenomena of 
hardening may be explained in this manner, which nearly 
agrees with what you have observed in your Lectures I, 
p. 644 : after a piece of steel has been raised to a proper 
temperature, a cooling fluid is applied capable of abstracting 
heat more rapidly from the surface than it can be supplied 
from the internal parts of the steel. Whence the contraction 
of the superficial parts round the central ones which are ex- 
panded by heat ; and the contraction of the central parts in 
cooling, while they are extended into a larger space than they 
require at a lower temperature, produces that uniform state 
of tension, which diminishes so much the cohesive force in 
hard steel. The increase of bulk by hardening agrees with 
this explanation ; and it leads one to expect, that any other 
metal might be hardened if we could find a means of 
abstracting heat with greater velocity than its conducting 
power. 
I am, Sir, 
Your most obedient Servant, 
Thomas Tredgold. 
To Dr. Thomas Young, 
fyc. Sj-c. 
