128 Remarks on the Cutting: of’ Sécel by Soft Iron. 
steel is broken or separated by fracture,* with much less force 
than when heated less or more, the requisite temperature va- 
rying probably in proportion to the carbon contained in the 
steel. 
The result of the copper wheel mentioned by MM. Da- 
rier and Colladon having no action on the steel, goes far to 
prove that the effect depends at least as much on heut soft- 
ening the steel, to a certain degree, as on percussion, copper 
having but little disposition to generate heat under any cir- 
cumstances, a fact duly appreciated by the manufacturers of 
gunpowder. 
The reason why ‘the heat should be nearly all concen- 
trated in the steel and scarcely perceptible in the iron,” F 
think to be this; the percussion against the steel is continual, 
but against any one part of the iron cutter, perhaps not more 
than from 53, to s$5 part of the time ; consequently the 
heat received by each would be in an inverse proportion of 
the thickness of the steel to the circumference of the iron, 
‘after making the proper allowance for what may be thrown 
off from the circular cutting iron in its passage through the 
nir, which must be considerable. 
P. S. As evidence of the absence of heat, it is stated in 
‘the memoir of MM. Darier and Colladon that the small 
particles of steel adhering to the edge of the cutter, “seen 
through a lens, did not appear as if untempered, and when 
tried with a file, were found as hard as the best tempered 
steel.” 
I have never observed the appearance of the particles, cr 
examined their temper, but have examined the burr raised 
in cutting a plate of steel, which before the operation was 
sufficiently soft to file with ease, but in the operation became 
hardened on the outer edge much harder than before, which 
was evidently caused by the great heat and by being sudden- 
ly cooied by the current of air caused by the motion of the 
cutter; the same would be the case with particles disen- 
gaged by heat, or when hot, and adhering to the edge of the 
* The disposition to be easily separated by fracture at a particulay 
heat exists in carbonized or cast iron, in the alloys of copper and of tin, 
is very perceptible in flint glass, and perhaps in all factitious metallic 
compcunds ; some requiring a moderate, and others a more intense.heat. 
