THE EOYAli AETiLLEEY INSTITUTION. 
151 
may have been successfully avoided, a great risk is run should there be 
any carelessness, especially when it is considered that there is only one 
coil to depend on. 
On the subject of overheating iron. Dr. Percy says, in his “ Metallurgy, 
Iron and Steel,” pp. 8-10 :— 
“The crystallisation of iron has excited much attention, especially amongst 
engineers; and although much has been talked and written about it, yet no small 
confusion respecting it still prevails. However, a careful examination of the 
subject will tend to remove this obscurity. Bar iron acquires a largely crystalline 
structure by long exposure to a temperature which, though high, is yet very far 
below the melting points of the metal. On the application of a certain amount of 
heat, the particles have sufficient freedom of motion to arrange themselves in 
crystals. We have previously had a striking illustration of this fact in the anneal¬ 
ing of sheet zinc, at a temperature bordering on, yet sensibly below its melting 
point. Hence we can readily understand why iron which has been frequently and 
strongly heated, or iron which has been forged into large masses, and which must 
necessarily have been subjected during a considerable time to a high temperature, 
should tend to become largely crystalline in structure. The operation of hammering 
iron while strongly heated and during cooling, to a certain degree will obviously 
interfere with the action of the forces which determine crystalline arrangement, and 
may consequently be expected to diminish the size of the crystals. But in the 
case of large masses, it will be difficult to affect the metal far below the surface, 
unless a very heavy hammer is employed, and very powerful blows are applied; 
and even then it is hardly possible to conceive that uniformity in the size of the 
crystals should be produced through the mass. For, when the exterior may be 
cooled down to redness, the interior must still be at a much higher temperature— 
it may be white hot; so that on subsequent cooling, after the cessation of the 
blows, the particles in one part of the mass will be in a condition to assume a 
more largely crystalline structure than those in another part. It is this which 
constitutes the difficulty in large forgings; and it cannot be overcome by continuing 
the hammering until the metal in the interior is sufficiently reduced in temperature 
to prevent the formation of large crystals in that part ; for if the metal on the 
exterior were hammered at too low a temperature, as would certainly be the case 
in the condition supposed, it would become brittle and tender. It must be borne 
in mind that these remarks relate to iron, and not to steel, or iron containing any 
sensible proportion of carbon. With reference to the size of the crystals, it should 
here be stated that the presence of phosphorus favours the formation of large 
crystals; and this element occurs in most commercial varieties of British iron. 
The rapid cooling of large forgings by immersion in water, might be expected to 
render the interior less largely crystalline. 
“ When iron is hammered cold—especially in various directions—the crystals of 
which it consists will obviously become more or less disaggregated, and therefore 
the strength of the metal will be diminished. The larger the crystals, the more 
easily will the iron break; for, as fracture will occur in the direction of least 
resistance, which is that of the cleavage planes, and of the planes of junction of 
contiguous crystals, it will be facilitated in proportion to the size of these planes. 
I have buttons of fused iron, in which the crystals are so large, that the cleavage 
planes extend completely across the fracture. On the other hand, when the crystals 
are comparatively small, they are, so to speak, more interwoven with each other; 
there are no large cleavage planes, and consequently there is less tendency to 
fracture. Whether the foregoing considerations be correct or not, it is well 
established in practice that largeness of crystal in a bar of iron indicates facility 
of fracture.” 
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