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POPULAR SCIENCE REVIEW. 
the Report observes, it is difficult to see bow it can possibly 
account for tempering, i.e., for tbe nice gradations of hardness, 
varying with each small variation of temperature, and showing 
each its characteristic colour, which have been described above. 
Moreover, as there is no known process of liquefying carbon, 
it is hard to believe that it is effected by the mere presence of 
hot air without the occurrence of any chemical reaction. 
Barba and Akerman advance a theory altogether different, 
viz., that the severe compression in the several layers, produced 
by their contraction during rapid cooling, retains a greater pro- 
portion of carbon in solution, instead of allowing it to separate 
out as graphite. Akerman supposes that the compression itself 
makes the metal more compact, and therefore harder, exactly 
as it is hardened in cold-rolling and wire-drawing. But, as the 
Report observes, the outer layers, which cool first, are brought 
into a state of tension, not compression, by their efforts to con- 
tract over the still heated core ; and yet it is precisely these 
outer layers which attain the greatest degree of hardness. There 
is, in fact, no reason to think that a piece of metal ( e.g ., a thin 
sheet of steel) heated and then cooled uniformly, would suffer 
any internal compression at all ; yet it would undoubtedly be 
hardened. 
The exact molecular changes that occur during the heating 
and cooling of steel were ably discussed by Chernoff in 1868. 
His view is, that there is a certain temperature, a, which must 
be overpassed before any hardening effect can be produced by 
cooling ; and a higher temperature, b, at which steel takes an 
amorphous, wax-like form, and on cooling from which it crystal- 
lizes into large crystals if the process is slow and undisturbed, but 
into small crystals if the process is rapid or disturbed by 
hammering. How for toughness and uniformity, in almost any 
metal, fine regular grain is essential ; hence steel is improved, 
while at the same time hardened, by heating above temperature 
b, and then cooling rapidly. If it is again heated, it begins to 
soften, without losing its quality ; but, so long as the heat is 
below temperature a, any required degree of softness can be 
permanently fixed, by simply quenching the steel as soon as it 
has reached the proper point, as indicated by the characteristic 
colour. The phenomena of tempering are thus in some measure 
explained ; but those of hardening still remain a mystery, 
since, although large crystals are unfavourable to the tough- 
ness, there is no reason to suppose that they affect the hardness 
of a metal. The existence, however, of some such molecular 
changes as Chernoff describes is confirmed by many other facts, 
e.g., the sudden and temporary expansions observed during the 
cooling of iron wire by Gore and Norris. 
Dissatisfied with the theories of their predecessors, the 
