It further appears that the rapidity of the first cooling, from the 1112° to 1292° F. (600° to 700° C), to which steel has commonly been heated, to 572° to 752° F. (300° to 400° C), has a manifold greater influence on the degree of hardness than the succeeding cooling. Thus, Jarolimek has shown that steel wire may be very well hardened both in watery vapour and in molten tin, lead, and even zinc, though the last - named metal does not melt under 752° F. (400° C), while the cooling of the same steel wire from 572° or 752o F. (300° to 400° C.) to 32° F. (0° C), does not cause any true hardening, however rapidly it may proceed. In order that steel wire may be hardened in this way, it is not, however, allowed to remain any considerable time in the molten bath of metal, for by long - continued heating, following such a hardening, the degree of hardening is afterwards diminished more and more. If it be taken out again after being dipped in the bath for quite a short time, and afterwards allowed to cool in the air, the degree of hardening for small articles is equal to that attained by ordinary hardening with the tempering following upon it.

Of the effects produced by hardening, it was in old times mainly the hardness on which attention was fixed, and from this is derived the old saying that a substance does not take hardening if it do not thereby become so hard that a common file can no longer exert any noteworthy influence upon it. From time immemorial a distinction has also been made between iron and steel in this way, that the former, with common hardening in water, is not hardened in the sense just indicated, while steel, on the contrary, is hardened. We sometimes hear it brought as an objection against the old way of distinguishing between iron and steel, that it is difficult to determine whether a piece, after common hardening in water, is to be considered as having taken true hardening or not. But such a reason is in fact quite unwarranted, because, according to the old view, only the varieties approximating most closely to each other of the hardest iron and the softest steel can be mistaken for each other, and such a mistake is indeed of little importance when compared with the great mistake just referred to, of soft iron for soft steel.

If it be wished wholly to avoid the possibility of making mistakes between hard iron and soft steel, this even ought to be attained very easily by the method of determination, in which a sharp - edged splinter of a certain mineral - felspar, for instance - scratches iron, although, after being heated to moderate redness, it has been suddenly cooled in cold water, while steel, after similar treatment, cannot be scratched by the same mineral. The substance which exerts the greatest influence on the increase of hardness by a certain hardening process is the content of combined carbon in the iron. Iron completely free from carbon is, even after hardening in mercury, as soft as before, and an otherwise pure iron, with at most J Per cent. carbon, does not become very much harder by hardening; but, on the other hand, as the content of carbon increases, the difference in the degree' of hardness before and after hardening increases more and more, so that the boundary - line between iron and steel lies in general at a content of carbon of about 0.4 per cent., this depending, however, upon the iron's content of certain other substances, which also exercise some influence on the degree of hardness.

In the closest connection with the increase of the hardness by hardening stand the raising of the limit of elasticity, the breaking strain or ultimate tensile strength, and the diminution in ductility. Unfortunately, the researches that have been carried out regarding these points are not yet numerous enough to enable us with figures to express completely all the changes in these respects which are caused by hardening in iron and steel with different contents of carbon, but sufficient experiments have already been made to give us somewhat satisfactory ideas on this point. A comparison snows that the effect of hardening is in general less in the case of the weld - iron, loose or open in its texture, than in that of the dense or compact ingot - iron; but in proportion as the former even is denser or freer of cinder, hardening has a greater effect upon it, as is shown by a comparison both of the more compact Lesjofors iron with the other sorts of iron refined in the open hearth, and of the more compact Surahammar with the other sorts of puddled iron. In order to augment considerably the strength of ordinary puddled iron, a French iron - manufacturing company - increases the hardening power of water by adding to it sulphuric acid.

The cooling effect of water is thereby raised, and thus also its hardening power; but in order to prevent the corrosion and rusting of the iron, it would be advisable to endeavour to attain the same result in some other way, as by the addition of some salt that would have less corrosire action upon the iron. Hitherto We have considered the influence of harden-ing upon iron, but if we proceed to in-vestigate its action on steel, we find that it is shown chiefly by an increase in its hardness and a diminution in its ductility greater in the same proportion as the steel is richer in carbon and the hardening fluid employed is more powerful in its action. At the same time that steel with an increased content of carbon becomes, through a certain hard - ening, all the harder, it becomes thus at the same time more brittle; and in the closest connection with this is the fact that in the hard steel, rich in carbon, the limit of elasticity is increased by hardening much more than the ultimate tensile strength, so that these in the strongly - hardened hard steel even coincide.

Provided the method of hardening is adjusted to the degree of hardness of the steel, so that it is less powerful in the same proportion as the content of carbon in the steel is greater, it may be asserted that the breaking weight is increased by hardening, even in the case of steel; but if the hardening be too strong, the ultimate tensile strength of hard steel is thereby diminished quite rapidly; or the steel breaks in pieces of itself either during the hardening or a short while after. It is on account of this brittleness or deficient ductility that the hardened steel is usually tempered or heated to 392° to 572° F. (200° to 300° C), for thereby its ductility is somewhat increased, but its hardness at the same time also diminished. This is the case most of all with the outer layer, which of course is that which it is desired should be hardest, and to avoid this and the trouble and loss of time connected with the process just mentioned, the hardening itself is sometimes instead so modified that its effect is equal to that of a more powerful hardening followed by tempering. For such a method, however, more than common skill and practice are required, and it is therefore comparatively seldom used.