Water should be kept clean, and it may be heated to about 100° F., 38° C, with a diminishing risk to cracking. Dip straight articles vertically, and here two opposite influences occur, for if the article is allowed to remain still, the spheroidal state follows - if the author may so put it - and consequently the plate is not so hard, whereas if moved about there is an unequal hardening produced, that portion leading being the hardest; hence, it has been found most beneficial to inject a full stream of water upon the article in specially constructed tanks. Undoubtedly, water that has been used for a considerable period has a superiority for hardening, which may be chiefly attributed to the fact that with use the dissolved air has been expelled. The ultimate hardness depends, first, upon the amount of combined or hardening carbon which may be taken as a true alloy of iron, and as the content of this is higher or lower - tenths per cent. - so is the degree of susceptibility to hardness; Secondly, upon the difference between the temperature of the article and the cooling medium, and also the conductivity of this medium. The existence of the carbon in different forms is a much discussed point, for by the action of solvents more than one modification is discovered, two of winch only it is proposed to deal with, as sufficient for our purpose. The effect of annealing appears to be the reduction of the combined carbon, Section II., Part II. p. 78 - a separation out in the interior, and a partial disappearance upon the exterior. Sudden cooling acts exactly opposite, retaining it in its combined form.

Compression appears to bear an important influence, for as the content of carbon increases, the elastic limit is also correspondingly increased, and therefore a more powerful and rigid compression takes place; that is, the outer layers, by virtue of their elastic limit, refuse to surrender to the resistance to compression offered by the inner layers, and therefore the greater the differences between the temperature of the article and the cooling medium, the more serious is the trial to the elastic limit; and consequently, in steels whose chemical composition has raised the elastic limit to he their ultimate strength, it is only natural to expect serious loss by cracking. Finally, hardness and brittleness go together, according to chemical composition, and tempering restores to a certain extent the elasticity, without destroying to a serious degree the hardness. The hardening is generally contined to the immediate vicinity of the edge, and as the heat travels forward the well-known colours develop indicating certain temperatures, and as the desired colour appears, its progress is finally stopped by total quenching. Although this colour may indicate quite definitely the temperature of the material in hand, it cannot possibly indicate the exact temper, because this depends upon the chemical composition, which condition must be known to the operator, or experience gained upon this point. The foregoing has been stated upon the assumption that hardening upon sudden cooling is due to a change or difference in the condition of the carbon, but the most recent theories propounded are, that hardening is due to an allotropic change in the condition of the iron itself - in fact, that iron is a polymorphus element like sulphur; but whether this phenomena is due to chemical reaction or allotropic changes, or both, it is not for the author to discuss in the present work.

Case-hardening is a short period cementation process, the theory of which is well known and unnecessary for discussion here, the benefit being a steely surface with a soft tenacious backing. It is produced by packing the articles to be hardened in an air-tight box with animal carbon and cyanogen compounds, then gradually raising the temperature and maintaining it at a red heat until the required depth and degree of hardness is obtained The crank-pins, and the end of the coupling-rods that require to be case-hardened, generally receive about twelve and eighteen hours' heating respectively; the motion and pins seldom require more than twelve hours, and in certain classes of steel for milling cutters about four to six hours, the cutters being quenched in oil. A small test sample is placed in each box, about ¾ inch diameter and 5 inches long, and each sample is registered, together with the articles case-hardened in the same box, which is heated in a small Siemens or other suitable furnace. Soda and lime or other alkaline substances assist the process, which in works other than locomotive, especially in relation to cotton machinery, is a large undertaking in itself, as much as 6 tons of bone alone being used in one week. A rapid production of this hardness may be obtained, but only to a slight degree in depth, by rubbing the heated surface with prussiate of potash and quenching. Annealing has been thoroughly dealt with in relation to steel castings, page 72, and toughening bears a direct relationship to annealing; but toughening by water quenching has been made the special study of a material with a high percentage content of manganese by Mr. R. A. Hadfield, of Sheffield.