nature as the hardening of steel. There is another fact, which is of great conse quence, and may perhaps afford the prin cipal ground for Rinman's apparent ex ception to the general rule concerning the hardening of steel. Hardness is that property of bodies by which they resist indendation, and rather break, than bend or suffer contusion. It is the opposite to softness. Tenacity, which is very fre quently confounded with hardness, is not the property by which they resist inden tation, but separation of their parts. It is the opposite to friability or brittleness. Now that steel, in common language, is said to be the hardest, which unites most eminently the properties of hardness and tenacity. But the effect of heat and sud den cooling is directly opposite in these two respects. Soft steel is the most tena cious, but the least hard. The operation of hardening diminishes its tenacity, and increases its hardness progressively to the maximum of heat the steel is formed to bear. There will be no difficulty then in conceiving, that the best state of steel, relative to any particular use, will be at some precise degree between the softest and hardest qualities. Thus, for springs, much tenacity and moderate hardness are required. For chissels and similar tools, which operate by a blow, a greater hard ness may be admitted. Razors, knives, and such tools as effect the intended pur pose by a gradual stroke, will be still more valuable the harder they are : but even in these the tenacity must not be too much diminished, otherwise the edge will be liable to break. They must all be capable of having the edge turned or bended on one side in the operation of whetting. Files are perhaps of all tools such as require the greatest hardness ; but in these, it is far from being the ut most the steel is capable of receiving. It is found, that the tenacity of steel is con siderably increased bycontinued hammer ing to a certain point. But the whole ef fect of this hammering is taken off by strong ignition. Good steel by hardening at a white heat may be rendered so brit tle, that it will break full as easily as a stick of the same dimensions, and its tex ture is then found to be coarse and large grained. As the subsequent annealing does not restore the effect of the ham mering, nor bring the grain of the steel to the state it would have possessed if a lower heat had been used at first, it is evi dent, that the most useful hardness is produced by that degree of heat, which is just sufficient to effect the purpose.
And, accordingly, tools made of cast steel, and intended to sustain a good edge for cutting iron and other metals, are not af terwards annealed, but the ignition is carefully regulated at first. Annealing ought only to be used where considera ble softness is required.
Iron may be hardened to a certain de gree by ignition and plunging in water, but this effect is confined to the surface ; except, as it very often happens, the iron contains veins of steel. These are no shall impediments to the filing and work ing this material. It sometimes likewise may happen, that the iron may have un dergone a process of the nature of case hardening from the fuel. We have been informed by a workman, that ignited iron, suddenly plunged into the soft leather of a shoe, becomes very hard on its surface, which must arise from an instantaneous effect of this kind.
The increase ofdimensions acquiredby steel in hardening is such, that in general such pieces of work as are finished soft will not fit their places when hardened. Rieman found, that bars of steel, six inch es long, six lines wide, and half an inch thick, were lengthened at least one line after hardening by a whitish red heat, which is about one seventieth ofthe linear dimensions; and supposing the dilatation to be proportional, Guyton Morveau com putes the bulks to be as 48 to 49. But the cubes of 72 lines and 73 lines are in proportion more nearly as 47 to 50. Va rious kinds of steel at different hardness must no doubt greatly differ in this re spect. The specific gravities, as given by Brisson, p. 366, art. GRAVITY, (specific), afflard a much less ratio. Rieman found, by his experiments on two different kinds of fine cemented steel, the specific gravi ty of which was 7.991, that one after hardening gave only 7.555, and the other 7.708. These numbers agree sufficiently near with the experiment of the six inch bar. Yet he once found Styrian steel aug mented in density, by hardening, in the ratio of 7.822 to 7.782. Morveau found, with pieces of steel 28 lines long, that the increase of length by hardening was about the 350th part.
The fineness of grain in hard steel, as exhibited in its fracture, is various, ac cording to the quality of the metal, and the temper it has received. The harder the steel the coarser the grain. But in like circumstances, fine steel has the closest grain, and is ever the most uni form in its appearance Workmen avail themselves much of this indication. In general, a neat curved line fracture, and even grey texture, denote good steel ; and the appearance of threads, cracks, or brilliant specks, denotes the contrary.
But the management of the forging and other circumstances of manufacturing will modify these indications i and the steel that is good for some purposes may be less suited to others.
The fluid into which ignited steel is plunged is of great consequence. All the facts seem reducible to these general con clusions. The hardness will be greater, 1. The hotter the steel is made, provided it be not decomposed : 2. The more con siderably its temperature is lowered in the cooling : 3. The shorter the time of cooling : and 4. The more favourable the fire or the cooling material may be to the steel-making process. But the most use ful combination of hardness and tenacity will be at a medium temperature in each kind of steel.
With regard to the first particular, lit tle need be said, but that the decompo sition of steel in heating will be prevent ed, and its surface somewhat improved, if it be bedded in charcoal, or the cement ing compound, during the application of the heat. The second and third, namely, the quantity and suddenness of cooling, require an attention to the doctrine of CA LORIC, as explained under that article. The cooling will be more sudden and ef fectual, the greater the quantity of heat absorbed in the same time. There are three circumstances which favour this effect, namely, a very low temperature of the body to which the hot steel is applied; that it should be a good conductor of heat ; or that it shall assume either the fluid or elastic state, which always de /nand a supply of heat for their mainte nance. Thus it is found, that steel is more effectually hardened in cold than in warm water, and, at like temperatures, more ef fectually in mercury than in water. It may also be remarked, that these two fluids cool the steel by different ener gies. The water is partly converted into vapour, which carries off the heat, and leaves the fluid much less altered in tem perature than mercury, which acts by its conducting property. This last fluid, not having evaporated in the process, is found to have acquired a much more elevated temperature by the immersion. Oil is found to harden the surface of steel much more than its internal part, so that it re sists the file, but is much less easily bro ken by the hammer. This effect arises from its imperfect conducting quality, and the elevated temperature it demands to be converted into the vaporous state ; to which we may also add, that a stratum of coal is formed round the steel from the burned oil, which still more effectually prevents the transmission of the heat A remarkable instance of this nature pre sented itself to our observation,in harden. inga small piece of steel two inches long, and a quarter of an inch diameter. Atthe time of ignition, the water nearest at hand had been used with soap. The steel made very little noise when plunged into the water, and remained hot for a consider able time ; but when taken out, was found to be scarcely at all hardened. It was covered with coally matter ; which being cleared off, and the process repeated with clean water, it became perfectly hard. The heat, in both cases, was a low red heat, proper for cast steel which is not intended to be afterwards annealed. We had very little doubt, but that alcohol and the essential oil of turpentine, which are good conductors of heat, and very volatile, would render steel very hard, if their inflammability, and the little neces sity there is for using them, were not an impediment to their application. N'arious artists avail themselves of different sub stances for the immersion of ignited steel. Some use urine, others water charged with common salt, nitre, or sal ammoniac. Tallow and oil are used for such works as are not required to be brittle, though very hard, the reason of which has just been mentioned ; but tallow differs from oil in the heat which becomes latent for its fusion ; and, accordingly, solid tal low is an excellent material for harshening drills and other small articles. It has been found by Reaumur, that' saline li quids produce rather more hardness than common water; and, in particular, that aquafortis possesses this property in an eminent degree, probably from its con. ducting power ; the makers of files co ver them with the grounds of beer and common salt, which assist their harden ing, and keep the surface from scorifying. The mucilage of the beer supplies a coal ly matter; and the fused salt seems not only to form a varnish in the fire and de fend the steel, but may also produce coal, by its sudden solution in the water at the time of immersion. Very small articles heated in a candle are found to be hard ened perfectly by suddenly whirling them in the cold air ; and thin bars or plates of steel, such as the magnetic needle of a compass, acquire a good degree of hard ness by being ignited, then laid on a plate of cold lead, and suddenly covered with another plate. 'These would be unequally hardened, and bend, if plunged in water.