One of the most serious evils in hardening steel, especially in thick blocks, or those which are unequally thick and thin, is their liability to crack, from the sudden transition; and in reference to hardening razors, a case in point, Mr. Stodart mentions it as the observation and practice of one of his workmen, "that the charcoal fire should be made up with shavings of leather;" and upon being asked what good he supposed the leather could do, this workman replied, "that he could take upon him to say that he never had a razor crack in the hardening since he had used this method, though it was a frequent occurrence before."

"When," says Mr. Stodart, "brittle substances crack in cooling, it always happens from the outside contracting and becoming too small to contain the interior parts. But it is known that hard steel occupies more space than when soft; and it may easily be inferred that the nearer the steel approaches to the state of iron, the less will be this increase of dimensions. If, then, we suppose a razor, or any other piece of steel, to be heated in an open fire with a current of air passing through it, the external part will, by the loss of carbon, become less steely than before; and when the whole piece comes to be hardened, the inside will be too large for the external part, which will probably crack. But if the piece of steel be wrapped up in the cementing mixture, or if the fire itself contain animal coal, and is put together so as to operate in the manner of that mixture, the external part, instead of being degraded by this heat, will be more carbonated than the internal part, in consequence of which it will be so far from splitting or bursting during its cooling, that it will be acted upon in a contrary direction, tending to render it more dense and solid.

* Great diversity of opinion exists respecting the cause of elasticity in springs; by some it is referred to different states of electricity; by others the elasticity is considered to reside in the thin, blue, oxidized surface, the removal of which is thought to destroy the elasticity, much in the same manner that the elasticity of a cane is greatly lost by stripping off its siliceous rind. The elasticity of a thick spring is certainly much impaired by grinding off a small quantity of its exterior metal, which is harder than the inner portion; and perhaps thin springs sustain in the polishing a proportional loss, which is to them equally fatal.

Mr. Dent stated at the British Association, 1841, that he found experimentally the bare removal of the blue tint from a pendulum spring, by its immersion in weak acid, caused the chronometer to lose nearly one minute each hour; a second and equal immersion scarcely caused any further loss. He also stated it as a well-known fact, that such springs get stronger, in a minute degree, during the first two or three years they are in use, from some atmospheric change; when the springs are coated with gold by the electrotype process, no such change is observable, and the covering although perfect may be so thin as not to compensate for the loss of the blue oxidized surface.

"The cracking which so often occurs on the immersion of steel articles in water, does not appear to arise so much from any decarbonisation of the surface merely, as from the sudden condensation and contraction of a superficial portion of the metal, while the mass inside remains swelled with the heat, and probably expands panda for a moment, on the outside coming in contact with the water."

The file-makers, to save their works from clinking or cracking partly through in hardening, draw the files through yeast, beer-grounds, or any sticky material, and then through a mixture of common salt and animal hoof roasted and pounded. This is corroborative of the above, as in the like manner it supplies a little carbon to the outside, and also renders the steel somewhat harder and less disposed to crack; the composition also renders the more important service of protecting the fine points of the teeth from being injured by the fire. An analogous method is now practised in hardening Jones's patent axletrees, which are of wrought iron, with two pieces of steel welded into the lower side, where they rest upon the wheels and sustain the load. The work is heated in an open foge fire, quite in the ordinary way, and when it is removed, a mixture, principally the prussiate of potash, is laid upon the steel; the axletree is then immediately immersed in water, and additional water is allowed to fall upon it from a cistern. The steel is considered to become very materially harder for the treatment, and the iron around the same is also partially hardened.*

These are, in fact, applications of the case-hardening process, which is usually applied to wrought iron for giving it a steely exterior, as the name very properly implies. Occasionally, steel which hardens but imperfectly, either from an original defect in the material, or from its having become deteriorated by bad treatment, or too frequent passage through the fire, is submitted to the case-hardening process in the ordinary way, by inclosing the objects in iron boxes, as will be explained. This in part restores the carbon which has been lost, and the steel admits of being hardened, but this practice is not to be generally recommended, although it is well employed for the purposes of transfer engraving explained at foot; a method introduced by Mr. Jacob Perkins, and which took its origin in the curious transfer processes of the calico works, wherein however copper is the material principally used, †

Various methods have been likewise attempted to prevent the distortions to which work is liable in the operation of hardening,

* These axletrees are used for Jones's Patent wrought-iron suspension wheels, which have iron naves made of chilled castings; to the prussiate is added one-third of the carbonate of ammonia; the effect of the carbonate is principally considered to be the more minute subdivision of the prussiate over the surface of the steel.