When the malleable metals are hammered, or rolled, they generally increase in hardness, in elasticity, and in density or specific gravity; which effects are produced simply from the closer approximation of their particles, and in this respect steel may be perhaps considered to excel, as the process called hammer-hardening, which simply means hammering without heat, is frequently employed as the sole means of hardening some kinds of steel springs, and for which it answers remarkably well.

After a certain degree of compression, the malleable metals assume their closest and most condensed states; and it then becomes necessary to discontinue the compression or elongation, as it would cause the disunion or cracking of the sheet or wire, or else the metal must be softened by the process of annealing.

The metals, lead, tin, and zinc, are by some considered to be perceptibly softened by immersion in boiling water; but such of the metals as will bear it are generally heated to redness, the cohesion of the mass is for the time reduced, and the metal becomes as soft as at first, and the working and annealing may be thus alternately pursued, until the sheet metal, or the wire, reaches its limit of tenuity.

The generality of the metals and alloys suffer no very observable change, whether or not they are suddenly quenched in water from the red heat. Pure hammered iron, like the rest, appears after annealing, to be equally soft whether suddenly or slowly cooled; some of the impure kinds of malleable iron harden by immersion, but only to an extent that is rather hurtful than useful, and which may be considered as an aeridental quality.

Steel however receives by sudden cooling that extreme degree of hardness combined with tenacity. which places it so incalculably beyond every other material for the manufacture of cutting tools; especially as it likewise admits of a regular gradation from extreme hardness to its softest state, when subsequently re-heated or tempered. Steel therefore assumes a place in the economy of manufactures unapproachable by any other material; consequently we may safely say that without it, it would be impossible to produce nearly all our finished works in metal and other hard substances; for although some of the metallic alloys are remarkable for hardness, and were used for various implements of peaceful industry, and also those of war, before the invention of steel; yet in point of absolute and enduring hardness, and equally so in respect to elasticity and tenacity, they fall exceedingly short of hardened steel.

Hammer-hardening renders the steel more fibrous and less crystalline, and reduces it in bulk; on the other hand, fire hardening makes steel more crystalline, and frequently of greater bulk; but the elastic nature of hammer-hardened steel, will not take so wide nor so efficient a range as that which is fire-hardened.

If we attempt to seek the remarkable difference between pure iron and steel in their chemical analysis, it appears to result from a minute portion of carbon; and cast-iron, which possesses a much larger share, presents, as we should expect, somewhat similar phenomena.* Moreover, as the hard and soft conditions of steel may be reversed backwards and forwards without any rapid chemical change in its substance, it has been pronounced

For the mode of analysis for ascertaining the quantity of carbon in cast-iron and steel, invented by M. V. Regnault, Mining Engineer, See Annales de Chime et de Physique, for January 1839; also Journal of the Franklin Institute, vol. 25, p. 327. It is stated that the analysis is very easy and exact, and may be completed in half an hour.

to result from internal arrangemnet or crystallization, which may be in a degree illustrated and explained by similar changes observed in glass.

A wine-glass, or other object recently blown, and plunged whilst red-hot into cold water, cracks in a thousand places, and even cooled in warm air it is very brittle, and will scarcely endure the slightest violence or sudden change of temperature; and visitors to the glass-house are often shown that a wine-glass or other article of irregular form, breaks in cooling in the open air from its unequal contraction at different parts. But the objects would have become useful, and less disposed to fracture, if they had been allowed to arrange their particles gradually, during their very slow passage through the long annealing oven or leer of the glass-house, the end at which they enter being at the red heat, and the opposite extremity almost cold.

To perfect the annealing, it is not unusual with lamp-glasses, tubes for steam-gages, and similar pieces exposed to sudden transitions of heat and cold, to place them in a vessel of cold water, which is slowly raised to the boiling temperature, kept for some hours at that heat, and then allowed to cool very slowly: the effect thus produced is far from chimerical. For such pieces of flint glass intended for cutting, as are found to be insufficiently annealed, the boiling is sometimes preferred to a second passage through the leer: lamp-glasses are also much less exposed to fracture when they have been once used, as the heat if not too suddenly applied or checked, completes the annealing.

Steel in like manner when suddenly cooled is disposed to crack in pieces, which is a constant source of anxiety; the danger increases with the thickness in the same way as with glass, and the more especially when the works are unequally thick and thin.

Another ground of analogy between glass and steel, appears to exist in the pieces of unannealed glass used for exhibiting the phenomena, formerly called double refraction, but now polariza-tion of light; an effect distinctly traced to its peculiar crystalline structure.

In glass it is supposed to arise from the cooling of the external crust more rapidly than the internal mass; the outer crust is therefore in a state of tension, or restraint, from an attempt to squeeze the inner mass into a smaller space than it seems to require; and from the hasty arrangement of the unannealed glass, the natural positions of its crystals are in a measure disturbed or dislocated.*

In the philosophical toy, the Prince Rupert's drop, this disruption is curiously evident to the sight, as the inner substance is cracked and divided into a multitude of detached parts, held together by the smooth external coat. The unannealed glass, when cautiously heated and slowly cooled, ceases to present the polarizing effect, and the steel similarly treated ceases to be hard, and may we not therefore indulge in the speculation, that in both cases a peculiar crystalline structure is consequent upon the unannealed or hardened state?

In the process of hardening steel, water is by no means essential, as the sole object is to extract its heat rapidly, and the following are examples, commencing with the condition of extreme hardness, and ending with the reverse condition.

A thin heated blade placed between the cold hammer and anvil, or other good conductors of heat, becomes perfectly hard. Thicker pieces of steel, cooled by exposure to the air upon the anvil, become rather hard, but readily admit of being filed. They become softer when placed on the cold cinders, or other bad conductors of heat. Still more soft when placed in hot cinders, or within the fire itself, and cooled by their gradual extinction. When the steel is encased in close boxes with charcoal powder, and it is raised to a red-heat and allowed to cool in the fire or furnace, it assumes its softest state; unless lastly, we proceed to its partial decomposition. This is done by enclosing the steel with iron turnings or filings, the scales from the smith's anvil, lime, or other matters that will abstract the carbon from its surface; by this mode it is superficially decarbonized, or reduced to the condition of pure soft iron, in the manner practised by Mr. Jacob Perkins, in his most ingenious and effective combination of processes, employed for producing in unlimited numbers, absolutely identical impressions of bank notes and cheques, for the prevention of forgery. These methods of treating steel will be hereafter noticed.

* Mr. Pellatt has shown experimentally, that a re-arrangement of the particles of glass occurs in the process of annealing, as of two pieces of the same tube each 40 inches long, the one sent through the leer, contracted one-sixteenth of an inch more than the other, which was cooled as usual in the open air. Tubes for philosophical purposes are not annealed, as their inner surfaces are apt to become soiled with the sulphur of the fuel; they are in consequence very brittle and liable to accident. For other interesting particulars concerning flint-glass, tee Minutes of Conversation, Inst. Civ. Eng., 1840, pp. 37 to 41.

A nearly similar variety of conditions might be referred to as existing in cast-iron in its ordinary state, governed by the mag-nitude, quality, and management of the castings; independently of which, by one particular method, some cast-iron may be rendered externally as hard as the hardest steel; such are called chilled iron castings; and, as the opposite extreme, by a method of annealing combined with partial decomposition, malleable-iron castings may be obtained, so that cast-iron nails may be clenched.

Again, the purest iron, and most varieties of cast-iron, may, by another proceeding, be superficially converted into steel, and then hardened, the operation being appropriately named case-harden-ing. I therefore propose to illustrate these phenomena collec-tively, under three divisions: first the hardening and tempering of steel; secondly, the hardening and annealing of cast-iron; and thirdly, the process of case-hardening.