To those, however, who are not interested in the known details of “how” and “why” hardening and softening of steel is possible and why hardening of pure iron and mild steels does not and cannot take place, we must say as would “Ali Baba”:—“Those who do not care to study it better hadn’t.” Anyway, the study of this rather intricate subject is conducive of “headaches,” and perhaps it is not extremely important when viewed from the non-technical standpoint of these articles. We have several times referred to the debt which civilization owes to iron and steel structural materials, machinery and tools and particularly to those tools which have hardened cutting edges. Almost every one knows that hardened cutting edges are imparted to tools by sudden cooling in water or oil from a good red heat. Probably most of us, too, know that the blacksmith can again soften such tools by reheating to the same red heat and allowing them to cool slowly. This he calls annealing. In this softened or annealed condition a piece can readily be sawed or filed, while in its hardened state a saw or file produces no result upon it. Now what are the facts, meaning and the cause of this dual life of the alloy, steel, without which we would be so greatly handicapped. To be better prepared to understand the answer, let us consider three or four accompanying and closely allied phenomena which close observation of the habits of steel has disclosed. The “Point of Recalescence” If we drill a hole in a small piece of carbon tool steel which we are about to put into the heating furnace, and if into this hole we insert the bare tip of an electric pyrometer, this heat-measuring instrument will indicate at all times the rising temperature of the piece of steel as it heats in the red-hot muffle or chamber of the furnace. As we watch the piece grow red, the pyrometer registers 900°, 1000°, 1100°, 1200° F.,—gradually and uniformly indicating higher and higher temperatures. [Illustration: APPARATUS FOR DETERMINING THE CRITICAL POINTS OF STEEL ] But lo! Something must be wrong! The pyrometer needle does not now move forward but is standing still. Though we know that in that hot furnace the piece must be absorbing heat at the same rate as before, yet the pyrometer needle does not budge! But, as our wonderment grows and we are still undecided as to the meaning, the needle again begins to advance and continues again regularly and uniformly to higher and higher temperatures as though it had never taken the vacation. With the piece now at a white heat, we have proceeded far enough with the heating. Turning off the electric current from the furnace and allowing it to cool we again watch the pyrometer needle as the temperature of the piece in the cooling furnace gradually falls. Lower, lower, lower swings the needle, always at a rate approximately uniform.