Aladdin Oven.
Matter Impressed by Its History.
Every property of matter seems universal. The best non-conductor of heat transmits a little heat; the best conductor is by no means perfect: the two classes of substances are joined by materials which gradually approach one end of the scale or the other. Nothing is so hard but that it may be indented or engraved, and where neither a blow nor severe pressure is employed, we may have, as in the photographic plate, an impression which is chemical instead of mechanical, displaying itself to the eye only when treated with a suitable developer. A bar of steel hammered on an anvil is changed in properties; as it becomes closer in texture its tenacity is increased. When that bar takes its place in a structure, the work it has to do, the shocks it bears, equally tell upon its fibres. Stresses and strains leave their effects upon the stoutest machines, engines, bridges; they are never the same afterward as before, and usually their experience does them harm. Says an eminent engineer, Mr. W. Anderson: “The constant recurrence of stresses, even those within the elastic limit, causes changes in the arrangement of the particles which slowly alter their properties. In this way pieces of machinery, which theoretically were abundantly strong for the work they had to do, have after a time failed. The effect is intensified if the stress is suddenly applied, as in the case of armor plate, or in the wheels of a locomotive. . . . When considerable masses of metal have been forged, or severely pressed while heated, the subsequent cooling of the mass imposes restrictions on the free movement of some if not all the particles, hence internal stresses are developed which slowly assert themselves and often cause unexpected failures. In the manufacture of dies for coinage, of chilled rollers, of shot and shell hardened in an unequal manner, spontaneous fractures take place without apparent cause, through constrained molecular motion of the inner particles gradually extending the motion of the outer ones until a break occurs.”
Sir Benjamin Baker says:—“Many engineers ignore the fact that a bar of iron may be broken in two ways—by a single application of a heavy stress, or by the repeated application of a comparatively light stress. An athlete’s muscles have often been likened to a bar of iron, but if ‘fatigue’ be in question, the simile is very wide of the truth. Intermittent action, the alternative pull and thrust of the rower, or of the laborer turning a winch, is what the muscle likes and the bar abhors. A long time ago Braithwaite correctly attributed the failure of girders, carrying a large brewery vat, to the vessel being sometimes full and sometimes empty, the repeated deflection, although imperceptibly slow and free from vibration, deteriorating the metal, until in the course of years it broke. These girders were of cast iron, but it was equally well known that wrought iron was similarly affected, for Nasmyth afterward called attention to the fact that the alternate strain in axles rendered them weak and brittle, and suggested annealing as a remedy, having found that an axle which would snap with one blow when worn, would bear eighteen blows when new or just after annealing. We know that the toughest wire can be broken if bent backward and forward at a sharp angle; perhaps only to locomotive and marine engineers does it appear that the same result will follow in time even when the bending is so slight as to be unseen by the eye. A locomotive crank-axle bends but 1⁄34 inch, and a straight driving axle but 1⁄64, under the heaviest bending stresses to which they are exposed, and yet their life is limited. Experience proves that a very moderate stress alternating from tension to compression, if repeated about a hundred million times, will cause fracture as surely as bending to a sharp angle repeated a few hundred times.”
Hence an axle, or other structure, should be tested by just such stresses as it is to withstand in practice. A steel bar may satisfactorily pass a tensile test applied in one direction, only to break down disastrously under alternating stresses each less severe.
Magnetization.
That matter virtually remembers its impressions is plain when we study magnetism. Steel when magnetized for the first time does not behave as when magnetized afterward. It is as if magnetism at its first onset threw aside barriers which never again stood in its way. If the steel is to be brought to its original state it must be melted and recast, or raised to a white heat for a long time. In quite other fields of channeled motion we remark that violins take on a richer sonority with age; their fibres, under the player’s hand, seem to fall into such lines as better lend themselves to musical expression.
In 1878 the late Professor Alfred M. Mayer of the Stevens Institute of Technology, Hoboken, New Jersey, published a series of remarkable experiments in the “American Journal of Science.” He there told and pictured how he had magnetized several small steel needles, thrust through bits of cork set afloat in water, the south pole of each needle being upward. As the needles repelled each other, or had their repulsion somewhat overcome by a large magnet held above them with its north pole downward, the needles disposed themselves symmetrically in outlines of great interest, which varied, of course, with the number of needles afloat at any one time. Three needles formed an equilateral triangle, four made up a square, five disposed themselves either as a pentagon or as a square with one magnet at its centre, and so on in a series of regular combinations, all suggesting that magnetic forces may underlie the structure of crystals.
Mayer’s floating magnets.