Page:Harper's New Monthly Magazine - v108.djvu/748

704 of irregular proportions and somewhat differing from each other in size, as cubes, the average length of the edges of such cubes would in the unheated condition be some 0.008mm. After half an hour's exposure to 1015° C. of heat, the average size of the crystal grains would correspond to a cube of 0.189mm. length of edge; and finally, after two and a half hours' exposure to the same temperature, we should get a length of edge amounting to 0.257mm. Observe the rapid growth of the crystals during the first half-hour, and the materially slower growth during the two subsequent hours. Similar processes are found in the other metals, especially in iron. Iron in general is the metal forming the transition between the inorganic and organic world. The life processes shown in iron, under varying conditions, are exceedingly manifold and, of course, of a more complicated nature. Iron, according to the manner of heating, cooling off, etc., can often assume such various forms, presenting so many different qualities, as to sometimes be difficult to recognize.

Let us for a moment imagine the working of a piston-rod, transmitting the steam pressure on to the piston of a locomotive, or some other engine or machine, to the connecting-rod, according to the direction the piston is moving, now carrying along with it the attached and resisting mechanism, now pressing the latter before it. In the one case a tensile stress is exerted on the piston-rod, in the other pressure is brought to bear upon it. Such interchangeable movements necessarily act upon the cells or crystals of which the steel of the rod is composed, which former either, under tensile strain, stretch slightly, like the human muscles, only to contract on the strain being removed, again assuming their original form and position, or under the effect of pressure are compressed, expanding again to original size after the pressure is removed. The processes are entirely similar to those in the muscles of the arm, only that the muscles in the iron piston-rod are so small as not to be visible to the naked eye. Should the strain exercised on these muscles exceed a certain limit, they will not return to their original form, but will, as it were, remain stretched or contracted. If, owing to excess of load, such change of traction and pressure should frequently occur, the small muscles will gradually become distorted and destroyed and the rod will break. The metal must first be exactly tested as to its powers of endurance before being put to use.

Unfortunately, also in the case of metals, to err is human. Such a metal may have stood such test, but may, during the further process of working up, fail to come up to the standard demanded of it at the time such test took place. In short, the capacity of such metal may, owing to symptoms of disease, be seriously impaired and rendered incapable of satisfying the demands made upon it.

The diseases of metals manifest themselves in a very varied manner. Knowledge of such diseases has progressed especially during the last few decades, although the technical physician is no nearer effecting a radical cure than his colleague of the medical faculty is to the curing of ills that flesh is heir to. In order to prove to the reader under what difficulties the engineer of the present day is laboring in this respect, we will cite a few examples of such diseases of metals.

Many metals show symptoms of poisoning, rendering them unfit for use. Thus steel can, by means of small quantities of hydrogen and under certain circumstances, be very seriously affected. Let us take two steel bars of the same material, both heated to a red heat, one surrounded by air, the other exposed to the influences of hydrogen or hydrogen gas, chilling both bars in water after heating; we shall find the bar heated in hydrogen to be brittle, whereas the other bar, heated in air, will turn out to be far superior. The hydrogen has in this instance acted like poison upon the heated steel, and very small quantities of such poisonous matter will suffice to produce very violent effects. The disease in question can be radically cured, it only being necessary to anneal the poisoned bar, repeating the process by heating exposed to air. The poisoned steel, by being allowed to lie for a long time, will, without any further expert treatment, show signs of improvement to a certain degree, the poison gradually