leaving it. A better treatment still is boiling in water or oil, which process may be compared to using warm compresses in the case of human beings.
Similar symptoms of poisoning, caused by hydrogen or gases containing hydrogen (as gas for lighting purposes), are apparent in copper when exposed to red heat. Not every kind of copper is susceptible to this poisoning in equal degree. Copper perfectly free of cuprous oxide is entirely exempt from poisoning. Most of the various coppers of commerce, however, contain cuprous oxide, formed during the smelting process while exposed to atmospheric influences. In such coppers, containing cuprous oxide, hydrogen causes a terrible disease on the copper being heated red hot. The copper bursts asunder and is permeated by cracks, as shown in Fig. 4—natural size. This disease is practically incurable, and can be eradicated by resmelting only. The results work destructively according to the amount of cuprous oxide contained in the copper.
Metals can become diseased from improper treatment, as, for instance, copper and steel when exposed a certain length of time to temperatures exceeding fixed limits. The copper in consequence loses a great part of its ductility and bending qualities. In steel the disease can become so virulent that a steel bar so infected can, on falling on the ground, break to pieces. The technical expert calls such disease "overheating."
Every tool-manufacturer knows how easily common tool-steel is apt to become diseased when, before hardening, it has been exposed to too high temperatures. In bad cases the disease will cause the steel to crack on being tempered. In light cases the cracks ensuing are not visible, but cause the edges of the steel to break off in use, besides giving rise to constant contention between consumer and producer of tool-steel. Recent investigation has succeeded in fixing certain sure characteristics to aid in determining the existence of this disease and to decide where to place the blame. Here the microscope affords us aid. The two microscopic photographs Figs. 5 and 6 will serve to make this plain to the reader. Fig. 5 shows the structure of a piece of tool-steel after passing through the proper hardening process. Fig. 6, on the other hand, gives an idea of the structure of the same steel when, in consequence of overheating before tempering, it has become diseased. The illustrations are enlarged 750-fold. The difference between the two will be readily apparent.
Figs. 5 and 6 do not represent fractures, which, owing to the manner of the inequalities, would not admit of being so strongly magnified. The figures represent polished surfaces. A neatly smoothed cutting surface through the metal is polished and this polished surface treated by a proper etching reagent. The surface thus prepared is observed under the microscope, whereby the etched opaque polished specimen itself serves as an illuminating mirror. In this manner we can magnify 2000-fold. These processes, first employed by Sorby and A. Martens, to-day form the base for the proper investigation of diseases of metals and their various stages of life, which processes have been further elaborated by scientists like Osmond, Robert-Austen, and others.
Thus a microscopic pathology of metals has been developed, similar to the microscopic pathology employed by Virchow in his study of human diseases. Both branches of science resort to the same means in attaining their end, with the only difference that in the case of the former the diseased or healthy cellular tissue or structure of metals is made the subject of investigation, in the latter the cellular tissue of parts of the human or animal body. Both branches of science convincingly teach us how the great and wonderful is often to be found in the small and seemingly insignificant; as from the investigation of these tiny cellular parts of animals or metals do we owe our knowledge of diseases which, in the one case directly and in the other indirectly, threaten mankind. By virtue of such knowledge, resulting from a diagnosis of the threatening peril, man is gradually led to devise efficacious means for combating the same, and in this manner humanity is rendered a signal service in two fields, seemingly so different, but really intimately related to each other.
