Page:Popular Science Monthly Volume 83.djvu/35

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The knowledge of the tin disease is no more modern than the knowledge of most other diseases.

In this connection it is interesting to speculate on the antiquity of the use of tin. This metal is one of the easiest to obtain from its ores, and may have been used far earlier in the history of mankind than is generally supposed. Evidence in the form of utensils, etc., would of course have been destroyed by the tin disease.

We are indebted to the investigations of Professor Cohen for a more striking example of a metastable metal, that of the "explosive" antimony. By passing an electric current through a solution of antimony chloride this metal may be deposited on platinum in the form of a thick metallic coating. This electrolytic antimony is in the metastable condition exhibiting the same state of passive resistance towards change as the metastable sulphur, sodium acetate and water. If scratched with a file it changes to the stable form of antimony with explosive violence, heat is given off and dense clouds of whitish vapor are evolved. The metal has changed to the ordinary antimony, used so much in manufacturing as a basis for bearing metals. The method of bringing this change about and the velocity of transformation reminds one forcibly of the transition of undercooled water to the stable form.

That many other metals have the property of existing in the metastable state is highly probable. In this connection the hardening of steel is of especial interest, particularly so since the manufacture of steel has played so important a role in the advance of civilization. The method of tempering steel has been the subject of numerous trade secrets. In a book of recipes published in the sixteenth century the reader is told that steel may be hardened by quenching it in rain water in which snails have been boiled: also that

Fortunately for this type of young man the modern steel manufacturer uses other methods for hardening steel.

The discovery of hardening steel by the quenching process is of course as much of a mystery as the method of raising bread by fermentation, we only know that it is an ancient process and moreover of great interest from the standpoint of delayed transformations.

If the alloy that we call steel is taken at a high temperature and allowed to cool very slowly it becomes soft and tools made from it will not have a cutting edge. Sudden chilling, however, produces in the metal a decided hardness. The results obtained by different rates of cooling have