The Romance of Modern Chemistry

METALS, COMMON AND UNCOMMON temperature at which this takes place is called the melting-point; if, on the other hand, we are thinking of the change of liquid into solid, the temperature at which this change occurs is called the freezing-point. The two temperatures are the same if the substance is pure. The third variety of iron, namely, steel, is intermedi- ate between cast and wrought-iron in regard to the amount of carbon which it contains. The remarkable thing about steel is that when it is heated and then suddenly cooled by plunging into cold water it becomes exceedingly hard, so much so that it has the power of scratching glass. Curiously enough, if this hard steel is again heated and then allowed to cool slowly, it is found to be nearly as soft as ordinary iron. By re- gulating the temperature to which the hardened steel is exposed the second time, any required degree of hardness may be attained. Articles made of steel, such as razors, scissors, and watch-springs, are therefore first hardened, and then “ tempered ” by heating them to a point between 430° and 550° Fahrenheit, the tempera- ture varying according to the purpose for which the article is to be used. A razor, for example, is heated only to 430°, a temperature at which the metal acquires superficially a pale yellow colour, due to the forma- tion of a film of oxide. Watch-springs or sword-blades, on the other hand, which should be softer and more elastic, are tempered by heating to 550°, and the colour of the surface film passes through various shades— yellow, brown, purple, and blue—as the temperature rises. The degree of heat attained in tempering may in fact be judged from the colour of the surface. Thus hardened steel which has been heated to 430*, and then 65 ‘ e