The Romance of Modern Chemistry

METALS, COMMON AND UNCOMMON of the total carbon present in cast-iron some has combined with the metal to form a compound known as a carbide, while the rest has crystallised out during cooling in the form of graphite. If the carbon is removed from cast-iron as com- pletely as possible we get wrought-iron, which contains only about one-tenth of 1 per cent, of carbon, and differs very notably in its properties from cast-iron. In the first place, wrought-iron can be welded—that is, if two pieces of this material are made red-hot they soften, and in this state may be hammered together. This cannot be done with cast-iron, which is a hard, brittle, crystalline substance. Again, cast-iron is much more easily melted than wrought-iron. The latter is very nearly pure metal, whereas the former contains an appreciable quantity of foreign material. Now it is a well-known fact that if a small quantity of a foreign body is added to a pure substance, the melting-point of the mixture is lower than that of the pure substance. Salt water, for example, contains much more dissolved matter than fresh water, and is more difficult to freeze; or, to put it the other way round, ice melts at a lower temperature in salt water than it does in fresh; in fact, a strong solution of common salt in water will not freeze even at 0° Fahrenheit. The fact that cast-iron melts more easily or has a lower freezing-point than wrought-iron is therefore an illustration of a very general principle. The reader will observe that the freezing and melting- points are to be regarded as the same temperature, and this is always so if we are dealing with a pure sub- stance. The difference is merely this, that if we are thinking of the solid being converted into liquid, the