The kind of furnace just described is, owing to the strong blast of air needed for its operation, called a "blast-furnace." One would be inclined to think that a fire so well supplied with oxygen, both from the blast and from the ore itself, would cause the fuel to be completely burnt up, yet such is not the case. The gases which ascend from the fire consist largely of "carbon-monoxide," a burnable gas with lots of heat still left in it. Years ago, and one may still see instances of it, this gas was allowed to escape at the top of the furnace, where it burnt in the form of a huge flame. In most modern furnaces, however, there is a kind of plug in the orifice at the top which, while it can be lowered in order to admit the ore and fuel, normally prevents the escape of the gases, which are led away through pipes. In some cases the gases are burnt under boilers to provide the works with steam, in other cases they heat other furnaces for metallurgical purposes, while in yet others they are employed to drive large gas-engines to generate electricity. It is sometimes a difficulty to find useful employment for the vast quantities of this "blast-furnace gas" which are produced at a large works. We see, then, how is obtained the pig iron from which the other kinds of iron and steel are made. It is not pure iron by any means; indeed, it is not sought to make iron pure, as is the case with most other metals, since, in its pure state, it is too soft to be of much use. All the familiar forms of iron and steel are really alloys of iron and carbon, a fact which tends to give iron its unique position among the metals, since by exceedingly slight variations in the percentage of carbon we can vary the properties of the iron to an amazing extent, thereby producing in effect a wide range of different substances each particularly suitable for a particular purpose. To make cast iron, such as the guns of the Crimea were made of, it is only necessary to melt up some pig iron and to pour it into a mould. There is scarcely a town in which there is not an iron foundry, either large or small, and that is the work carried on there. A smaller form of the blast-furnace, known as a "cupola," melts the pig iron, and the moulds are generally made of sand. The process of pouring the melted metal into the moulds is called "casting" and the things so produced are "castings," and are said to be made of "cast" iron. [Illustration: AN 18-POUNDER IN ACTION. The crew consists of six men. No. 1 (the sergeant) gives instructions. No. 2 stands at the right of the breech. No. 3 fires the gun. No. 4 holds the shell ready for placing in the bore. No. 5 adjusts the fuse and hands the shell to No. 4. No. 6 prepares the ammunition and hands it to No. 5. In this picture only three of the crew are left.]