The humble earth of the fields—the clay which is used in our potteries, also contains aluminium, that brilliant metal which is as malleable as silver, and unspoilable as gold. When clay is submitted to the influence of sulphuric acid and chloride of potassium, we obtain alum, which is a sulphate of alumina and potash. Alum is a colourless salt, which crystallizes on the surface of water in beautiful octahedrons of striking regularity. Fig. 308 represents a group of alum crystals. This salt is much used in the colouring of fabrics; it is also used for the sizing of papers, and the clarification of tallow. Doctors also use it as an astringent and caustic substance. When alum is submitted to the action of heat in an earthenware crucible, it loses the water of crystallization which it contains, and expands in a singular manner, overflowing from the jar in which it is calcined (fig. 309). Iron, the most important of common metals, rapidly unites with oxygen, and, as we know, when a piece of this metal is exposed to the influence of damp air, it becomes covered with a reddish substance. In the well-known experiment of the formation of rust, the iron gradually oxidises without its temperature rising, but this combination of iron with oxygen is effected much more rapidly under the influence of heat. If, for example, we redden at the fire a nail attached to a wire, and give it a movement of rotation as of a sling, we see flashing out from the metal a thousand bright sparks due to the combination of iron with oxygen, and the formation of an oxide. Particles of iron burn spontaneously in contact with air, and this property for many centuries has been utilized in striking a tinder-box; that is to say, in separating, by striking a flint, small particles of iron, which ignite under the influence of the heat produced by the friction. We can prepare iron in such atoms that it ignites at an ordinary temperature by simple contact with the air. To bring it to this state of extreme tenuity, we reduce its oxalate by hydrogen. We prepare an apparatus for hydrogen as shown in fig. 310, and the gas produced at A is passed through a desiccative tube, B, and finally reaches a glass receptacle, C, in which some oxalate of iron is placed. The latter salt, under the combined influence of hydrogen and heat, is reduced to metallic iron, which assumes the appearance of a fine black powder. When the experiment is completed the glass vessel is closed, and the iron, thus protected from contact with the air, can be preserved indefinitely; but if it is exposed to the air by breaking off the end of the receptacle (fig. 311), it ignites immediately, producing a shower of fire of very beautiful effect. Iron thus prepared is known under the name of _pyrophoric iron_. Iron is acted upon in a very powerful manner by most acids. If some nitric acid is poured on iron nails, a stream of red, nitrous vapour is let loose, and the