Fig. 308.—Group of alum crystals.
If six hundred grams of this chloride of magnesium are mixed with one hundred grams of chloride of sodium, or kitchen salt, and the same quantity of fluoride of calcium and metallic sodium in small fragments, and the mixture is put into an earthenware crucible made red-hot, and heated for a quarter of an hour under a closed lid, we shall find on pouring out the fluid on to a handful of earth, that we have obtained instead of scoria, forty-five grams of metallic magnesium. The metal thus obtained is impure, and to remove all foreign substances it must be heated in a charcoal tube, through which passes a current of hydrogen.
Magnesium is now produced in great abundance, and is very inexpensive. It is a metal endowed with a great affinity for oxygen, and it is only necessary to thrust it into the flame of a candle to produce combustion; it burns with a brightness that the eye can scarcely tolerate, and is transformed into a white powder—oxide of magnesium, or magnesia. Combustion is still more active in oxygen, and powder of magnesium placed in a jar filled with this gas produces a perfect shower of fire of very beautiful effect. To give an idea of the lighting power of magnesium, we may add that a wire of this metal, which is 29/100 of a millimetre in diameter, produces by combustion a light equal to that of seventy-four candles.
Fig. 309.—Calcined alum.
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).
Fig. 310.—Preparation of metallic iron.
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 oxidised iron is dissolved in the liquid to the condition of nitrate of iron. This experiment is very easy to perform, and it gives an idea of the energy of certain chemical actions. We have endeavoured to represent its appearance in fig. 312. Fuming nitric acid does not act on iron, and prevents it being attacked by ordinary nitric acid. This property has given rise to a very remarkable experiment on passive iron. It consists in placing some nails in a glass, into which some fuming nitric acid is poured, which produces no result; the fuming acid is then taken out, and is replaced by ordinary nitric acid, which no longer acts on the iron rendered passive by the smoking acid. After this, if the nails are touched by a piece of iron, which has not undergone the action of nitric acid, they are immediately acted upon, and a giving off of nitrous vapour is manifested with great energy. Lead is a very soft metal, and can even be scratched by the nails. It is also extremely pliable, and so entirely devoid of elasticity that when bent it has no tendency whatever to return to its primitive form. Lead is heavy, and has a density represented by 11·4; that is to say, the weight of a quart of water being one kilogram, that of the same volume of lead is 11·400 k.
