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. [Illustration: Fig. 311.—Pyrophoric iron.] Fig. 313 represents cylindrical bars of the best known metals, all weighing the same, showing their comparative density. [Illustration: Fig. 312.—Iron and nitric acid.] Lead, like tin, is capable of taking a beautiful crystalline form when placed in solution by a metal that is less oxydisable. The crystallization of lead, represented in fig. 314, is designated by the name of the _Tree of Saturn_. This is how the experiment is produced: Thirty grams of acetate of lead are dissolved in a quart of water, and the solution is poured into a vase of a spherical shape. A stopper for this vase is made out of a piece of zinc, to which five or six separate brass wires are attached; these are plunged into the fluid, and we see the wires become immediately covered with brilliant crystallized spangles of lead, which continue increasing in size. The alchemists, who were familiar with this experiment, believed that it consisted in a transformation of copper into lead, while in reality it only consists in the substitution of one metal for another. The copper is dissolved in the liquid, and is replaced by the lead, but no metamorphosis is brought about. We may vary at will the form of the vase or the arrangement of the wire; thus it is easy to form letters, numbers, or figures, by the crystallization of brilliant spangles. [Illustration: Platinum Density 21.50 Gold D. 19.25 Mercury D. 13.56 Lead D. 11.35 Silver D. 10.47 Bismuth D. 9.82 Copper D. 8.78 Nickel D. 8.27 Tin D. 7.29 Iron D. 7.20 Zinc D. 6.86 Aluminium D. 2.56 Magnesium D. 1.43 Sodium D. 0.97 Fig. 313.—Representation of bars of metal, all of the same weight.]