Closely connected with this question is the phenomenon of allotropy. This word, which signifies " other form," is applied to the existence of elements in more than one condition. Thus phosphorus, which is usually a yellow, waxy substance, with a low melting-point, changes its appearance when heated, and becomes converted into a red amorphous powder, insoluble in the usual solvents for phosphorus, such as carbon disulphide, and melting at a much higher temperature than the yellow variety; moreover, the red form is much less easily inflamed than the yellow form. These two forms are said to be allotropic, and the element is said to display allotropy.
The elements which display allotropy are:—carbon, silicon, tin, phosphorus, arsenic, antimony, oxygen, sulphur, selenium, iridium, ruthenium, rhodium, silver, gold, and iron. These will be considered in their order.
Diamonds, as was discovered by Lavoisier, yield on combustion nothing but carbon dioxide; their identity with carbon was thus proved. When pure, they are colourless ; they are the hardest of all known substances, and possess a density of 3.514 at 18°. When heated in absence of air in an electric arc, a diamond changes to a coke-like black substance. Diamonds of any appreciable size have not been formed artificially, but minute diamonds have been made by Moissan by dissolving carbon in molten iron heated to its boiling-point in an electric furnace, and then suddenly cooling the iron by plunging it into molten lead; the external surface of the iron solidifies, and encloses a molten interior. As iron possesses a greater volume in the solid than in the liquid state, the molten iron, containing carbon in solution, when it solidifies is under great pressure, for it is confined and hindered from expanding by the crust of solid iron ; under this pressure the carbon separates out in the liquid form, and in solidifying crystallises in octahedra with curved facets characteristic of natural diamonds. If, on the other hand, the iron is allowed to cool without any device to compress the interior, the carbon crystallises out in the form of graphite or plumbago, or, as it is sometimes termed, " black-lead. " This variety of carbon is also found native; it forms hexagonal plates, is soft, and is slippery to the touch. Lastly, many compounds of carbon when heated to redness decompose, and leave the carbon in an amorphous or noncrystalline form. Varieties of these are gas-carbon, deposited in the necks of gas-retorts; oil-coke, left as a residue after the distillation of certain oils; sugar-charcoal, the residue on heating sugar in absence of air ; and wood-charcoal, the product of the distillation of wood. All of these are black, more or less hard substances. When heated to whiteness in an electric arc, they are transformed into graphite. They all contain a trace of hydrogen, from which they can be freed by heating to redness in a current of chlorine. At the temperature of the electric arc, carbon volatilises without fusion and condenses as graphite; it is only when it is heated under pressure, as described, that it can be made to melt.
This element exists in three forms, two of them crystalline, the third amorphous. The amorphous modification when dissolved in molten zinc or aluminium crystallises out in either black lustrous tablets resembling graphite or in iron-grey prisms. It is not known what circumstances determine the formation of the one or the other form. Silicon melts at a bright red heat, and can be cast into rods; they have the graphite-like crystalline form.
This metal, when kept at a low temperature— about —30°—changes to a grey powder. On heating the powdery modification to above 20% it is converted back into ordinary metallic tin, the more quickly the higher the temperature. If the powder be left in contact with ordinary tin at atmospheric temperature, the metal is slowly changed into its allotropic modification, and articles of tin fall to pieces.
Three forms are known for phosphorus. The first, or ordinary form, is a waxy solid, melting at 44.40. It is soluble in carbon disulphide, and crystallises from it in rhombic prisms. It is luminous in the dark in presence of air, but if the pressure of the air be raised it ceases to shine; it is also non-luminous in oxygen. It is very easily inflamed, and burns to its oxide, P2O5. It is poisonous when swallowed. The liquid obtained by melting it is nearly colourless. When this variety is heated to 240° in a vessel from which oxygen is excluded, it changes to a red substance, generally termed amorphous phosphorus. This body is insoluble in carbon disulphide and the other solvents which dissolve ordinary phosphorus. It is not luminous in the dark, and is not easily oxidisable. When heated to a temperature higher than 240°, it volatilises and condenses to ordinary phosphorus, and if air be present it takes fire. It is soluble in lead, and when the molten lead cools it crystallises out in nearly black crystals. Indeed, its colour depends on the temperature at which it is formed. If produced at 260°, it is deep red, and has a glassy fracture ; at 440° it has a granular fracture and is orange; at 550° it is grey, and it fuses at 580°, and on solidifying it forms red crystals. It is possible, though not probable, that a mixture of several allotropic forms is the cause of all these changes.
When arsenic is distilled, it passes directly from the gaseous to the solid state on condensing. The portion which cools most quickly is a black powder; that which condenses in the warm part of the tube has a grey metallic lustre. The black variety can be converted into the crystalline metallic variety on heating to 360°. When arsenic is heated in an indifferent atmosphere under great pressure, its boiling-point is raised above its melting-point, and it melts; on solidification, it forms the metallic variety.