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 in flamed than the yellow form. These two forms are said to be allotropic, and the element is said to display allo tropy.
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.
