The Romance of Modern Chemistry

ELEMENTS WITH DOUBLE IDENTITY it the picture of another substance with the following different characteristics:—a red powder which cannot be melted, which can be heated in the open air to a temperature of 450° Fahrenheit without taking fire, which does not phosphoresce in the dark, is not poisonous, and not soluble in carbon disulphide. Nobody looking casually at these two substances would dream of regarding them as anything else than quite separate and distinct. And yet the fact is they are both the same element—phosphorus. The chemist has learned how to convert the ordinary yellow phosphorus into the red, and how the reverse change of the red into the yellow may be effected. Besides, the compounds which are prepared from the red variety are exactly the same as those obtained from the yellow form, so that there is no doubt that phosphorus is an element with a double identity. How is it that a given element is able to assume different characteristics ? How is it that such totally distinct properties can be associated with one and the same kind of matter ? There are two possible causes for this curious phenomenon, and if we build on the foundation already laid in a previous chapter, we may be able to make the explanation clear. It was said there that the smallest particles of a substance which can exist by themselves are called mole- cules, each of these containing one or more atoms. In the case of an element the atoms which go to make up the molecule are certainly all of one kind, but a further question arises about the number. And the first possible cause of the phenomenon that an element exists in two absolutely different forms, like red and yellow phosphorus, is simply this, that the molecules in the two cases contain 52