The Romance of Modern Chemistry

 ELEMENTS WITH DOUBLE IDENTITY There is, however, one notable illustration of an element existing in two forms which differ in respect of the number of atoms in the molecule. That is the common element oxygen. The molecule of this gas contains two atoms, but under certain circumstances it is possible to induce three atoms of oxygen to club together in a mole- cule, and then we have ozone. Long before anybody knew about this curious substance, a peculiar smell had been noticed whenever an electrical machine was at work, and people adopted what seemed the simplest explanation and regarded it as the “ smell of electricity.” We now know that an electrical discharge, either as a spark from an induction coil, or in the shape of lightning, converts some of the oxygen in the air into another substance, ozone, which is responsible for this peculiar smell. To speak of ozone as “ another substance11 is both right and wrong. It is right because, in regard to the pro- perties which it possesses, ozone is quite distinct from oxygen. In some respects it behaves like intensified oxygen, oxidising things which that gas cannot touch. An illustration of this is the extraordinary effect which it has on mercury. The merest trace of ozone introduced into a vessel containing the metal seems to scare it out of its usual behaviour; the bright, lustrous surface becomes dull and unresponsive ; instead of moving about freely, it sticks to the glass as if it were greased. In a second sense it is wrong to speak of ozone as “another substance'” than oxygen, for they are simply two forms of the same element—“ allotropic ” forms, as the chemist calls them. The existence of phosphorus and carbon in more than one modification was attributed to a different arrangement of the molecules, but such an ex- planation could not possibly be correct in the case of 58