To explain various chemical and physical phenomena the scientists consider that matter consists of certain small molecules and atoms.
If a drop of water be divided and sub-divided indefinitely, it is conceivable that a point would come when it could not be divided further by physical means. This final bit of water is called a molecule. It would be far from visible by the most powerful microscope. From calculation which we will not go into, we learn that a few hundred million ordinary sized molecules would cover the space of a pin head.
If the water is broken up by some powerful force as by the electric current, we have seen that two different substances are obtained - oxygen and hydrogen. Consequently the molecules of water must have been made up of other still smaller particles and these are called atoms. The atoms of a chemical element, then, are of the same kind, for from an elemental substance like oxygen, only oxygen can be obtained by any means now known.
The atoms may be likened to the letters of our alphabet and the molecules to the words. From a few different kinds of atoms (letters) can be made a great variety of molecules (words).
Fig. 31. Section Through a Leaf