The Structure of Matter.—It is now a well known fact that all matter is made up of.submicroscopic particles. These particles which are the smallest into which matter can be subdivided and still retain the properties of the original substance, are called molecules.

Molecules of different substances vary greatly in complexity, ranging from extreme simplicity in some substances to very great complexity in others. All molecules, however, may be broken up into simpler constituents called atoms, of which there are more than ninety distinct kinds known, each representing one of the chemical elements from which all matter is constructed.

Only a few elements, however, appear in the molecules of any one of even the most complex substances. An element, then is a fundamental substance composed of only one kind of atom. In some elements, the molecules are composed of single atoms; in other elements, two or more like atoms are associated together to form the molecule. Of the more common elements are hydrogen, oxygen, nitrogen, carbon, iron, copper, etc.

Carrying the analysis still further, atoms are well known to have complex structures. In accordance to the most widely accepted modern physical picture of the atom, it corresponds roughly to a miniature of our solar system. Corresponding to the sun in the solar system is theciucleus of the atom which, in general, is a very small, compact structure composed of a combination of extremely minute particles called protons, neutrons, positrons and electrons.

Fig. 1.—Structure and electronic orbit of the hydrogen atom.

The proton, whose mass may be taken as the unit of atomic weight has a positive charge equal in magnitude, but opposite in sign, to that of the electron. Its mass is very large compared with that of the electron or of the positron.

The neutron has very nearly the same mass as the proton, but is uncharged. The positron may be regarded as the ultimate unit of positive charge just as the electron is the ultimate unit of negative charge. The positron has the same magnitude of charge as the electron and very nearly the same mass. Practically all the mass of the atom is associated with the small, dense nucleus. Revolving about the nucleus in orbits at relatively large distances from it, are one or more electrons.

The simplest of all atoms is that of hydrogen, whose nucleus consists of a single proton with a single electron revolving about it. The two charges revolve about each other in space much like a whirling dumbbell,except that there is no rigid connection between them. See fig. 1.

The next atom in simplicity is that of helium, whose nucleus consists of four protons and two electrons bound together in a compact central core of great electrical stability. Revolving about this compact nucleus are two electrons. See fig. 2.

Atoms of other elements become increasingly more complex by the successive addition of one electron to those revolving about the nuclei, and with the progressive addition of protons, neutrons, positrons and electrons to the nuclei. In every case the normal atom has an exactly equal number of positive and negative elementary charges, so that the atom as a whole is neutral; thàt is, it behaves toward electrified bodies at some distance from it as though it had no charge at all.

Positively and Negatively Charged Substances.—With reference to the picture of the neutral atom, it will be easy to understand what takes place when a substance is electrically charged.

Assume that by some means one of the external electrons of the neutral helium atom is removed as shown in fig. 3, the result will be an unsatisfied atom in so far as the balance o


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