Matter, it is held, consists of minute indivisible particles or atoms, of which eighty-one different varieties are at present recognized. These correspond to the chemical elements, oxygen, carbon, sulphur, iron, gold and the rest, by the combination of which all other substances or compounds are found. The atoms of each element are exactly alike, while those of the various elements differ in mass. Thus atoms of sulphur are twice as large as atoms of oxygen, silver atoms are nearly twice as heavy as those of iron. The largest atom known—that of the rare element uranium—is over two hundred times as massive as the smallest, the atom of the elementary gas hydrogen, which is taken as the unit of comparison.
These atoms are indestructible and can not be converted into one another. However, they are not, like the atoms of Lucretius,
Solida pollentia simplicitate,
"strong in their solid singleness," but are of complex structure, capable of vibrating in many different ways. From the evidence of the spectroscope we learn that each kind of atom has its own modes of vibration and is distinguishable from others by these no less than by its mass.
While the atoms are the fundamental units they can not in most cases exist in isolation, but are drawn together by the forces of chemical affinity into groups which we call molecules. The atom bears to the molecule the relation of the letter to the word on a printed page. While the number of kinds of atom is limited, that of the varieties of molecule is practically unlimited, there being as many kinds of molecule as there are substances, or words in the chemical dictionary. The number of atoms in a molecule varies greatly. In a few exceptional cases the atom and molecule are identical. This is the case, for example, with mercury and with rarer gases of the atmosphere. These elements are the a, I and of chemistry. The inorganic molecules with which we begin our chemical studies are appropriately words of one syllable, containing but a few letters; while some of the organic molecules, with their hundreds or even thousands of atoms, surpass even the creations of Aristophanes and would require the mouth of Gargantua to utter.
This distinction between atom and molecule is one of the most important characteristics of the theory. The atom it is often said is the unit of the chemist, the molecule of the physicist. To determine the relations of the atoms in the molecule is one of the problems of chemistry; while it is the task of the physicist to form from the interactions and motions of the molecules a consistent theory of physical phenomena. To be sure, the boundary between the sciences thus laid down is somewhat arbitrary, and we need not be surprised to find it often overstepped from either side. There is in fact a whole borderland occupied by troops of marauders who style themselves physical chemists or chemical physicists, according to their predilections, and
