of the other kind were similarly picked out they would form a non-metal called chlorine. Each of these smallest portions of table salt, which if divided are no longer salt, is called a molecule of sodic chloride, and each of the parts into which the molecule is divisible is called an atom, of sodium or of chlorine. In dealing with the dimensions of these very small portions of matter an inch or a centimeter is too clumsy a unit. To express the size of an atom in fractions of an inch is worse than stating the diameter of an apple in fractions of a mile. Every one knows what is meant by a millimeter; it is nearly one twenty-fifth part of an inch. A meter is equal to a thousand millimeters. Suppose a millimeter divided into a thousand parts. Each of these is called a micron and denoted by the Greek letter μ. This however is still too large a unit of length for measuring the size of atoms, so we again divide the micron into a thousand parts and call each a micromillimeter or micromil, and denote it by the symbol μμ. Lord Kelvin's estimate of the diameter of a molecule is that it lies between one hundredth of a micromil and two micromils, that is between .01 μμ and 2 μμ. This is certainly a very wide estimate, but it is the best yet to hand, and for present purposes we may take it that an atom is a small portion of matter of approximately one millionth of a millimeter or one micromil (1 μμ) in diameter. On the same scale the wave length of a ray of yellow light is about 0.6 μ or 600 μμ. that is six hundred times the size of an atom. We know nothing as yet about the relative sizes of different kinds of atoms. In the next place as regards the number of molecules in a given space, various distinguished physicists, Maxwell, Kelvin, Boltzmann, Van der Waals and others, have given estimates for the number of molecules in a cubic centimeter of air at ordinary temperature and pressure, which vary between 1018 and 1021 or between a million billion and a thousand million billion. All we can do is to take a rough mean of these different values, and we shall consider that in one cubic centimeter of hydrogen or other gas at 0° C. and 760 mm. or freezing point and ordinary pressure there are about 2 1019 or twenty million million million molecules. To understand what this enormous number means we must realize that if we could pick out all the molecules in one cubic inch of air and place them side by side in a row, small as they are individually, the row would extend nearly twice the distance from the earth to the sun.
Having provided ourselves with a rough idea of the sizes and numbers of the molecules of any gas we proceed to obtain an idea of their weight or mass. Since 11,162 cubic centimeters of hydrogen gas at 0° C. and 760 mm. weigh one gram, it follows from the above facts that each molecule of hydrogen has a mass of nearly 1/1023 of a gram. To weigh these tiny atoms we must therefore take a
