ATOMIC WEIGHTS AND ISOTOPES.1

By F. W. ASTON, M. A., D. Sc., F. R. S.,
Fellow of Trinity College, Cambridge.

That matter is discontinuous and consists of discrete particles is now an accepted fact, but it is by no means obvious to the senses.

FIG. 1.-Cubes 11 to 15 compared with familiar objects to scale.

The surfaces of clean liquids, even under the most powerful microscope, appear perfectly smooth, coherent, and continuous. The

1Abstract of a summary of a series of lectures given before The Franklin Institute, March 6-10, 1922. Reprinted by permission from the Journal of the Franklin Institute, May, 1922.

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merest trace of a soluble dye will color millions of times its volume of water. It is not surprising therefore that in the past there have arisen schools who believed that matter was quite continuous and infinitely divisible.

The upholders of this view said that if you took a piece of material, lead, for instance, and went on cutting it into smaller and smaller fragments with a sufficiently sharp knife, you could go on indefinitely. The opposing school argued that at some stage in the opera

FIG. 2.-Cubes 17 to 21 compared with minute objects to scale.

tions either the act of section would become impossible, or the result would be lead no longer. Bacon, Descartes, Gassendi, Boyle, and Hooke were all partial to the latter theory, and Newton in 1675 tried to explain Boyle's Law on the assumption that gases were made up of mutually repulsive particles.

The accuracy of modern knowledge is such that we can carry out, indirectly at least, the experiment suggested by the old philosophers right up to the stage when the second school is proved correct, and the ultimate atom of lead reached. For convenience, we will start with a standard decimeter cube of lead weighing 11.37 kilograms,

and the operation of section will consist of three cuts at right angles to each other, dividing the original cube into eight similar bodies each of half the linear dimensions and one-eighth the weight. Thus the first cube will have 5 cm. sides and weigh 1.42 kilograms, the second will weigh 178 grs., the fourth 2.78 grs., and so on. Diminution in the series is very rapid and the result of the ninth operation is a quantity of lead just weighable on the ordinary chemical balance. The results of further operations are compared with suitable objects and a scale of length in Figures 1, 2, and 3. The last operation possible, without breaking up the lead atom, is the twenty-eighth. The twenty-sixth cube is illustrated in Figure 3. It contains 64 atoms, whose size, distance apart, and general arrangement can be represented with considerable accuracy, thanks to the exact knowledge derived from research on X rays and specific heats. On the same scale are represented the largest atom, cæsium, and the smallest atom, carbon, together with molecules of oxygen and nitrogen, at their average distance apart in the air, and the helical arrangement of silicon and oxygen atoms in quartz crystals discovered by X-ray analysis. The following table shows at what stages certain analytical methods break down. The great superiority of the microscope is a noteworthy point.

Just as any vivid notion of the size of the cubes passes out of our power at about the twelfth-the limiting size of a dark object visible to the unaided eye-so when one considers the figures expressing the number of atoms in any ordinary mass of material, the mind is staggered by their immensity. Thus, if we slice the original decimeter cube into square plates one atom thick the area of these plates will total one and one-quarter square miles. If we cut these plates into strings of atoms spaced apart as they are in the solid, these decimeter strings put end to end will reach 6.3 million million miles, the distance light will travel in a year, a quarter of the distance to the nearest fixed star. If the atoms are spaced but one millimeter apart the string will be three and a half million times longer yet, spanning the whole universe.

Again, if an ordinary evacuated electric light bulb were pierced with an aperture such that one million molecules of the air entered per second, the pressure in the bulb would not rise to that of the

air outside for a hundred million years. Perhaps the most striking illustration is as follows: Take a tumbler of water and-supposing it possible-label all the molecules in it. Throw the water into the sea, or indeed, anywhere you please, and after a period of time so great that all the water on the earth-in sea, lakes, rivers, and clouds has had time to become perfectly mixed, fill your tumbler again at the nearest tap. How many of the labeled molecules are to be expected in it? The answer is, roughly, 2,000; for although the

FIG. 3.-Cube 26 showing atoms with scale of reference.

number of tumblers full of water on the earth is 5 by 1021, the number of molecules of water in a single tumbler is 10 25.

From the above statements it would, at first sight, appear absurd to hope to obtain effects from single atoms, yet this can now be done in several ways, and, indeed, it is largely due to the results of such experiments that the figures can be stated with so much confidence. Detection of an individual is only feasible in the case of an atom moving with an enormous velocity when, although its mass is so minute, its energy is quite appreciable. The charged helium atom shot out by radioactive substances in the form of an alpha ray possesses so much energy that the splash of light caused by its impact