Popular Science Monthly/Volume 76/March 1910/The Structure of the World-Stuff
|THE STRUCTURE OF THE WORLD-STUFF|
UNIVERSITY OF PENNSYLVANIA
SCIENCE, and the humanities. How often are they placed in opposition. There is doubtless a utilitarian aspect of science which though admirable in itself tends to foster a spirit antagonistic to culture. But science is many-sided. And in the single-minded seeking for the truth amidst clouding obscurities, in the searching out the laws of the development be it of an atom, a tree, a man or a star, in the aim to express that unity which we instinctively feel is the key to the interpretation of nature's marvelous complex, I feel that she earns an honored seat among the immortals. And so I need make no apology for speaking to you upon a scientific subject, one which lies at the very basis of natural science, one whose development has demanded not only zealous, strenuous research but calm judicial, wise speculation,—the subject of the constitution of matter, the stuff of which the physical world is made.
The ultimate structure of the material world around us must always have been a problem of deep interest to thoughtful minds, and has formed a fruitful subject of speculation from the time of Thales to the present day. But it is not of the philosophical aspect of the question that I venture to speak. I can not claim to be a philosopher—save such a one as is characterized by Touchstone as a "natural philosopher"—but only a student of physics; and it is therefore to the physical side of the problem that I shall confine myself. The substance and the form of Aristotle, the monad of Leibnitz, the strife between idea and thing-in-itself, and other metaphysical contributions toward the interpretation of the universe, important though they be in the history of thought, are beyond the limitations of the present speaker and of the present occasion. Our attention is rather to be directed to the physical theories which have been framed as to the constitution of matter, especially to the one which has won almost universal acceptance, that known as the atomic theory; its development from the past, its modern form, and its promise for the future.
For the hypothesis of atoms is not a product of modern science. Indeed the question of the divisibility of matter must necessarily arise in the early stages of scientific thought. In our youth when we inquire as to the structure of things we are told that
Little drops of water,
Little grains of sand,
Make the mighty ocean
And the wondrous land,
and we doubtless speculate as to whether the sand grain and the water drop are not likewise divisible into smaller portions, and whether these smaller portions differ in quality from the larger. And so in the youth of science we find some philosophers maintaining the infinite divisibility of matter, and on the other hand the school founded by Leukippos and Demokritos, to whom we owe the conception and the word atom, the indivisible (or at least never divided) particle which forms the ultimate structure of matter. In these atoms, their ceaseless motion and their various groupings, is to be found the interpretation of the manifold phenomena of nature.
Both of these schools of thought have contributed to modern science. From the former we obtain the conception of a continuous medium which has developed into the theory of the all-pervading ether. From this school too we received the doctrine of the limited number of elementary substances from which all things are formed; a number which has grown from the four of Empedokles—earth, air, fire, water—through many vicissitudes into the eighty or so of the present day. But to the opposing school we owe a far greater debt, a debt which we can not lightly repudiate with Clifford by saying "The atomic theory of Demokritos was—no more than a guess—which was more near the right thing than the others." The atomic theory is much more than a guess. Incorporated into the system of Epikuros, and expounded in the marvelous poem of the Roman Lucretius, it forms a well-reasoned and well-balanced system of thought which it is true lacked in definiteness but was not without marked success in furnishing a framework on which to erect an image of nature. So successful was it that after two millenniums it has suffered little modification. As an illustration let us compare the atom of Lucretius with that of Newton.
These are the words of the Roman poet:
While the description by Newton is as follows:
A comparison of these passages shows how the two conceptions are essentially the same. Indeed in some respects the older view has the advantage, as it lays greater stress upon the motion of the atoms; a forecast of the modern kinetic theory of matter.
The lack of development of the atomic theory is to be ascribed largely to the adverse criticism of Aristotle. The overwhelming influence of the Aristotelian philosophy was thrown against it, and it made little headway down through the middle ages. Not until the downfall of scholasticism do we find any extensive revival of the system; a revival culminating in the seventeenth century school of atomists, among whom are to be noted Gassendi, Boyle and, as we have seen, Newton. But still another century of stagnation was to elapse before it was to be transformed and modernized at one stroke by the genius of the English chemist John Dalton.
The modern atomic theory founded by Dalton and developed during the nineteenth century must not be regarded merely as an extension of the older theory, but as a new structure built upon the old one as a foundation. That was speculative, this was scientific. That was vague, this was definite. That was based merely upon observation and introspection, this upon experiment and calculation. The theory of the elements and the theory of atoms was blended into a single comprehensive whole. The prime distinction between the different kinds of atoms was found in a single property—that of their relative mass. The older theory was not inadequate in the early days of science; but it failed when the quantitative relations of phenomena were brought into prominence by the development of experimental methods; and such was the case when the principle of the indestructibility of matter was raised by Lavoisier from a philosophic dogma to a scientific truth, and emphasis was thus laid upon mass as the fundamental property of matter.
I need not detail to you the marvelous growth of the theory during the past century; how it met every demand made upon it by modern chemistry, and indeed inspired much of the development of that 6cience; how, on the other hand, it has lent its aid to the progress of physics and especially how by the founders of the kinetic theory of gases,
the flaring atom streams
And torrents of her myriad universe
Ruining along the illimitable inane
were marshaled to the defense of the great principle of the conservation of energy, and the science of heat was annexed to the domain of mechanics. Let me rather recall to you the salient points of the theory as held by the close of the century, for comparison on the one hand with the theory of the past and on the other with its developments in the future.
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,
"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 who make frequent raids impartially into either territory, usually carrying off rich spoils.
It is natural to inquire as to the size of these molecules and atoms of which we are thus assured the world is made. The question of the relative size is accurately answered by chemical analysis. We know, for instance, that the atom of oxygen weighs 15.88 times as much as that of hydrogen, and so on. But this gives no answer to the question as to the absolute size. It may seem that it would be impossible—even presumptuous—to attempt to estimate the size of particles which must be far beyond the reach of the most powerful microscope; but this has been accomplished. Time would not permit me even to outline the methods of wonderful ingenuity by which this problem has been attacked. The study of the laws of expansion of gases, the phenomena of the soap bubble, the action of the electric current, the blue of the sky, the settling of fine drops of mist or of specks of dust, these and other classes of phenomena have all contributed to the solution; and the evidence from such varied sources has been strikingly concordant.
Let me give you the results. Small indeed are these atoms, but not immeasurably small. So small that when they are expressed in ordinary units the mind shrinks from the attempt to grasp them. But the scientist is not limited to a single unit of measure. The geographer uses a mile, the carpenter a foot. The astronomer's staff with which he gauges the motions of the planets in their courses stretches from the earth to the sun; while in estimating the distances of the fixed stars the unit is the far greater distance traversed by light in a year. And so in the world of the little a convenient standard of comparison is the wave of light, some fifty thousand of which are contained in an inch, of the order of the thickness of a brightly colored soap bubble or of the smallest things that we can see with our best microscopes. Measured in these units, we find the diameter of a hydrogen atom to be about one two thousandth part of a wave of light, or, in our ordinary measure, a hundred-millionth of an inch. We hear much of millions, especially in the daily press, though perhaps we have but a vague conception of them. For example, we heard not long since of a celebrated fine of $27,000,000. If that fine had been paid, and paid in dollar bills, and the bills laid end to end, they would have reached from Maine to California. (I do not suggest this as a desirable method of laying out money, though we often meet with suggestions of even less merit; but to help in expressing the magnitude of the quantities with which we are dealing.) Now if each bill were replaced by an atom, and the line closed up, it would extend a quarter of an inch. Or we may express the result in another way: The diameter of an atom bears the same relation to that of a tennis ball that the tennis ball does to the earth; and the masses are in about the same proportion. If we emulate Archytas,
"the measurer of the innumerable sands," and estimate the number of molecules in even a drop of water, we obtain a result far beyond our powers of realization; a number requiring 22 figures for its expression. It is surely not to be reckoned among the least achievements of science that it has determined the order of this enormous quantity and has even made us reasonably certain of the first figure.
Such is the modern atom. It would seem impossible to penetrate farther into the details of so minute a structure, one too whose elements defied attack by physical and chemical agencies. It was felt, however, that a system based upon some eighty distinct kinds of primordial matter could hardly be an ultimate solution of the problem; and the suggestion was early made that the atoms are complex groups of a fundamental atom—possibly that of hydrogen, the smallest known. This hypothesis, suggested and supported by the fact that many atoms are very nearly exact multiples of the hydrogen atom in mass, has proved attractive to those who saw in the orderly succession of properties among the elements (known as the periodic law) indications that matter has reached its present state of multiplicity through some process of evolution. Similar indications were thought to be found by some in the study of the spectra of the stars. But these views were speculative, and direct evidence was lacking; and little light was thrown upon the subject until just before the close of the last century new lines of investigation were opened which greatly extended and modified our views as to the nature of the atom. This expansion was determined by the simultaneous development of the modern or what might be called the atomic theory of electricity, usually known as the electron theory.
That electricity, like matter, consists of indivisible units or atoms had long been suspected, since experiments of Faraday had shown that the quantities of electricity carried by atoms were always either equal to or exact multiples of a single charge—that carried by the hydrogen atom; and the term electron had been suggested as a name for the atom of electricity. As early as 1878 the great Dutch physicist Lorentz had based an explanation of the refraction and dispersion of light upon the presence in matter of equal discrete particles or atoms of electricity, and this hypothesis was afterwards developed into a complete framework of a theory of electrical and optical phenomena. But in the absence of experimental confirmation little attention was paid to these theories until the investigations to which I alluded brought the electrons themselves forcibly before the scientific world.
In 1897 J. J. Thomson was investigating the electrical discharge in highly rarefied gases—the so-called kathode rays. These had been proved to be negatively charged particles, and were supposed to be atoms or molecules of the gas. Thomson showed that they were identical in their properties whatever was the gas used; and while he was not able to determine their mass nor charge directly he succeeded in measuring the ratio of their charge to their mass and showed that either their charge was much greater than the atomic charge, or the mass much less than that of an atom. Of these alternatives he chose the latter as the simpler and more probable—a choice justified by subsequent research. Later investigations showed that particles identical in properties were emitted from metals under the influence of light, and from incandescent solids. If we assume that the unit charge is carried by these particles the mass is calculated to be about an eighteen-hundredth part of that of a hydrogen atom. They are also shown to be enormously more concentrated, as their diameter is estimated to be only a few millionths of the atomic diameter.
Let us note as the result of these investigations three highly significant facts. First, that here we are presented with bodies smaller—much smaller—than atoms. Secondly, that from whatever source they are derived—gas, metal or hot lime—their properties are identical. Finally that they are associated with a definite charge of electricity, and that this charge is negative. Here is the electron, the atom of electricity demanded by the theory of Lorentz.
But what of the positive electron? Search for this has often been made, but the mass of the positively charged particles has always proved to be comparable with that of an atom. There are some indications that positive electrons may be identical with positively charged atoms of hydrogen or of the gas helium whose atomic mass is four times that of hydrogen; but for the present we can say only that positive electricity is never found dissociated from matter of atomic dimensions.
These conclusions received confirmation from two other sources. The year before Thomson's measurement of the electron marks the discovery of radioactivity and the beginning of the researches which speedily led to the discovery of radium and similar substances. The properties of the rays emitted by these substances were carefully investigated and it was proved that they consist partly of negatively charged particles identical with the electrons, and partly of positively charged particles having a mass equal to that of a helium atom.
The other confirmation to which I alluded, while less direct, penetrates even more deeply into the structure of matter. In this same year (1896) a minute effect of magnetism upon light, discovered by Zeeman, was shown to be completely concordant with the theory of Lorentz, and to lead to the conclusion that the light of a luminous vapor was due to negatively charged particles circling in or about the molecules, of dimensions similar to those almost simultaneously discovered by Thomson. Later developments of optical theory stimulated by this discovery indicated the presence in molecules of positively charged particles as well, but that these were atomic in size.
Thus three independent lines of investigation have almost simultaneously converged to furnish a basis for a new theory of electricity, which we need follow only so far as it affects the theory of matter. Of the two kinds of electricity we find but one—the negative—that can be detached from atoms. We find the negative electron too as a constituent of the atom. The electron, whatever its source, is always the same, while the positive charge partakes of the varying nature of the matter with which it is associated. Emphasis is thus laid upon the electrons as forming the true electrical fluid, the positive electricity playing a subordinate part. According to this theory a neutral atom, which we know contains some electrons, contains also enough positive charge to exactly neutralize them. If one or more additional electrons become attached to it, it becomes negatively charged with the atomic quantity or its multiple. If, on the other hand, it loses some of its electrons, it becomes positively charged. The terms positive and negative have here exchanged their usual roles. It is the positive electricity that is
"the spirit of negation." Except for this exchange (due to the unfortunate original allotment of the terms) the theory bears a remarkable resemblance to the single fluid theory of Franklin.
Some atoms normally contain too many electrons, others too few. These will attract each other, forming neutral molecules. Thus an oxygen atom, which normally holds two extra electrons, will attract to itself two hydrogen atoms which each lack one, and thus will form a molecule of water. By the number of electrons in excess or deficiency the combining power of an atom with others is determined. Such considerations have proved efficient in disentangling many puzzling questions connected with chemical combination.
The most salient point of this theory is that we seem to be confronted with a dualism, matter and electricity, atoms and electrons. A closer study of the electron has suggested a possible way of escaping this, or rather of turning it, in what is called the electrical theory of matter.
Long before electrons were observed J. J. Thomson had shown theoretically that a body when charged with electricity would by mechanical tests appear to have a slightly greater mass than when uncharged, and the smaller the body the greater the effect. Thus it would require more work to stop a moving charged body than if it were uncharged; a greater force would be needed to deflect it from its path. But even an atom is not small enough for the difference between its charged and uncharged states to be appreciable. With an electron, however, it is different. Application of the theory has shown convincingly that the entire observed mass of the electron may be accounted for by its electrical charge and that there is no evidence of any other mass apart from its charge. An electron is thus literally a disembodied spirit—a concentrated charge, and nothing more.
But what of the atom? We have seen that its mass can not be accounted for by its positive charge. We may, however, meet the difficulty in another way. Let us imagine a structure of the following nature. Scattered through the volume of a sphere of the size of our microcosmic tennis ball let its suppose a congeries of some 1,800 electrons. To get the scale of our image correct we shall have to magnify it still more and we shall then see this number of fine shot scattered through a space the size of a large hall. Let the equivalent neutralizing positive charge be uniformly diffused throughout the sphere. The electrical mass of such a system would be that of its electrons, in other words would be equal to that of a hydrogen atom. It is, therefore, unnecessary to attribute to such an atom any additional substance, "matter," distinct from the positive and negative charges.
This is the electrical theory of matter. I do not say that it has been established. It is at present only a fruitful speculation. But it strongly appeals to those who seek for unity in science and who prefer to have a single interpretation of a phenomenon rather than two separate hypotheses to account for the same thing. Some of the mass of atoms must be electrical. Why not all?
Let us call upon the scientific imagination and attempt to picture the atom of the twentieth century for comparison with the atoms of the earlier theories. We see a large number of electrons immersed in what may be called a positive jelly. In some cases, if not in all, the atom is partly at least compounded of sub-atoms of the size of the hydrogen or helium atom. Of the electrons some may be vibrating about neutral positions, or circling in closed orbits, and in doing so sending forth waves of light; others may be more firmly fixed. We may even have systems of electrons revolving in concentric rings like the rings of Saturn. A few, especially if the atom is that of a metal, are so loosely attached that they readily escape, leaving the atom positively charged. Sometimes under the action of light-waves a vibrating electron is so violently shaken that it breaks its bonds and escapes. High temperature or a powerful electrical field may produce the same effect.
If our atom belongs to the group of radioactive elements such as radium, thorium, etc., we shall see from time to time, if we watch attentively, a kind of explosion. Perhaps an electron will be hurled forth with enormous velocity, perhaps one of the sub-atoms, sometimes both. The positively charged sub-atom, after it has given up most of its energy by collisions, will attract to itself a pair of neutralizing electrons and settle down, a staid helium atom. The remainder of the original atom rearranges itself into a new condition of more or less stability, and we have a new atom. It is no longer an atom of radium, for instance, but an atom of something else; another element with an atomic mass some four units less, and differing from radium as gold does from mercury. Its spectrum will be different; its properties will be different. It may perhaps be a gaseous atom instead of an atom of a solid. And we shall see this process continuing at irregular intervals, the atom gradually becoming smaller until a state is reached which is so stable as to seem permanent.—And all these processes are taking place within the bounds of our diminutive tennis ball.
Here we have the transmutation of the elements of which the alchemists dreamed. It is true that these changes now seem to go on "like the stars without haste without rest" uncontrollable by human agencies, but one would be rash to predict the impossibility of such control.
I have pictured a radioactive atom. But need we make that limitation? The intervals at which these transformations occur vary greatly. Thus we are told that the average life of a radium atom is about 2,000 years, that of its first product but four days, and a similar product of another element, actinium, lasts but a few seconds. It is estimated that an atom of uranium or thorium lasts some thousand million years, but still eventually changes into another form. In our imaginary picture we need set no limits to our measurement of time. The 200,000,000 years that we are told the earth has endured may be but a mere incident in the life of an atom; and an element surpassing uranium as much as that does some of the more rapidly disintegrating substances would appear permanent by all known tests.
The atoms would thus appear to be crumbling, perishing—indeed their death-knell has already been sounded. I find it in a recent number of a scientific journal. I do not know the author, but the initials appended to it—W. R.—are those of the foremost chemist of England.
Old Time is a'flying; the atoms are dying;
Come, list to their parting oration:
"We'll soon disappear to a heavenly sphere
On account of our disintegration.
"Our action's spontaneous in atoms uranious,
Or radious, actinious or thorious;
But for others, the gleam of a heaven-sent beam
Must encourage their efforts laborious.
"For many a day we've been slipping away
While the savants still doz'd in their slumbers,
Till at last came a man with gold leaf and tin can,
And detected our infinite numbers."
Thus the atoms in turn, we now clearly discern,
Fly to bits with the utmost facility.
They wend on their way, and in flitting, display
An absolute lack of stability.
'Tis clear they should halt on the grave of old Dalton
On their path to celestial spheres,
And a few thousand million—let's say a quadrillion—
Should bedew it with reverent tears.
But lest these views seem too somber and devoid of hope for the future, we must not forget that we may be looking at but one side of the mighty rhythm of nature. There may be also, still veiled from us, the compensating process by which atoms are formed and developed. This the author seems to feel and to express in his final verse:
There's nothing facetious in the way that Lucretius
Imagined the Chaos to quiver
And electrons to blunder, together, asunder,
In building up atoms forever.
The imaginative sketch of the atom which I have drawn must not be regarded as an accurate photograph. Many details are imperfectly known, many doubtless erroneous. But in its general outlines it reproduces the views of the foremost investigators, and it has proved eminently successful in unraveling the most extensive and perplexing body of facts that has ever been accumulated in so short a time.
But even when all difficulties shall have been smoothed away, and the electron enthroned above the atom, we shall not have reached the end. Even now we begin to hear discussions as to the shape of the electron, some holding it spherical, others flattened like the earth. There is, in fact, no final theory. "Every ultimate fact," says Emerson, "is only the first of a new series. Every general law only a particular fact of some more general law presently to disclose itself." We live in a succession of infinities. The earth with all its multiplicity is but a small part of the solar system; that is but an insignificant unit in a mighty stellar group. And so within the smallest sensible particle of matter is the world of atoms, within the world of atoms the world of electrons. Who shall set a boundary in the one direction or in the other?
- Address delivered before the joint meeting of Phi Beta Kappa and Sigma Xi, University of Pennsylvania, June 16, 1909.
- I., 609, and II., 333.
- Opticks, 4th edition, p. 375.
- The electrical mass of a hydrogen atom due to its atomic charge is calculated to be about one hundred-millionth of its whole mass.
- It may be said that the dualism still remains. But it is now a dualism of form rather than of fact. Positive and negative electricity, like action and reaction, are but two sides of the same phenomenon. We can not have one without the other. But this would lead us too far into speculations as to the nature of electricity.
- Nature, 73, 132.