of that matter of which the universe is composed. It is true that our modern theory of atoms, at first sight, seems to resemble closely that of these two philosophers; for in the air of a room we suppose billions of atoms; we believe in the continuity of matter, and therefore that all matter is ultimately made up of atoms. The ancients' conception of atoms was a flight of the imagination, but the modern theory is supported by measurements of weight, magnitude, and speed.

I wish, however, to call attention to the matter of speed, for in regard to it we have advanced far beyond the highest flight of imagination of the ancients. The greatest speed known to the Greeks or Romans was that of an athlete, a horse, or a dart. To them the earth was at rest, and the stars fixed in space. They would be appalled, if they should revisit the earth, by the speed of an express train. They never conceived, in their theories of atoms, of infinite collections of minute particles aggregated into the sphere of our earth, which is spinning on its axis with such speed that we on its circumference at the equator are traveling from night to morning with a speed of seventeen miles a minute - the velocity of a cannon-ball; and that this earth, this collection of atoms, is traveling through space about the sun, from month to month, from summer to winter, with a velocity of nineteen miles a second, seventy-five times the speed of a cannon-ball. The average man of to-day, I venture to say, is in the mental attitude of the ancients in respect to the realization of great speed. To him the world is moving with a velocity which he cannot measure, and therefore does not realize. Indeed, I cannot call to mind any poet who sings of great speed. It is true that in Milton's Paradise Lost there are

the following lines descriptive of the fall

from Heaven of Mulciber:

"From morn

To noon he fell, from noon to dewy eve, A summer's day, and with the setting sun Dropt from the zenith, like a falling star, On Lemnos, the Ægean isle."

A simple calculation will show that in the space of fourteen hours the time mentioned - he would have attained a final velocity of over three hundred miles a seconda velocity of nearly twenty times that of our progress through space. The poet, in correcting an ancient fable, and in striving for accuracy, is not restricted in his flights of imagination by a consideration of the heat which would have been developed by impact. When Vulcan was headlong sent, with his industrious crew, to build in Hell, could there have been a prophetic physical conception in Milton's mind of a generation of heat which would have fitted the objects of Heavenly wrath for their future abode ?

The chief characteristic of modern physical science is its development of knowledge of the consequences which follow from changes in or cessation of great speed. A cannon-ball by its impact can raise a steel plate to a white heat; the earth striking another heavenly body, not a comet, might instantly become a fiery furnace.

It is related of a late professor in Harvard University that he was invited to deliver lectures on astronomy in a town not far from Boston, in the days when ly ceum courses on high topics had not been supplanted by stereopticon shows. The selectmen said that the town was too poor to give him the fee he asked, and he finally agreed to deliver the course of lectures for half the sum he had originally demanded. In those lectures he proceeded to enlarge upon the terrible catastrophes which might arise from a possible disturbance of the equipoise of the earth. At the conclusion of the course the selectmen offered him the other half of the sum he had demanded, if he would show how the equipoise would probably be maintained. This he did, and gained the sum he had originally asked.

While the earth spins on and pursues its path without collisions, modern physi cists are occupied in the study of countless atoms in disturbed orbits, and in con

We are beginning to have a realizing sense of the effect on matter of great speed, for until now it has been impossible in laboratories to experiment on matter moving faster than a rifle-ball — perhaps - perhaps fifteen hundred feet a second. We soon reach the point of rupture of a steel disc when we attempt to revolve it with such an angular velocity that a point on its rim moves only three times faster than the swiftest express train or three miles a minute. The resistance of the air is a powerful factor which we have to overcome. To obtain an idea of this resistance, an interesting experiment consists in attaching an ordinary newspaper to an axle, and endeavoring to flatten it into a disc by a rapid revolution of the axle. The experiment requires the expenditure of at least ten horse-power. It is said that the Wright brothers found no facts on record which could aid them in their efforts to construct and manage an aeroplane; they were obliged to make their own experiments.

One can conceive that matter might assume an entirely different aspect if it should move with a velocity of many thousand miles a second. Since it is impossible to experiment with visible objects, such as those we measure on an ordinary balance, it remained to discover an invisible particle of infinitesimal weight, to which could be communicated an enormous velocity. We must bear in mind that a body of infinitesimal size moving with a velocity of many thousand miles a second can develop by impact a considerable and measurable amount of heat, for energy is proportional to the

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the velocity. Such a body has been discovered, and the analysis of its behavior while traveling many thousand miles a second is the most remarkable result obtained from radium. The latter substance shoots forth a particle which is called an electron,- for it is charged with electricity. I have said that it is one one-thousandth of the size of the hydrogen atom. A statement of enormously large or infinitely small dimensions, with which science is concerned, often transcends our power of apprehension; but I will endeavor to give some idea of the size of the electron.

It is estimated that 250,000,000 hydrogen molecules in line measure an inch. A cube having a side of one one-hundredthousandth of an inch-the minimum visible under the most powerful microscope - contains from sixty to a hundred millions of molecules of oxygen or nitrogen, and more of hydrogen. If one could magnify a drop of water to the apparent size of the earth seen from a distance at which a small orange is just visible, we could see its molecules. The electron is the smallest body we have recognized in the universe, and it is certainly a wonderful proof of the fineness and accuracy of modern scientific methods that an invisible particle of such almost inconceivable minuteness can be isolated, and its mass and velocity measured.

I have dwelt at some length upon the subject of speed, in order to lead the mind of the reader to what I consider the most significant fact in modern physical science; and the fact is this: the mass of the electron increases as it flies with a velocity of over fifty thousand miles a second. The physicists are asking themselves, “Is this increase in mass a true increase, or is it an electrical phenomenon?" If it is the latter we have, so to speak, a new hold upon the ether which is supposed to fill all space, for we can study its commotions if we cannot weigh it while it is at rest. It is only through the phenomena of light that we have recognized the necessity of

an ethereal medium for the transmission of light waves, and Maxwell has shown that electromagnetic waves are propagated from the sun by the same medium. Wireless telegraphy depends upon these electrical waves, which are transmitted with the velocity of 180,000 miles a second the velocity also of light.

The electron has another remarkable manifestation. It is capable of producing by its motion the phenomenon of magnetism, and we are moving toward a theory of magnetic activity on the sun which may enlighten us in regard to the cause of the magnetism of the earth. This invisible particle is also changing our views in regard to the probable age and future duration of the sun. Physicists and geologists no longer dispute over the age of the earth, and the length of time which will elapse before the sun is extinguished. The discovery of radium and the electron have made us mute.

To radium we owe a great extension of the theory of ionization, that is, the break

ing up of a liquid or gas under the effect of electricity or radium into small particles, which are called ions. This new development of physical science is called radioactivity. The great volume of investigations on the subject of ionization and its allied subject, radioactivity, may induce some future physicist to rewrite Tyndall's epoch-making book entitled Heat as a Mode of Motion, which appeared nearly fifty years ago. The new treatment of energy will consist, not, as in Tyndall's treatise, mainly of a discussion of the heat developed by motion of tangible bodies, but of a treatment of the heat and radiations which result from the impact of invisible particles. The new treatise should be written by one gifted with scientific imagination, controlled by a sense of exactness; and the reading pub

lic will be fortunate if the author should possess Tyndall's power of exposition. Physical science, however, is advancing so fast that no competent scientific man would think of undertaking the task at present; for the book would have to be

rewritten before it had been published a year. The university lecturer is now compelled to rewrite his lectures on energy and radiations from month to month.

In our conception of the action of electrons we seem to be reviving, in a certain measure, Sir Isaac Newton's corpuscular theory, which supposed that light was caused by the motion of infinitely small particles called corpuscles; and also Benjamin Franklin's one-fluid theory of electricity, which taught that negative electricity is due to a deficit of something which appears as an excess in positive electricity. In the modern theory of the electron, the negative charge may be regarded as the state of the positive charge after the electron has been detached from it. A distinguished English physicist said to me lately that there is a mine of suggestions in Franklin's electrical work. The latter, however, had no conception of an interaction between invisible particles and a universal ether.

I have said that wireless telegraphy should excite less wonder than the transmission of an ordinary telephonic message over a copper wire. We have abandoned all electrical fluids, and have in their place waves in the ether and swiftly moving electrons. These electrons can be weighed and their speed can be determined, but the ether is thoroughly intangible; even the earth, in its swift motion, causes no commotion in it. We must reflect, however, that until now we have not been able to study the behavior of this ether toward matter in motion under a speed greater than that of a point revolving on the surface of the earth. It seems probable that future knowledge of the properties of the ether will come from the study of the behavior of electrons moving through it with velocities many thousand times greater than the speed of the earth on its axis. The study of elec trons and that of the ether must be pursued together. But progress in the study of the ether advances very slowly. Lord Kelvin made many attempts to conceive of its structure and its behavior under

strain; but his powerful mind, aided by all the mathematical knowledge of his time, failed in the attempt. The only new fact in regard to it that has been discovered during the last fifty years is that light exerts a pressure in the direction of its propagation; and a distinguished physicist has hazarded the suggestion that this pressure may carry germs of life to remote parts of the stellar universe. It may be that we shall adopt a hypothesis of the aggregation of these invisible particles or germs into visible forms. This hypothesis would be indeed a nebulous

one.

In considering the theories of modern physical science, and the mass of facts which have been obtained, do we not wonder at the flights of imagination of scientific men, and at the results they obtain by following the lead of the faculty of imagination? Imagination is the greatest moving force in the world. In saying this. I am merely repeating a remark of Disraeli's, and to prove the strength of his conviction I will repeat a story Mr. Lowell told me when he was minister to England. It is the custom of the Royal Academy of Painters to hold a private view of their pictures before the public exhibition. Disraeli, walking arm in arm with Browning through the galleries, said, "What strikes me most forcibly here is the lack of imagination; " and he proceeded to enlarge upon the power of imagination, declaring it to be the greatest force in the world. In responding to a toast at the banquet which followed the private exhibition, he dwelt upon the wealth of imagination in evidence on the walls about him, and again expressed his conviction that imagination is the moving force in the world. Browning repeated Disraeli's first remark to Gladstone, who sat beside him, and he muttered, “The Devil!"

If I paused here the reader might conclude that considerations of speed form the chief characteristic of modern physical science. But the factor of time underlies the factor of speed. It is only by

thought of time that we measure speed; we always speak of so many feet a second, so many miles an hour. Time is an ultimate consideration.

In this rapid review of modern physical science I should be remiss if I did not refer to the effect that physical research is having upon contemporaneous thought. Physicists have apparently reduced matter to a possible whirl in the ether, and the ether is intangible. The discussion of the meaning of matter, speed, and time brings the physicists into the field of the philosophers, who claim that the former, in their late theories, are steadily approaching the views of the scholastic philosophers. The world is an illusion of the senses; matter, speed, and time have no objective reality.

This article may be read by a philosopher who is delighted that a smug physicist is approaching his appropriate field, and who would be pleased to have me enlarge further upon the fleeting illusions of physical science. I am reminded of the old Scottish woman who asked the distinguished preacher Irving to sit down beside her and “ gang over the essentials." All true physicists, however, should decline to discuss with philosophers the actuality of matter, of speed, and of time.

The function of the scientific man is to measure and to generalize from facts. The measuring worm is safe so long as it confines its progress to its continent, -the top of a table, - but when it reaches the edge it finds an unfathomable abyss. While the physicist is measuring the specific gravity of lead to the sixth place of decimals, the philosopher has written a volume to prove the nonexistence of matter. When a physicist ventures into philosophy he is in the condition of a man who steps in the dark over a precipice.

In the new subject, too, of psychology physical science is having a dominant influence. Emerson says, "The human heart concerns us more than peering into microscopes, and is larger than can be