there are hundreds of electrons in a single atom of most of the common substances and the volume actually occupied by these electrons themselves is probably as small in comparison with the volume. of the atom, as is the volume of the sun and the planets in comparison with the whole volume of the solar system.

Scientists have long believed that electricity was just as definitely atomic as any other substance-a mass of iron, for instance. Professor Millikan has given additional proof of the correctness of this theory. He has demonstrated that there is a measurable and ultimate limit to the divisibility of an electric charge, and has accurately determined the value of the ultimate electric atom.

An ion is a molecule carrying one free electron, or atom of electricity. The diameter of a molecule is .000000007874 of an inch. It would take 1,000 molecules laid side by side in a row to make a speck long enough to be seen in the most powerful microscope. The diameter of an electron is about one hundredthousandth that of a molecule. This means that one hundred millions of them would have to be placed in a row and then magnified a thousand or more times before the speck they made could be seen.

To illustrate again, let us suppose that one hundred million people began to count the number of electric atoms necessary to generate by electrolysis sufficient hydrogen to fill an ordinary toy balloon. If they counted at the rate of one hundred a minute, and kept steadily at it, never stopping to eat, sleep, or rest, it would take these hundred million hustlers something like four million years to complete the job.

Professor Millikan has not only solved a seemingly impossible problem, given additional proof of the atomic theory of electricity, and shown that a very satisfactory ocular demonstration is possible of the perpetual dance of the molecules of matter, but probably has made by far the most accurate of all determinations of the value of the fundamental unit. Nor is this all, for the improved value of this unit also carries with it corresponding corrected values of other things, such as the number of atoms per unit weight of any given element, and the mass of the unit itself.

The history of the atomic nature of electricity which has culminated in Professor Millikan's recent discovery dates back almost a century. Faraday was the first to prove that when an electric current is passed through a water solution of any one of certain substances, the substance itself is taken out of the solution. The extent to which this action takes place is dependent upon the quantity of electricity so passed, and upon the nature of the substance dissolved.

If the same current is passed in series through the water solution of such salts as copper sulphate sodium sulphate and silver nitrate, the weight of the metal deposited, or permanently removed from the solution, is directly proportional to the atomic weight of the metal itself, and inversely proportional to the number of hydrogen atoms necessary chemically to take the place of one atom of the metal. Repeated experiments proved that this proportionality held true under all conditions, and it was therefore regarded as certain that to each ion in a solution conveying an electric current there belongs a definite electric charge.

The natural inference drawn from these experiments was that electrical charges also are definitely atomic in their nature. For many years, however, there was no way to test this inference except in the case of any quantity of electricity used in the decomposition of an electrolyte, and even the most refined measurements could directly detect nothing less than the aggregate of countless millions of elementary charges.

Similar difficulties presented themselves in connection with the so-called cloud method of determining the elementary electrical charge, discovered by Sir Joseph Thompson about twelve years ago. This method, with modifications by H. A. Wilson, consists in observing first, the rate of fall under gravity of a cloud produced in an ionized fog-chamber, and second, the rate of fall of a like cloud when a vertical electrical field is superposed upon gravity. Here, too, all the deductions relating to the elementary charge were drawn from observations of the average behavior in electrical and gravitational fields of swarms of charged particles, and consequently the value of the unit charge was not determinable.

It was while experimenting with the cloud method about two years ago that Professor Millikan succeeded in balancing individual charged drops by an electrostatic field, a feat which led directly to his discovery of a way to isolate an ion. The first process he employed consisted essentially in catching ions on droplets of water or alcohol, in then isolating by a suitable arrangement a single one of these droplets, and measuring its speed first in a vertical electrical and gravitational field combined, then in a gravitational field alone.

There were, however, several sources of error or uncertainty still attached to this method.

The substitution of oil for water or alcohol entirely freed the method from all of these limitations.

Through the study of the behavior in electrical and gravitational fields of this oil drop, carrying its captured ions, it was also possible to present direct and tangible demonstration of the fact that all electrical charges, however produced, are exact multiples of one definite, elementary electrical charge; to make an exact determination of the value of the elementary charge which is free from all questionable theoretical assumptions; to observe directly the order of magnitude of the kinetic energy of agitation of a molecule, and thus to bring new direct and most convincing evidence of the correctness of the kinetic theory of matter; and to demonstrate that the great majority, if not all, of the ions of ionized air, of both positive and negative sign, carry the elementary electrical charge.

By means of a commercial atomizer a cloud of fine droplets of oil is blown with the aid of dust-free air into a dust-free air chamber. One or more of the droplets of this cloud is allowed to fall through a pin-hole into the spaces between two plates of a horizontal air condenser, and the pin hole is then closed by means of an electromagnetically operated cover. The pin hole is closed to prevent air currents from passing through it and producing irregularities. The two plates of the air condenser are heavy, circular, ribbed brass castings about 8.5 inches in diameter having surfaces which are ground so nearly to true planes that the error is nowhere more

than .008 inch. These planes are held about 6 inch apart by means of three small ebonite posts held firmly in place. by ebonite screws. A strip of thin sheet ebonite passes entirely around the plates, thus forming a completely closed air space.

Three glass windows about .5 inch square are placed in this ebonite strip at the angular positions of 0°, 165°, and 180°. A narrow parallel beam of light from an arc lamp enters the condenser through the first window and emerges from the last. The other window serves for observing, with the aid of a short focus telescope placed about two feet distant, the illuminated oil droplet as it floats in the air between the plates. The appearance of this drop is that of a brilliant star on a black background. It falls, of course, under the action of gravity, toward the lower plate; but before it reaches it, an electrical field of strength between 7,500 and 20,000 volts per inch is created between the plates by means of a battery, and if the droplet had received a frictional charge of the proper sign and strength as it was blown out through the atomizer, it is pulled up by this field against gravity, toward the upper plate. Before it strikes it the plates are short-circuited by means of the switch and the time required by the drop to fall under gravity through the distance corresponding to the space between the cross hairs of the observing telescope is accurately determined. Then the rate at which the droplet moves up under the influence of the field is measured by timing it when the field is on.

This operation is repeated and the speeds checked an indefinite number of times, or until the droplet catches an ion from among those which exist normally in air, or which have been produced in the space between the plates by any of the usual ionizing agents like radium or X-rays. The fact that an ion has been caught, and the exact instant at which the event happened are signalled to the observer by the change in speed of the droplet under the influence of the field. From the sign and magnitude of this change in speed, taken in connection with the constant speed under gravity, the sign and exact value of the charge carried by the captured ion are known.

AHE fable of the goose and the golden egg is as old as the hills-as some of our younger hills, at least-but we seem never to have taken it to heart. Our ever-present greed is forever driving us on to new killings of our future welfare, just the same as if we had never smiled in conscious superiority at the

impatient owner of that other goose. How future generations will smile at us if they can spare time from cursing us for some of our bequests.

Here's a new one! Our gutta percha, that priceless material for its purposesgolf-balls, toilet-articles, submarine cable insulations and other similar and dissimilar necessities-is almost gone, because

we have cut down the trees to get in a hurry the precious sap from which it is made! A famine of gutta percha exists in the markets of the world, because we've been killing another goose, as it

were.

It is a serious matter. And the only salvation for us-if we would continue to play golf and to cable home for more money is "to raise more geese." The American government is leading the way in the propagation of this sort of poultry, with the Philippines as a poultry-yard. Which, being interpreted, is to say, that Uncle Sam is going to farm for gutta percha trees in this latest addition to his acreage.

Artificial growth of these trees is ex

ceedingly important; and luck is with Uncle Samuel, as is usual with the energetic, in that he has found the greatest little gum-tree ever, growing wild in his own domain, just waiting for somebody's kindly hand to be extended in encouragement, to grow widely and well. The hand is being extended. The little gumtree is being cultivated and gives big promise of being a very useful resource in course of a very reasonable time.

Gutta percha is absolutely the only substance yet found that will effectually keep our cross-seas messages from slipping off their charted courses and going adrift. It is the only really truly insulator. The demand for it, for this purpose alone, has been so great within the past few years that it is little wonder that the natives of the Malay Archipelago, where it mostly grows, have hunted it with eager rapacity that has run away with forehandedness. They gather it as a wild crop exclusively and their method is the method of the savage always-like his method of hunting or his way of war. In it he demonstrates his savagery: he kills for what he wants. He chops down the trees and leaves all but the gum which will bring him the immediate gold. Probably he does not even know that he does not need to do this, but his ignorance does not affect the outcome of his plan of procedure. But our blame is no less than his, for we have let him do it-because we were in a hurry, too.

A trifling little 25,000 pounds of the product was all that the year 1845 brought forth. Twelve years later, the Malay Archipelago exported 530,000 pounds. Twelve years after that, Sumatra alone sent out 300,000 pounds, and Borneo more than nine times as much2,864,000 pounds. And it is estimated that, in order to collect so vast a quantity of the gum, the natives must have ruthlessly sacrified more than 5,000,000 trees.

Now the trees grow big, when they are allowed to get their growth at all; but they are very slow in the process. A good idea of their size may be gained from the fact that the lowest branches of some of the most stalwart of them are one hundred and forty feet from the ground, and that their girth is not infrequently as much as twenty feet. At the present time, however, the full-grown tree is a rarity, because the best have been slaughteredthat is the proper word-for the moment's profit. And the thing that has the sting in it, in the light of the present scarcity, is that the slaughter was unnecessary as would be the killing of a sheep for its wool. Tapping the trees for the gum is quite as easy as running a pneumatic clipper-and as effective.

Experiments, having in view the planting of gutta percha trees on a large scale, are now being made with good hopes of success. For this purpose the seeds have first to be procured. A wild tree of a desirable species having been found, all the jungle in its immediate neighborhood is cut away, leaving a small clearing about it-the object of this expedient being to keep off the monkeys, which are very fond of the pulpy plum-like fruits that contain the seeds. To prevent monkeys from climbing the tree, a sheet of tinned iron is fastened about the lower part of the trunk in such a way as to cut short any such attempt on their part. When the fruits are ripe, Malay boys, who are expert climbers, are sent up to gather them.

It then remains only to plant the seeds and grow the seedlings, from which, by the usual process of selection, superior varieties may be expected in the course of time to be obtained. This work is now being carried on at Buitenzorg-in Java -where is located the greatest botanical garden in the tropics, and also at Singa

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