Still more curious to relate, Bohr has followed in the steps of Newton and shown how Newton's own laws with certain modifications apply in the interior of an atom.

Most people know that all material things are made up of a number of chemical elements. There are now known to be some 89 of these and the existence of 3 more has been deduced. Probably this exhausts the list, although, as many can occur in certain slightly varied forms, there may be some 200 or so distinct varieties of atom.

What is an atom? It is the smallest individual unit of an element. In its usual condition the space occupied by an atom is rather more than a hundred millionth of a centimetre in diameter. In certain stars it may be much smaller and in others much larger, but this is the usual size. The outer parts of an atom are in a continuous state of motion and spectrum analysis of radiation shows that when the atoms are "cooling down" they give off vibrations (light, radiant heat, etc.) which have characteristic pitches or frequencies of vibration, and can arrest and absorb passing vibrations of these same pitches. Each kind of atom has its own set of vibrations by means of which its presence is known.

A spectium of the light from a heated gas shows a number of distinct lines, each corresponding to a certain pitch and the number and complexity of these lines has for many years presented an insoluble problem, to which any simple solution appeared impossible. Any simple mechanism in the atom seemed incompatible with the multiplicity of the vibrations and am atom has been compared to a grand piano.

Gradually it was found that in certain substances (Hydrogen gas especially) the lines were grouped in mathematical series and certain relative simple rules were found for some of these series.

Furthermore experiment showed that if the source of the light was placed in a strong magnetic or electric field individual lines could be split, and hence it was deduced that the particles whose motions generate the vibrations were electrical in nature. The hypothesis was suggested that the particles revolved in orbits about an attracting center and that the pitch of the vibrations was the same as the frequency of the revolution. Herein we see the dawning of the idea that an atom might be like the solar system, but what could be the particles which correspond to the satellites, and what their central sun, was by no means clear.

Experiments made by passing electricity through glass tubes from which almost all the air had been pumped show that under certain conditions minute particles could come from the negative terminal, and since these particles were affected by magnetic and electric fields they must be electrified. By an ingenious piece of apparatus their velocity and the relation of their electrical charge to their mass was found, and by other methods the charge and mass were separately measured. In this way it was found that all these particles were indentical in mass (very small compared with that of an atom) and charge and were produced from all materials indifferently, and that the charge on them was exactly the same as was computed to come from single atoms in the action of an electric battery cell. These particles are the electrons. They are all negatively charged, and it has been found possible to frame an almost complete theory of all electrical phenomena with them as the principal agents. Madam Curie's curious discovery, radium, the stuff which can keep itself warm for thousands of years, was found to give off similar electrons, but also to produce other heavier particles which were positively charged, both hinds having prodigious velocities, but especially the electrons. By studying the transparency of thin plates to these heavier charges from radium, it was found that in all matter there are certain very minute positively charged centers which are widely separated relative to their own diameters. On the basis of all these and various other related facts Rutherford propounded the theory that the atoms consist wholly of electrical particles, a central heavy nucleus which is positively charged and a number of the electrons whirling around it at distances very great compared with the size of the nucleus.

The number of the electrons and the magnitude of the central charge were conceived to follow the chemical order of the atoms which has been known for some years.

Returning to the experiments in vacua, it should be mentioned that by making a hole in the negative terminal it was found possible to get positively charged particles which on investigation, by the same methods as were applied to the electron, proved to have charges which were equal in magnitude (although opposed in sign) to those of the electrons or else a simple number of times greater, but the 'n8 proved to be much greater and identical with those of the atoms of the various substances present in the tube.

It was also found that when moving electrons impinged on to a solid surface, the now well known X-rays emerged therefrom; that these rays were identical in character with light although of 1,000 times higher pitch; that series of spectral lines could be got from them; and that these series of spectral lines were simpler than those of ordinary light rays and could be arranged according to pitch in a very simple relation to the weight of the atoms from which they came.

The above is a rough sketch of the general data, which were most difficult to reconcile with one another. It was impossible to suppose that the whirling particles in the atoms gave off light rays by gradually falling into the center, just as the earth would heat a gas filling all space whose friction caused it to fall into the sun, because in that case the pitches of the vibrations would become higher and higher and not have constant values, such as the atoms actually show in spectra.

It happens that in studying the radiation of heat it was discovered that when the quantities of radiation are very small, the energy is absorbed or released in definite minute parcels or "quanta" dependent in magnitude on the pitch of the vibrations, and this idea was the clue which led to the solution of the atom mystery.

Bohr started with the hydrogen atom, which is known from various considerations to be, relatively speaking, the simplest in its behaviour. Only one electron can be obtained from each hydrogen atom; the charge on the positive hydrogen particles is equal to that of the electron; and the mass is the smallest known for an atom or positive particle. He assumed that the hydrogen positive center or nucleus together with one electron constituted the whole hydrogen atom; that they attracted one another according to the known laws of static electricity; and that the electron revolved about the nucleus just like the earth does about the sun, being prevented from falling in by centrifugal force. Thus far there was nothing new in his concept, but to these assumptions be added three more. Firstly, he supposed that so long as an electron remained in the same orbit, it did not radiate energy. This is quite contrary to electromagnetic theory; but he presumed that this theory does not apply to the revolving electron, however true it may be for larger masses.

Secondly, he assumed that an electron of any one orbit had a non- fractional number of quanta of angular momentum, i.e., mass times velocity times distance from the center.

Thirdly, he assumed that as a result of disturbance an electron could jump from one orbit to another and liberate or absorb exactly one quantum of radiant energy in so doing.

All these assumptions are arbitrary, and in some respects contradictory to classical mechanical and electrical theory; but the extraordinary fact is that they work!

Calculations based on these assumptions give an adequate explanation of all the spectral phenomena shown by hydrogen and agree with all the known dimensions of that kind of atom.

Applied to positively charged helium, the next heaviest atom having only one electron, they also agree. Applied to twice positively charged lithium, the next atom with only one electron, they agree as well as can be expected under the more complex conditions.

Applied to other heavier complex atoms with one mobile surface electron definite indications of agreement are also seen.

Applied to the spectral lines of X-rays for alt the elements they give a very fair agreement in principle and confirm the hypothesis that these radiations come from deep seated electrons near to the nucleus of the atom.

Furthermore, the computed electrical energy required to 'ionize" the atoms (i.e. separate the charges) from various states agrees. In fact as yet there is no serious disagreement at all.

Encouraged by his first attempt, Bohr together with a German, Sommerfeld, has proceeded much further. The theory of elliptic instead of circular paths has been worked out and proved a means of developing atomic models for all the elements, and provides a most powerful tool in the explanation of many atomic phenomena.

Undoubtedly years of work are still necessary to follow out all the consequences of this theory, just as has been the case with the law of gravitation but the first steps are so secure that there can be practically no doubt that the discovery is one of the highest importance.

Just as Newton's laws enable the astronomer to compute the motions of the sun, moon and planets with great precision for years ahead and even indicate the conditions of stability after millions of years, so it is hoped that Bohr, by providing a rule which enables the paths of the particles in an atom to be computed, will enable the scientist to prophesy the behaviour of matter under new conditions. The test of science is prophecy and this test has already been applied to Bohr's theory with much success.

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