associated with electrical charges or, possibly, being nothing else but electrical charges. The variety of the elements can then be accounted for by variations in the number and arrangement of these component parts. Even if the component parts were merely electrical charges, the inertia and momentum of matter could be explained by the principles of electromagnetism.

§ 6. Radio-active Substances. It used to be thought that the molecules of all substances were absolutely incapable of any change, but now it has been found that some substances such as uranium, radium and thorium, which according to most tests behave as elements, suffer transformations into other forms which again appear to be elements. This, of course, only strengthens the belief that all matter is only a single substance under a great variety of forms. The theory that molecules are built up of minute parts associated with electrical charges promises to account for these transformations and for the remarkable effects which are found to accompany them.

Of the radio-active substances, radium is perhaps the most amazing. As radium is being transformed into its child, the radio-active gas known as radium emanation, it emits vast numbers of positively electrified particles. In a single second one milligramme of radium emits about thirty millions of these particles, that is, one particle for each inhabitant of England. The activity of the radium decays because some of the radium ceases to be radium and becomes emanation, which in its turn suffers further transformations, but 1,800 years would pass before half the radium would be transformed. In spite of the excessive smallness of the emitted particles, Rutherford has found a way of observing an effect due to a single one.

To say that these particles are emitted gives a very faint notion of the stupendous velocity with which they are shot out, for their velocity is about one-fifteenth of the speed of light or about ten thousand miles per second. Some of the other radio-active substances shoot out negatively charged particles whose speed rises, in some cases, to nearly the speed of light. The impact of these projectiles upon the surrounding matter produces heat and thus a radio-active substance, such as radium, maintains itself by self-bombardment at a temperature above that of its surroundings. In a single hour one gramme of radium produces enough heat to raise one gramme of water from the freezing to the boiling point.

These experimental facts of radio-activity have given us

almost entirely new ideas as to the character of matter, and hence demand consideration in any account of what is known about the universe. Their bearing on our subject will be considered presently.

The

§ 7. Abrupt Changes.-The elements differ one from another by abrupt steps just as polygons of 3, 4, 5 sides differ. We cannot pass from one element to another by an infinite series of infinitely small steps. If we could, there would be no science of chemistry. The idea has been held that living organisms have been derived from earlier forms by a continuous process of evolution, but nothing like this occurs among the elements, for there the steps are abrupt. transformations of the radio-active substances appear to be due to abrupt changes in the number of electrified particles in the molecule. These abrupt steps and others we shall meet in our survey are of great interest. Thus the abrupt changes of the molecules of radio-active elements warn us that deductions based upon observed uniformity may be unsound, even though the period of observation may have extended over hundreds of years. If we start to-day with a gramme of radium, there will still be half a gramme left after 1,800 years, and if we were to observe for this period one of the molecules forming part of the remaining half gramme, we should naturally conclude that this molecule would continue for all time "unbroken and unworn." But, if we maintained our watch for another century, we might witness the catastrophe which results in the expulsion of a positively charged particle and our earlier conclusion would then be proved to be false.

§ 8. The Universe as a single System.-The view that all matter is built up of a single primordial substance is a great step in advance, but it does not at once replace complexity by simplicity, for the fact remains that the number of molecules in the universe is inconceivably great. You will not think the word inconceivable to be inappropriate in view of the estimate that a drop of water no larger than a grain of mustard seed contains enough molecules to supply each inhabitant of the earth with one molecule every second for many thousands of years. Who then shall grasp the number of molecules in the whole universe?

But the results of scientific investigation lead us to regard all these molecules in their vast array less as so many separate entities than as forming one great and indivisible whole. One instance will make this clear. We believe that if there were only two molecules in the whole universe the force of gravita

tion would still draw each towards the other.

There is therefore

some connexion or relation between the two molecules, and to speak of them as two separate systems is only a convenient mode of speech which does not express the whole of the conditions. If they were two entirely independent systems, the motion of one molecule away from the other would have no effect upon the latter. But the law of gravitation assures us that the second molecule would experience a distinct effect, for the attractive force acting on it would gradually diminish as the distance increased. An extension of this idea leads us to realise that all the molecules in the universe are so linked together by gravitation as to form but a single system. Yet gravitation is not the only link, for electric and magnetic actions between molecules produce their effects, whatever the distance between the molecules. In addition, there are other actions which are sometimes practically in abeyance, as when two molecules, one of oxygen and one of hydrogen, are too far apart for chemical combination to take place. Nevertheless, the power of combination remains ever ready to do its work, when the distance between the molecules is sufficiently reduced and certain other conditions are fulfilled.

We could, of course, suppose that these actions between molecules arise from something inherent in the molecules themselves, and that the intervening space has nothing to do with the affair. But the facts of optics and of electromagnetism compel us to recognise the existence of an all-pervading medium to which the name of ether has been given. This medium is conceived to extend through all space, and there are good reasons for the belief that the forces between electrified bodies are in reality due to stresses in it. The ether enables radiant energy to be transmitted from one body to another, as when the earth receives heat from the sun, or telegrams are sent, with or without the aid of wires, from one station to another. There is thus a most intimate connexion between molecules and the ether, and hence the ether may be regarded as the substance, if it can be called substance, which binds the whole universe together.

The rate at which energy is supplied to the earth by radiation from the sun is very great. On each square yard of illuminated surface, energy is supplied at the rate of about one-fifth of a horse-power. For the whole of the illuminated hemisphere, this amounts to something like twenty million horse-power.

Thus we come to recognise that the whole tribe of molecules is linked together by the ether in such a way that they and the

ether form a single indivisible system. The word atom was originally coined to express the belief that certain minute particles are incapable of a physical division into smaller parts. But in the light of modern science the whole universe is to be regarded as an atom, or in other words, as something which cannot be divided.

The conception of the unity of the universe, to which modern science leads us, must of necessity have a most important place in any speculation concerning the origin of the universe.

§ 9. The Origin of the Universe.—All the evidence is against the idea that the existence of the constituent parts of molecules is due to any physical or chemical actions occurring in the present state of the universe: we are thus compelled to believe that they have been created, unless indeed, we suppose that they are self-existent or in other words, that there never was a time when they did not exist in their present forms-a supposition which has no place in the conception of the universe in the minds of modern physicists.

The fact that all the molecules of any given element have absolutely identical properties makes it clear that matter has been made on some plan, and the certainty that there is no molecule which is not associated with energy indicates that both plan and energy come from the same Source.

The uniformity of the molecules of any given element is the basis of chemistry. The spectroscope also bears witness to this uniformity, for the lines in the spectrum would be broad and not narrow if among the molecules of the substance under examination there were appreciable difference of the periodic time of the particular vibration corresponding to each line of the spectrum. Schuster has illustrated in a very forcible manner the conclusion that if there are inequalities in the periodic time of thallium, corresponding to the green line in its spectrum, these inequalities must be exceedingly small. He states that the want of uniformity is greatly over-estimated, if we say that twelve per cent. of the molecules differ from the average by one part in two millions in periodic time, and he brings out the meaning of this statement in the following way" If you had a great many clocks and found that, taking their average rate to be correct, not more than one in eight would be wrong by a second in twenty-three days, that would represent the maximum amount of variation which our interpretation of the experiment allows us to admit in the case of molecular vibrations. But would any maker undertake to supply you with a number of clocks satisfying that test

Though Sir John Herschel's saying that atoms possess the essential character of manufactured articles is still correct, yet no manufactured article approaches in accuracy of execution the exactitude of atomic construction. We may conclude with Maxwell that "each molecule throughout the universe bears impressed upon it the stamp of a metric system as distinctly as does the Metre of the Archives at Paris."

This exactitude of atomic construction is not merely of academic interest, but is of real importance in the very practical work of maintaining definite standards of length. The metre of the Archives has been measured in terms of the wave-length corresponding to a particular line in the spectrum of cadmium, and it is to the constancy of this wave-length that we now trust rather than to the constancy of the length of the metal bar known as the metre of the Archives.

In thinking about the creation of the universe, we shall perhaps be helped if we first consider what would be involved in the creation of a single new molecule at the present day. This event would not only require the creation of new matter but would also involve the establishment of relations between the new molecule and the countless millions already in existence, and this would change all those molecules to the extent of enabling each of them to act upon the new molecule. If we speak in terms of the ether, we may say that such a connexion must be established between the new molecule and the ether that the molecule is able to cause disturbances in it which produce effects throughout the whole of space.

The phenomena of radio-activity have disclosed far more of the skill of the great Architect and Electrician than was even suspected a few years ago. For the formation of a molecule of uranium involves not only the construction of the minute electrified particles which it contains, but the assembling of them together and the supply of that vast store of energy which will enable the molecule at the right moment, perhaps a thousand million years after the formation of the molecule, to shoot out an electrified particle at a terrific speed. But this is not all, for the design of the uranium molecule is such that the modified molecule, which remains after the expulsion of the particle, will after a few days in its turn shoot out a particle and so on for several stages, the time of halting in each stage being sometimes large, as with the 1,800 years of radium, and sometimes small, as with the four days of radium emanation.

The Power which is capable of creating a single molecule is