change the chemical and physical properties of the body; you make a new element.

The electrons (each of which is probably the same in size whatever atom it occurs in) contribute little to the weight of the atom. The electrons are supposed each to have a weight of only 1 of that of the lightest atom, so that their weight is negligible. The seat of the weight of the atom, as was mentioned above, lies in the nucleus. Each positive charge of electricity corresponds to from 2 to 2 units of atomic weight, depending upon whether the atom be light or heavy. Oxygen, for example, has an atomic number of 8 and an atomic weight of 16; tin, an atomic number of 50 and an atomic weight of 119; lead, an atomic number of 82 and an atomic weight of 207.

The atomic number is very easily obtained. One simply arranges the whole of the elements in the order of their atomic weights, as they are arranged in the usual periodic system of classification, and numbers them off from the lowest like soldiers. Hydrogen is the element of lightest atomic weight. Its atomic number is therefore 1, its nucleus has charge of 1 positive unit of electricity, it has 1 electron. Helium is the next lightest. Its atomic number is therefore 2, its nucleus has a charge of 2 positive units, it has 2 electrons. Lithium is the third lightest, hence, etc. Iron stands twenty-sixth in order of lightness of atoms. It has therefore a charge of 26 positive units on its nucleus, and 26 electrons. The statement applies to every element. Uranium, the heaviest of the elements, stands ninety-second in order. Its nucleus has 92 positive charges, and round it are its 92 electrons.

The atom as thus conceived was undoubtedly a fine piece of imaginative thinking, and it appeared to explain experimental work in a remarkably precise manner. It remained to investigate first the arrangement of the outer electrons on which the chief physical and chemical properties of an element depend, and second the structure of the nucleus where the mass and radio-active properties have their origin. So far only the first of these has been worked out in detail.

There is every reason to suppose that the electrons circle round the nucleus with a great velocity analogous Vol. 241.-No. 479.

X

to the motion of a planet round the sun, and that they must be arranged in groups so that the periodic system of classification of elements might be explained. But here arose a difficulty. For the ordinary laws of dynamics leave no escape from the conclusion that such an atom as Rutherford pictured could not exist for long even if it could be put together for a time.

On the ordinary theory an electron should be able to revolve in any circle which has the nucleus as its centre, or in any oval which has the nucleus as its focus. If it receives impetus from outside it ought to widen its circle or oval gradually in its course round the nucleus; if it radiates energy as it does when the atom gives spectrum lines, it should narrow its circle gradually also. Further, if the electron be moving and be left to itself it ought eventually to fall into the nucleus and stop there, and then the atom ceases to be an atom. Hence on ordinary dynamical principles if the atom be what it is thought to be there should be no atoms left in the universe at all. In consequence, workers in science were temporarily led by their theories to a position not dissimilar to that of the man in gaol who was repeatedly assured by his lawyer-who knew the law was on his client's side-that the authorities simply could not put him in gaol, and who could only reply, 'But I am in gaol.'

If we decide to accept the Universe' we must agree there is a fallacy somewhere in this knowledge of the atom.

Fortunately almost before the difficulty was realised it was very satisfactorily resolved by the work of Dr Niels Bohr, the Danish mathematical physicist. Bohr applied what is known as the Quantum theory to Rutherford's atom with astounding success. This theory was then only a dozen years old and had already established for itself a great reputation in explaining difficulties. Its salient features may be ventured here in outline in so far as it applies to the simplest atom.

Bohr imagined, first of all, that when an electron goes round and round the nucleus in one particular track it is behaving merely as though it were at rest. It does not then require any energy to keep it going, nor does it lose any energy in its course. This is a surprising assumption, but all goes well if it is made; and atoms ought not to

exist unless an assumption like this is made. Bohr next imagined that when the electron receives energy it jumps instantaneously into a wider circle with a radius four times as large as the one it left; it does not bridge the distance between gradually. If it wishes to leave this second one and go into another circular track it must jump to one with a radius nine times the original one, and so on; the possible tracks in which it may revolve not being unlimited, but simply circles of 4, 9, 16, 25, 36, 49, and so on times the radius of a certain minimum circle. The speed of the electrons on this theory depends upon their distance from the nucleus round which they revolve. The outermost electrons move with a speed of at least 600 miles per second, whilst those nearest the nucleus may attain a speed of 90,000 miles per second. From these assumptions Bohr has so developed a picture of Rutherford's atom as to give a convincing and complete explanation of a host of intricate and baffling detail. His theory, for example, gives a quantitative explanation of the spectra emitted by glowing bodies, and Sommerfeld has applied Einstein's theory of relativity to Bohr's theory of the motion of the electron, to give a beautiful explanation of the existence of what spectroscopists call the 'fine structure' of the hydrogen spectrum lines. The theory explains also more fully than before the existence of the periodic variations in the properties of elements. In particular it predicted the chemical properties of the missing element of atomic number 72, and this led very soon afterwards to the discovery of the element hafnium by a Danish and a Hungarian chemist in Copenhagen.

When we leave the electrons to consider the nucleus we leave a world of negative electricity for a very different one, that of positive electricity. The first thing that is apparent is the contrast between negative and positive electricity. The mass of the electron is only 1830 of the mass of the hydrogen atom, but no experimental evidence has been found for the existence of a positive electron of small mass like the negative. In no case is positive electricity found associated with a mass less than that of an atom of hydrogen. This is a surprising difference and constitutes an enigma, but that is how it is. The nucleus of the hydrogen atom-that is, the hydrogen atom deprived of its single electron-is

the unit of positive electricity and is called the proton. There is further a second unit of positive electricity, which may possibly be merely a secondary unit, the helium nucleus-that is, the helium atom deprived of both of its electrons. This unit has twice the charge a proton bears and has four times its mass. It is possible to explain the mass of atoms of all known elements in terms of these two units without making any assumptions that are illogical or far-fetched.

The helium nucleus certainly resides in the nucleus of the radio-elements because it is identical with the a-particle which these elements emit. It is reasonable to infer the possibility that this nucleus is also a constituent of other elements.

A number of elements have atomic weights which are nearly exactly whole numbers that are divisible by 4, which is the atomic weight of helium: for example, carbon (12), oxygen (16), neon (20), sulphur (32), titanium (48), chromium (52), iron (56), tungsten (184), thallium (204), thorium (232). It is a possible assumption, in consequence, that these elements are built up exclusively of helium, i.e. that carbon is three helium atoms bound in a very fundamental way, oxygen four, iron fourteen, and thorium fifty-eight.

A simple speculation of this kind includes only those elements which have whole numbers divisible by 4 for their atomic weight, but if we consider a few proven experimental facts we can extend it to a great many more elements. The first of these is that the atomic weights of all elements are really whole numbers although experiment seems to contradict this simple view. Now every whole number when divided by 4 gives a remainder of 0, 1, 2, or 3. In the first case it is supposed that the element's nucleus is built up exclusively of helium nuclei, in the second of helium nuclei and one nucleus of hydrogen, in the third of helium nuclei and two nuclei of hydrogen, and in the fourth of helium nuclei and three nuclei of hydrogen. This is, of course, mere arithmetic and need not represent reality in the least, but two sets of experimental work exist which make it plausible, and certainly the simplest explanation that is at present possible; first the work of Rutherford on the artificial disintegration of the atom, which reveals

the presence of hydrogen nuclei in the nuclei of certain light elements, and second the work of Aston and others on 'isotopes,' which establishes the fact that the atomic weights of elements are really whole numbers.

Rutherford and his colleague have observed that hydrogen nuclei are released from the light elements boron, nitrogen, fluorine, sodium, aluminium, and phosphorus, when they are bombarded by swiftly moving a-particles, and there is little doubt that these nuclei form an essential part of the nuclear structure. The elements which are disrupted by this bombardment have all atomic weights which when divided by 4 give a remainder of 2 or 3, so that these are exactly the elements most likely to submit to the process, since the 2 or 3 is supposed to refer to the weight of the hydrogen constituents of the nucleus. These experiments are also of interest for this reason, that they reveal for the first time the disintegration or transmutation of elements by man. This transmutation has, however, none of the sensational accompaniments of this over-praised idea, for the process takes place on a minute scale only. Rutherford has calculated that although an a-particle encounters more than 100,000 atoms of aluminium in its path, only about one a-particle in a million gets close enough to the nucleus to cause the disintegration. The observation of these effects has been made by counting the number of impacts of the hydrogen nuclei on a glass plate coated with sulphide of zinc.

The proof of the view that all elements have atomic weights which are essentially whole numbers is due partly to workers on radio-activity and partly to the experimental work of Aston on positive rays. Since the hydrogen atom weighs unity on the scale on which atomic weights are measured, this work appears merely to be a confirmation of the old and discarded hypothesis of Prout. But there is a difference. The element chlorine's atomic weight is 35-45, and this, in no way, can be reckoned a simple multiple of unity, and it was because of facts like these that Prout's hypothesis was discarded long ago. But Aston and others have shown that, for example, chlorine is really a mixture of two bodies with almost exactly similar chemical and