nge the chemical and physical properties of the aly; you make a new element.

bir The electrons (each of which is probably the same in the whatever atom it occurs in) contribute little to the hght of the atom. The electrons are supposed each to of weight of only 1830 of that of the lightest atom, that their weight is negligible. The seat of the weight the atom, as was mentioned above, lies in the nucleus. the positive charge of electricity corresponds to from 2 2 units of atomic weight, depending upon whether tom be light or heavy. Oxygen, for example, has atomic number of 8 and an atomic weight of 16; an atomic number of 50 and an atomic weight of lead, an atomic number of 82 and an atomic weight

ther

ttor

or

207.

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

The atom as thus conceived was undoubtedly a fine of imaginative thinking, and it appeared to explain perimental work in a remarkably precise manner.

It

mained to investigate first the arrangement of the er electrons on which the chief physical and chemical perties of an element depend, and second the structure the nucleus where the mass and radio-active proties have their origin. So far only the first of these been worked out in detail.

There is every reason to suppose that the electrons le round the nucleus with a great velocity analogous

Jol. 241.-No. 479.

X

to the motion of a planet round the sun, and that i must be arranged in groups so that the periodic sys of classification of elements might be explained. here arose a difficulty. For the ordinary laws of dyna leave no escape from the conclusion that such an ato Rutherford pictured could not exist for long even could be put together for a time.

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

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

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

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

ist unless an assumption like this is made. Bohr ext imagined that when the electron receives energy it amps instantaneously into a wider circle with a radius our times as large as the one it left; it does not bridge he distance between gradually. If it wishes to leave his 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 10 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 mass less than that of an atom of hydrogen. This is a urprising difference and constitutes an enigma, but that how it is. The nucleus of the hydrogen atom-that 3, the hydrogen atom deprived of its single electron-is

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

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

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

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

e presence of hydrogen nuclei in the nuclei of certain ght elements, and second the work of Aston and others n '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 -particles, and there is little doubt that these nuclei form an essential part of the nuclear structure. The lements which are disrupted by this bombardment have all atomic weights which when divided by 4 give 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 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 n 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 artly to workers on radio-activity and partly to the perimental work of Aston on positive rays. Since he hydrogen atom weighs unity on the scale on which tomic weights are measured, this work appears merely be a confirmation of the old and discarded hypoesis of Prout. But there is a difference. The element alorine's atomic weight is 35-45, and this, in no way, n be reckoned a simple multiple of unity, and it was cause of facts like these that Prout's hypothesis as discarded long ago. But Aston and others have own that, for example, chlorine is really a mixture of o bodies with almost exactly similar chemical and