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projected with high velocity; the number of a particles from a given quantity of radium have been counted, and the volume of helium which they produce has been measured. In this direct way it has been shown that about 2-7 Xio 19 particles or atoms of helium are required to form one cubic centimetre of helium gas at normal pressure and temperature. Not only is it feasible to detect the effect of a single atom of matter in special circumstances but also to show the path of a swift a particle or electron through a gas. This has been made possible by the discovery of C.T. R. Wilson that under suitable conditions the charged ions produced in gases by a or ft rays become centres for the condensation of water vapours, and are thus rendered visible as the nuclei of visible drops of water. The photographs of these droplets show in a most striking way the track of the particle through the gas, and illustrate with extraordinary detail the main effects produced by the passage of ionizing radiations through gases.

The essential correctness of the kinetic theory of matter, which assumes that the molecules of matter are in vigorous but ir- regular motion, has been clearly demonstrated by the experi- ments of Perrin and others on the motion and equilibrium of small spheres of matter in suspension in fluids which show the Brownian movement. At the same time the atomic or discrete nature of electricity, which had been implicitly assumed in many theories, has received complete experimental verification, and the magnitude of this fundamental unit of charge has been measured with precision. The most accurate experiments on this subject have been made by Millikan by measuring the electric field required to support a small, charged droplet of oil or mercury. The charge on the drop was varied by ionizing the gas in its neighbourhood. In this way he has been able to show that the charge always varies by integral multiples of a fundamental unit. The charge given to a drop by friction or any other method is always an integral multiple of this unit charge. This fundamental unit is the same both for positive and negative electricity, and is numerically equal to the charge carried by the negative electron, the positive and negative ions produced in a gas by X rays, and also to the positive charge carried by the hydrogen atom in the electrolysis of water. The magnitude of this unit charge, combined with electrochemical data, gives a most reliable method of measuring a number of important and molecular magnitudes. The value of the fundamental unit of charge and thus the mass of the individual atoms of matter are now known with an accuracy of certainly within one per cent and possibly within one-tenth of one per cent. The data found by Millikan are given in the following table :

Fundamental unit of charge. e = 4'774Xlo'el2ctrostatb units The AvogaJro Constant, i.e. the

number of molecules in one

gramme molecule. . . JV = 6-o52Xio 23 The number of molecules perc.c.

ofanygasato C. and 760 mms. n =2-7O-,Xi3 ia

Mass of hydrogen atom in grammes m = i -662 X lo" 24

From these data the number of atoms in one gramme of any element can be determined. While the average distance apart of the atoms or molecules can at once be deduced, the actual dimensions of the molecules or sphere of action of the molecules can only be approximately estimated with the aid of other and much less precise data.

Structure of the Atom. Since the proof that the negative electron of small mass is a constituent of all atoms of matter, there has been a vigorous attack on the fundamental problem of the structure of the atom. After passing through a number of phases the general ideas on this subject have crystallized into a fairly definite form, and it is now generally believed that the atom is composed of a massive positively charged nucleus of minute dimensions surrounded at a distance by a compensating distribution of negative electricity in the form of negative elec- trons. Since electricity is atomic the resultant positive charge on the nucleus must be an integral multiple N of the fundamental unit of charge e and is given by Ne. In order for the atom to be electrically neutral it must be surrounded by a distribution of N negative electrons. The value of N for each of the atoms is a fundamental constant, for on it depends the magnitude of the

electric field surrounding the nucleus and the arrangement of the external electrons which in turn determine the main physical and chemical properties of the atom. The idea of the nuclear struc- ture of atoms arose initially from a study of the scattering of a particles in their passage through matter. On account of its great energy of motion the charged a particle penetrates the structure of some of the atoms and comes under the influence of the intense repulsive field of the nucleus. Assuming that the law of force is that of the inverse square the a particle describes a hyperbolic path, and the angle of deflexion depends on the near- ness of approach to the nucleus. From a close study of the scat- tering of a rays by Geiger and Marsden it was concluded that the number of a particles scattered through different angles was in close accord with the idea of the nucleus atom, while the actual number scattered through a given angle gave information on the magnitude of the charge carried by the nucleus. The pre- liminary experiments indicated that for the heavier atoms the value of N was about half the atomic weight in terms of hydro- gen. A notable advance was made by the fundamental experi- ments of Moseley on the X-ray spectra of the elements. He found that the X-ray spectrum was similar for all elements, and that the frequency of vibration of corresponding lines in the spectrum was proportional to the square of a number which varied by unity in passing from one element to the next. He concluded that the nuclear charge in fundamental units was equal to the atomic or ordinal number of the elements when arranged in increasing order of their atomic weights. On this view the lightest element, hydrogen, has a nuclear charge i, helium 2, lithium 3, and so on up to the heaviest element, uranium, of ordinal number 92. This is a generalization of great importance and simplicity which has guided all subsequent work on the structure of atoms. The essential correctness of Moseley's con- clusion has been directly verified in the case of a few representa- tive elements by Chadwick by accurate measurement of the nuclear charge based on the scatteringof a rays. Moseley showed that with few exceptions all values of the nuclear charge between i and 92 were represented by known elements. The missing elements were of ordinal numbers 43, 61 and 75, corresponding to positions in the Periodic Table where the existence of additional elements had been suspected. Moreover, when the atomic weight of the element in Mendelecf's classification was replaced by its ordinal number certain irregularities were removed. For example, the positions of ar^on and potassium, cobalt and nickel, iodine and tellurium were interchanged a result in complete accord with their chemical properties (see CHEMISTRY).

It thus follows that the main physical and chemical properties of an element arc defined by a whole number which represents both its nuclear charge in fundamental units and the number of external electrons. The atomic weight of an element is in a sense a secondary property, for, as we shall see, elements can exist of the same nuclear charge but of different atomic weights. The number and position of the external electrons, on which the or- dinary chemical and physical properties of an atom depend, are defined by the nuclear charge. The mass of the atom which resides mainly in the nucleus exercises a subordinate effect on the external arrangement of the electrons.

Isotopes. On Moseley's classification only 92 elements of ordinal numbers i to 92 are possible, assuming that uranium (92) is the last of the elements. We shall now briefly discuss some recent advances which clearly show that in some cases several elements can exist with the same nuclear charge but of different atomic masses. Information on this point was first obtained from a study of the radioactive bodies. It was early observed that a number of products which showed different radioactive properties were inseparable from one another by ordinary physi- cal and chemical methods. For example, ionium and thorium, radium and mesothorium, radium D and lead cannot be separated from each other, and appear to be identical in chemical proper- ties. Elements so closely alike in chemical properties were called " isotopes " by Soddy, since they appeared to occupy the same position in the periodic arrangement of the elements. Viewed from the standpoint of the nuclear theory isotopes are elements