I propose to give some account of the steps which have led in our time to the reversal of this view.

Although speculative doubt always persisted in relation to the view that all matter must be conceived as constituted of a very considerable number of fundamentally distinct elements until the last decade of the nineteenth century no single definitely ascertained fact was known which made untenable the current chemical conception of the irreducibility and inconvertibility of the atoms of the different chemical elements although some facts were known which seemed to indicate certain family relationships within various groups of elements. In the last 30 years a multitude of new facts have been discovered which have made it impossible any longer to regard all the chemical elements as fundamentally distinct from one another and which have given new life to the older conception of transmutation. Moreover they have led to a theory of the constitution of the atom which although much remains to be done before it can be regarded as a completely verified conceptual scheme has made remarkable advances in its power of representing a very considerable complex of phenomena. In accordance with this theory the electron or definite unit electric charge is conceived as a most important part if not the whole of the fundamental constituents out of which all atoms are built up. Should this theory be fully established it would appear that the goal would be reached the instinctive desire for which had inspired all the speculations relating to the unity which may underly the diverse forms of matter.

In the years 1815 and 1816 the attention of chemists was directed by Prout to some facts which tended to show that there is a relationship between the properties of various elements and their atomic weights. He called attention to the fact that the three magnetic elements iron nickel and cobalt had in accordance with his estimate the same atomic weight double that of nitrogen; from this and other instances of approximate equality of atomic weights he concluded that substances having nearly the same atomic weights resemble one another in properties and can combine more readily with one another. From an examination of the atomic weights he made the suggestion that all the other elements have atomic weights which are integral multiples of the atomic weight of hydrogen and he suggested that they are all compounded of hydrogen and oxygen; in his later writings he put forward the view that hydrogen is the primordial element. In this order of ideas Prout had various successors amongst whom Newlands who promulgated in 1865 his Law of Octaves is the most prominent as a predecessor of Mendeléeff. Newlands arranged the elements in a table in which all the elements are consecutively numbered and appear nearly in the order of their atomic weights; they are arranged in a number of groups each group containing seven members but in some cases two elements are coupled together so as to occupy one place in the table. The elements in each group are placed in a vertical column and all the columns are arranged in parallel so that the first eighth fifteenth etc. elements appear in a horizontal line. In this arrangement all the elements which are in a horizontal line are analogous having similar properties. In the table the relation between elements of the same family is the same as that which in music exists between the notes at the extremities of one or more octaves. In some cases analogous elements whose atomic weights are consecutive appear with consecutive numbers in the table but in general the numbers attached to analogous elements differ by 7 or a multiple of 7. Various objections in detail were raised to this classification one of which was that it left no room for the inclusion of elements which might be discovered later. The fact that caesium thallium indium and rubidium had been discovered only a few years earlier made this objection a cogent one. Both Newlands' arrangement of the elements and an earlier one embodied in the so-called telluric helix propounded by De Chancourtois in 1863 indicated a periodic variation of properties among the elements when arranged in the order of their atomic weights.

The Periodic Law of the elements as it is now known is the system published by Mendeléeff of Petrograd in 1869. He showed that when the elements are arranged in order of their atomic weights elements which have a given property occur periodically; thus for example chlorine bromine and iodine have similar fundamental properties although their atomic weights differ widely from one another. Again elements which have very similar chemical and physical properties have atomic weights of nearly the same magnitude unless they increase regularly. Further when the elements are arranged in the ascending order of atomic weight they are in their proper order as regards valency. The elements such as hydrogen oxygen nitrogen etc. which are most widely distributed have the smallest atomic weight; consequently Mendeléeff called hydrogen and those elements included in the second series of his table “typical elements.” The general principle on which Mendeléeff insisted may be expressed in the form that the physical and chemical properties of an element are periodic functions of its atomic weight. The table which Mendeléeff published in 1871 has been since accepted as an authoritative illustration of the Periodic Law. The gaps in his table were taken by Mendeléeff to indicate the existence of elements which had not yet been discovered and he was enabled to predict the characteristic properties of such unknown elements. The success of these predictions was conspicuous in the cases of Scandium Gallium and Germanium which were discovered a few years after the publication of the table; and this success was important evidence of the value of the Periodic Law. In the case of some elements Mendeléeff employed the law to correct their atomic weights. Thus the atomic weight of uranium had previously been regarded as 120 but Mendeléeff gave reasons founded on his theory for supposing that the number should be 240; and this surmise was shown by Zimmermann in 1881 to be correct. Very important work was done by Lothar Meyer in the establishment of the theory of the Periodic Law; he expressed his results in the form of a curve in which the atomic weights are plotted as abscissae and the atomic volumes as ordinates; in this curve there are four well-marked maxima and the elements arrange themselves in six divisions. He considered in detail the bearing of the tabulation upon the electro-positive and the electro-negative properties of the elements and upon magnetic and diamagnetic properties. The connection of the melting and boiling points of compounds with the places of the elements of which they are formed in the table of atomic weights was the subject of extensive researches by Carnelley between the years 1879 and 1885. Many applications of the Periodic Law in connection with chemical and physical properties and especially relating to the spectra of the elements were made by a considerable number of researchers.

In 1887 Sir William Crookes published papers dealing with the Genesis of the Elements. He suggested the hypothesis that all the elements including hydrogen have been evolved from one original substance which he proposed to name protyle. He suggested also that our knowledge of any particular element is that only of an average specimen of such element so that even as regards atomic weight there may be possible variations between limits for one and the same element. The Periodic Law and the detailed confirmation of its value which it received from the results obtained by a great mass of research seemed to make it highly probable that some community of structure and of composition of all elements must be recognized which would account for the law but it was not until 1896 that a direct breach was made in the seeming fixity and inconvertibility of the different kinds of atoms.

The acceptance of the electromagnetic theory of light naturally led to the conclusion that the radiation from hot bodies must have its origin in the vibration of electric systems and thus that the atoms of such bodies must contain electric charges capable of setting up such vibrations. This suggested an electron theory of matter developed by Larmor and Lorentz in accordance with which matter is composed of electric charges and its mass is to be regarded as electromagnetic inertia. On these lines it was suggested that the movement of the Electric charges would be changed by a magnetic field and that this would displace the spectrum of the substance. That this is actually the case was shown by Zeeman in the case of the spectral lines of sodium which were split into two under the influence of a strong magnetic field.

When an electric discharge is passed between electrodes through a glass vessel so exhausted that the air pressure is extremely small luminous rays known as cathode rays proceed from the negative electrode or cathode. When they strike an insulated conductor they impart to it a negative electric charge. It was shown by Sir J. J. Thomson that these rays can be deflected from the rectilinear path by magnetic force or by electric force in the same manner as a stream of negatively electrified particles would be deflected. From this it was concluded that the cathode rays consist of a stream of negatively electrified particles in rapid motion. By measurement of the deflections employing the highly probable assumption that all the particles are equally charged Thomson estimated that their velocity is about one-tenth that of light and that their mass which appeared to be independent of the nature of the electrodes is about one seventeen-hundredth part of the mass of a hydrogen atom. The estimate has been later the subject of revision. This amounted to a discovery of a particle of very much smaller mass than that of the lightest particle hitherto taken to exist viz. the atom of hydrogen; and such a particle might be taken to be a component of all of the various substances which could be employed as electrodes. Since electricity in motion possesses electromagnetic energy its effects are comparable with those of mechanical inertia; moreover the electromagnetic energy of a moving charge is increased as the velocity is increased. It appeared then that the whole or a part of the mass of material substances might be of electromagnetic origin. It seemed possible to regard the corpuscle as simply an electric charge of definite amount and to identify it with the electron of Larmor and Lorentz. As the charges of the corpuscles in the cathode rays are all negative there remained for consideration the question of the nature of positive electrons and the part they might play in the phenomena of matter; further facts were required for the elucidation of these questions.

In 1896 a most important discovery that of a radioactive substance was made by Becquerel in Paris. He found that compounds of the metal uranium which has the highest atomic weight of all the elements continually emit rays capable of penetrating opaque screens and of affecting photographic plates. The intensity of this effect was measured by utilizing the fact that the rays from the uranium convert the air through which they pass into a conductor of electricity. It was thus found that the radio-activity of the substances depends solely upon the amount of uranium which they contain. It was soon afterwards shown by M. and Mme Curie that thorium the element of next highest atomic weight to uranium possesses the property of radio-activity; but other of the known elements failed to exhibit the same property. On experimenting with pitch-blende and other natural minerals in which uranium occurs M. and Mme Curie found that the radio-activity of pitch-blende is greater in amount than what could be attributed to the uranium that it contains and they then succeeded in chemically separating out of the pitch-blende compounds of a new I and intensely radio-active substance to which the name radium was given only a minute amount of which is contained in the pitch-blende. They also found other very radio-active substances in the pitch-blende. Radium was found to be a chemical element with a well defined place assigned by the Periodic Law and it resembles the I element barium in its powers of entering into chemical compounds; it has a characteristic line spectrum and its radio-activity is about two million times as great as that of uranium; its atomic weight was found to be next less than that of thorium. The amount of energy given off spontaneously by pure radium compounds and capable of being transformed into light and heat is so great that it was estimated to be sufficient to heat a quantity of water equal to the weight of the radium from the freezing-point to the boiling-point every three-quarters of an hour.

The urgent question at once presented itself as to the source of this great amount of energy which is continually being given out by the radio-active substance. It was found that in radium and the other radio-active substances the intensity of the radiation is independent of such physical conditions as temperature and pressure and that the radiation cannot be inhibited by any known means. The discovery was made by Sir E. Rutherford in 1899 that the radiation from thorium contains three distinct kinds of rays which are known respectively by the names α-rays β-rays and λ-rays. The investigation of the character of these rays was carried out by Sir E. Rutherford and Professor Soddy; from their investigations and those of others a large number of facts as to the radio-activity of thorium and the other radio-active substances were disclosed. Of the detailed investigations of this order I can give no account but must confine myself to a statement of the main facts which emerged as the result of many elaborate and difficult experiments. It was found that the α-rays when tested by means of magnetic and electric fields behave as positively electrified material particles and that they are stopped by a sheet of paper. These particles carry two atomic charges of positive electricity and they travel with a velocity of from one-twentieth to one-fifteenth of that of light. At first there was some doubt whether these particles should be regarded as atoms of hydrogen or of helium the latter of which is always to be found in minerals containing uranium and thorium. It was ultimately shown by a spectroscopic examination of the light from a few milligrams of radium that the α-rays must be taken to consist of a stream of helium atoms. It was later shown that the α-rays from uranium thorium polonium actinium and other radio-elements all consist of helium atoms.

The history of the discovery of helium is a remarkable one. It was first made in 1868 by observation in the spectrum of the solar chromosphere of a bright yellow line which had not been observed to exist in the spectrum of any known terrestrial substance; from this it was concluded that there exists in the sun an element hitherto unknown and to this the name helium was given by Lockyer who was one of the first to observe it. Other lines were observed in the solar spectrum to accompany the yellow line and the same spectrum was observed in the light from many stars. But it was not until 1895 that helium was discovered to exist on the earth. After the discovery by Lord Rayleigh and Sir W. Ramsay of the element argon in the atmosphere a search for argon in mineral substances was undertaken and in the course of this search a gas was found to be given off by solutions of minerals containing uranium. On examination of the spectrum of this gas by Lockyer it was found to be identical with that of the helium discovered nearly thirty years earlier in the solar spectrum.

The β-rays have been shown to be negative electrons; they are deflected both by magnetic and electric fields and by means of measurement of such deflections their identity with the cathode rays in the vacuum tube has been established. These β-rays ravel with great velocity approaching that of light and much more rapidly than the cathode rays; they have great penetrative power being able to pass through a considerable thickness of tinfoil or of glass without losing their power of affecting the photographic plate. They consist of unit charges of negative electricity just half as great as the positive charges carried by the α-atoms. The effective mass of a β-electron is only a very small fraction of the mass of one of the α-atoms; and it is the latter which contain most of the energy emitted in the radiation. The γ-rays are not deflected by magnetic and electric fields but they have much greater power of penetration than the β-rays. They have been identified with the Roentgen rays emitted outside the vacuum-tube when the cathode rays are passing through the interior of the tube.

Besides the rays given out by these radio-active substances radium and thorium also give out gases called emanations which are themselves radio-active substances. It was found that the radio-activity of these emanations suffers a gradual decay whilst the walls of the vessels in which they are contained become radioactive. But when the walls are washed with certain acids they lose their radio-activity and this is transferred to the acids. The inference from this is that the emanation gives rise to a new radio-active substance which is deposited on the wall of the vessel and is soluble in the acid. It has been shown that in a radioactive substance chemical changes are going on which result in the production of small quantities of new substances. By chemical processes minute quantities of these new radio-active substances have been separated out from uranium and thorium and it is found that they have properties different from those of the original substance. At first these new substances are highly radio-active whilst there is a diminution of the radioactivity of the original substance. After some time the radio-activity of the new substances decays whilst that of the original substance gradually goes back to its original amount. Rutherford and Soddy who worked together by the examination of the phenomena connected with the radio-activity of thorium were led to the conclusion that a whole series of new chemical substances are produced successively the whole process being accompanied by the liberation of energy. The atom of uranium in twelve successive changes appears to expel 7 atoms of helium and 5 electrons one such atom or electron at each change. The activity of a radioactive substance is found to be independent of the presence and nature of any other substance with which it may be in chemical combination; from this it can be inferred that radio-activity is to be regarded as an atomic not a molecular phenomenon. From the point of view of the atomic theory the changes in the nature of a substance are to be regarded as a series of successive changes in the constitution of the atoms. These atoms become dissociated into simpler parts and the process is accompanied by the liberation of some of the internal energy of the atom. The amount of energy given out in these atomic changes is enormously greater than the amount liberated in ordinary chemical changes which only involve molecular changes that is alterations in the grouping of atoms whilst the atoms themselves are unaltered. It has been estimated that an ounce of radium in the course of its average life of about 2500 years gives out as much energy as is evolved by burning ten tons of coal.

We have before our eyes in the case of a radio-active substance an actual transmutation of elements the possibility of which had as I have already stated often been speculatively asserted. This transmutation theory of radio-activity was formulated by Rutherford and Soddy in 1903 and soon afterwards some of its consequences were verified in a striking manner. It was shown that when a small quantity of radium emanation was watched during its slow decay the spectrum of the element helium could be seen; and later the presence of helium in the α-rays was demonstrated spectroscopically. The successive products of the disintegration of radium were examined with minute care by Rutherford who traced out a long series of such disintegration products most of which are so small in amount that they can only be detected by means of their radio-activities. Some of these products give out only α-rays one gives out both β-and γ-rays and one gives out all three kinds of rays; two of them give out no rays. They show great differences in their rates of decay; one of them decays to half its amount in three minutes another in 143 days another in 40 years whilst radium itself takes about 2600 years for its half decay. Results of a similar character have been obtained as regards uranium and thorium and their radio-active products. As the result of various experiments by R. J. Strutt and B. B. Boltwood there is strong reason for believing that radium is a descendant of uranium although probably not the direct product. An immense amount of work has been done by a large number of Physicists in the detailed investigation of the phenomena of radio-activity and of the properties of the new substances discovered as the products of disintegration.

Various hypotheses have been advanced as to the mode in which the phenomena are to be conceived; and these may be divided into two classes. In the first of these the energy emitted from the radio-elements is regarded as being obtained at the expense of the internal energy of the atom. In the second type of theory the energy is regarded as derived from external sources the radio-elements being regarded as mechanisms for the transformation of such energy into the forms manifested in radio-activity. The evidence appears to be very strongly in favour of the theories of the first type; since no experimental evidence appears to have been obtained that the energy is obtained at the expense of any external source. The theory of Rutherford and Soddy assumes that on an average a definite proportion of the atoms of each radio-active substance become unstable at a given time and when this instability occurs there is a disintegration of the atom usually of a violent character. The disintegration is supposed to consist of the expulsion either of an α-particle or of a β-particle or of both simultaneously but in some cases the change in the atom does not appear to involve the expulsion of either kind of particle. When an α-particle of atomic weight 4 is expelled the remaining atom is lighter than before and a substance composed of such atoms has chemical properties different from those of the original substance. This new substance becoming again as regards some of its atoms unstable another α-particle may be expelled; and this process may be repeated through a series of stages. As the disintegration proceeds the substance consists of a mixture of atoms of the original type and of the new type; that this is the case with radium is evidenced by the fact that the original spectrum of radium persists unchanged whilst the disintegration very slowly proceeds. The difference between the chemical properties of the original substance and the new substance formed by its disintegration is strikingly illustrated by the case of radium. Radium itself is an element closely allied to barium and it has a definite spectrum of bright lines similar to the spectra of the alkaline earths; it is non-volatile at ordinary temperatures. But the emanation is a chemically inert gas which in its spectrum and in the absence of definite chemical properties resembles the group of inert gases to which argon belongs; it condenses at -1500 C. This emanation is unstable and emits α-rays that is atoms of helium the residual atoms then form a new substance called radium-A which behaves like a solid and is deposited on the surface of bodies. The question arises what are the final substances which appear as the result of the whole process of successive formation of new substances? One of these final products is helium; this is formed by the accumulation of the α-rays given out during the successive transmutations. The other final product is lead which is always a constituent of uranium minerals the atomic weight of which appears to agree with the atomic weight of the substance which would be obtained from radium by the expulsion of 5 α-particles; one such α-particle is known to be expelled in each of five out of the eight successive transformations of radium into lead. The whole line of descent of uranium through ionium and radium in a considerable number of successive transformations has been traced out; the rates at which these transformations proceed show enormous variations ranging from a few seconds to thousands of years.

It was natural that attempts should be made to devise models of the atom which should be capable of representing the facts which have been discovered relating to the instability of radio-active substances. A model of this kind was suggested by Lord Kelvin and investigated in a detailed manner in 1904 by Sir J. J. Thomson. The atom is supposed to consist of a uniform sphere in which is a positive electric field and throughout which are distributed a number of negative electrons in motion. The total positive charge in the sphere is taken to be balanced by the charge of the electrons when the atom is electrically neutral. The idea is that the properties of the atom depend upon the number of the electrons upon their distributions in their orbits and upon the stability of such systems. The possibility presented itself of connecting in this manner the various possible types of atoms with the Periodic Law of the elements. Sir J. J. Thomson showed that the electrons moving under the electric forces upon them must distribute themselves in a number of concentric shells of differing radii. In order to simplify the mathematical problem of determining possible arrangements of the electrons Thomson worked out in detail the case in which the electrons move in rings in one plane the electrons being arranged at equal angular intervals. He determined the number of rings and the number of electrons in each ring which are such that stability of the system is ensured. It appears that such an atom containing a large number of revolving electrons may radiate energy extremely slowly. There must however come a time when this small but continual loss of energy from the atom results in a rearrangement of its electrons into a new system or in the expulsion of one or more of the electrons from the atom. It was suggested that the disintegration of the atoms of the radio-active elements is due to this result of the gradual loss of energy by radiation. It was shown that only a very small part of the mass of the atom was due to its electrons the rest being presumably associated with the positive electric field. In order to account for the scattering of the α-particles when they penetrate a thin film of matter this theory of the atom was modified by Rutherford.

In accordance with the theory of the atom which was advanced by Rutherford the atom contains a charged nucleus of dimensions exceedingly minute compared with the whole volume of the atom that is with what is called the atomic domain; and this nucleus replaces the sphere of positive electrification in Thomson's theory. The nucleus consists of a number of charged elements some of which are positive and others negative their resultant being a positive charge. Surrounding this nucleus there are in the atomic domain a number of electrons; that number being equal to the number of resultant positive unit charges in the nucleus so that in the neutral atom so constituted the total number of unit charges positive and negative is zero. The resultant positive charge of the nucleus is taken to be about half the product of the atomic weight multiplied by the fundamental unit charge. The existence of the small massive nucleus was suggested by the fact that the α- or helium-atoms when they penetrate into matter are divided through a considerable angle; the existence of an intense field of force in the interior of the atom being deduced from this fact. The properties of an atom are regarded as dependent mainly on the resultant nuclear charge and not on its mass; this is taken to explain many facts connected with the Periodic Law of the elements. The electrons in the outer part of the atom are arranged at distances from the nucleus and are controlled by the forces due to the resultant charge of the nucleus and their own electric fields. The nuclear electrons form a close combination with the positively charged units that make up the main part of the mass of the nucleus. It is regarded as probable that in the region just outside the nucleus an electron cannot be in stable equilibrium. Rutherford makes the statement as regards the electrons internal to and external to the nucleus that¹:

In accordance with this theory radio-active change originates always in the nucleus of the atom the expelled α- and β-particles coming from the nucleus. There is evidence that when the rapidly moving α-particles or helium atoms pass through dry nitrogen they give rise to rapidly moving particles that closely resemble hydrogen. This is shown by observations of their brilliant scintillations when they are allowed to strike a zinc-sulphide screen. It is thus suggested that by a breaking up of the nitrogen atoms hydrogen has been obtained; if this inference is fully confirmed a very important step has been taken in the theory of the transmutation of the elements. The hypothesis has naturally been made that the atoms of all the elements are built up of hydrogen nuclei and of electrons. In accordance with this view the helium nucleus is composed of four hydrogen nuclei and two negative electrons with a resultant electric charge of two positive units. This is a modification of Prout's hypothesis to which I have already referred that all the elements are built up of hydrogen as a fundamental constituent.

Rutherford's theory of the nuclear constitution of the atom has been further developed by Dr Niels Bohr of Copenhagen who studied the optical spectra of hydrogen and of helium regarded as the simplest types of atoms. His theory takes account of and employs the new quantum-theory in accordance with which radiation only occurs discontinuously in quanta or definite unit losses of energy at moments when one stable configuration of the atom changes into another such configuration. This theory has been further elaborated by Sommerfeld and others and appears to have had a considerable amount of success in coordinating various phenomena connected with hydrogen and helium and especially in accounting for the complexity of the lines of their spectra. The mathematical theory of the hydrogen atom has been worked out; the calculation of the spectral lines of hydrogen known as the Balmer series is in full accord with observation; and even the displacement of the lines which occurs when the hydrogen is in an electric field has been successfully determined. It is a remarkable fact that this theory of the hydrogen atoms makes use of the theory of Abelian functions a purely mathematical theory which had been worked out without any expectation that it would ultimately have any such application. This is an example which has many parallels in Physical Science of the fact that the most abstract Mathematics which of course has its ultimate roots in the physical domain does not really lose its connection with its original source however long the time may be before the fact of such connection attains explicit recognition.

The development of the theory of the constitution of the atom is still in progress. Complete success in devising a model of the atom which would make possible the calculation of the detailed varieties of configuration of which such a model might be capable would be a considerable step towards the goal of turning Chemistry into a deductive science in accordance with which all possible elementary forms of matter might be ascertained and the possible nature of compounds with their chemical and physical properties predicted.

The known radio-active elements in spontaneous disintegration are few in number most of the elements being so stable that no such disintegration of them can be detected. We do not at present possess any fully established means of exciting artificially the process of expulsion of α- or β-particles from the atom and of thus liberating energy of the enormous stock which appears to be internal to the atom; but efforts are being made to accomplish this. Sir E. Rutherford by bombarding the atoms of nitrogen with α-rays has as I have already remarked succeeded in producing hydrogen atoms from a small proportion of the nitrogen atoms. If the means of solving this problem of transmutation are in the future discovered the practical use of such a method would lie more in the utilization of the internal energy of the atom as a source of energy than in the solution of the ancient problem of transforming one kind of matter into another. We should be in possession of a source of energy by which quantities of energy would be liberated and made available enormously in excess of the amounts obtainable by the ordinary chemical processes of combustion in which only molecular energy is utilized and in which the vast store of energy in the atoms remains unaffected.

The existence of the radio-active minerals is of much importance in connection with attempts to estimate the age of geological strata in which they are contained as we know the rate of change of uranium and its various products into the final product lead. As Soddy has written¹:

It will be observed that the processes going on in radio-active substances consist exclusively of the breaking up of more complex atoms into lighter and less complex parts and so far as our experience goes these processes are irreversible. Accordingly no light is thrown upon the possibility of the present complex elements having been built up out of simpler components such as is involved in the view that a simple primeval form of material once existed from which the present elements were evolved by processes in which there was a gradual complication of the original material. If we are to conceive that our present forms of matter were so evolved we must imagine that such evolution took place under conditions very widely different from those which we are able to observe at present. Passage from the more complex to less complexity is what we observe at present and the idea that change in the reverse order once took place is at present a speculation resting upon no evidence obtained from observation.

A very important discovery has been made in connection with the investigation of radio-activity that of the existence of isotopes the name given to elements which have identical chemical properties and occupy the same place in the Periodic Table but have different atomic weights. If an α-particle or a β-particle is expelled from an atom the new atom has different properties from the original one. If it is the α-particle that is expelled the element after this expulsion passes in the Periodic Table to the place next but one to that of the original element the atomic weight being diminished. If it is the ft-particle that is expelled the change of place is into the next place in the table in the opposite direction. But if an α-particle and two β-particles are expelled in any order the element returns to its original place in the table; its atomic weight is diminished by four units but its chemical and spectroscopic properties are the same as before the expulsions took place. The elements in the original and final form are in fact isotopes. The places in the Periodic Table appear to represent the integral nett charges of electricity in the atomic nucleus. The α-particle having a charge of two units of positive electricity when it is expelled from the atom moves the element through two places in the Periodic Table whereas the expulsion of a β-particle with its unit negative charge moves the element one place in the opposite direction. Isotopes such as thorium and ionium have identical chemical and spectroscopic properties although their radio-active properties are different. The lead obtained as the final product of the series which commences with uranium has a different atomic weight 206 from the lead which is the final product of the thorium series for which the atomic weight is 208. Ordinary lead has the atomic weight 207.2 and this suggests that it is a mixture of the two isotopes.

There appear to exist also isotopes which have the same atomic weight as well as identical chemical properties although the internal energy in the atoms of the isotopes is different in the two cases. Such isobaric isotopes are obtained when as in the case of thorium there is a branching off of the successive disintegrations into two series each with a final product.

By means of a new method of deflecting the rays which consist of streams of charged particles F. W. Aston has been able to determine the masses of a number of atoms with a great degree of accuracy. By this method he has been enabled to show that some of the elements formerly supposed to be homogeneous consist of a mixture of two or more isotopes with different atomic weights. When the atomic weight of oxygen is taken to be 32 so that the atomic weight of hydrogen is 1.008 it has been found that the atomic weights of many of the elements are represented exactly by integral numbers. In other instances such as chlorine in which this is not the case the elements have been found to consist of a mixture of isotopes each with an atomic weight represented by an integer; the fractional atomic weight of the element as a mixture being due to the fact that the element contains portions of different atomic weights. Thus for example chlorine of which the atomic weight is 35.46 obtained by chemical methods is a mixture in the proportion of 3 to 1 of two isotopes of which the atomic weights are 35 and 37 respectively. These investigations have led to the theory which has been to a great extent verified that the fractional irregularities of the atomic weights in the Periodic Table are due to the existence of isotopes.

The known facts appear to be consistent with the view that all matter is built up of electrons and atomic nuclei; all the electrons being alike and the atomic nuclei being constructed of fundamental units all alike and of electrons. The electrons and nuclei must be regarded as postulated concepts subject to defined interrelations and which from the point of view of this theory are irreducible.

As I have already indicated in an earlier lecture for a long period chemical and physical investigations were carried on independently of one another and they had but few points of contact. An important effect of the great discoveries of the last quarter of a century relating to the composite character of the atom has been to break down the barrier which had long separated the sciences of Physics and of Chemistry. The two great departments have in our day at last joined forces; their meeting-place has been within what was long regarded as the impregnable fortress of the atom.