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sun, and the gifts of the sun are conveyed to us by the energy possessed by these lines of electric force per cubic æther. It is to the sun that we owe, not merely night centimetre, divided by the square of the velocity of light. and day, springtime and harvest, but it is the energy of But though lines of electric force carry some of the æther the sun, stored up in coal, in waterfalls, in food, that along with them as they move, the amount so carried, practically does all the work of the world.

even in the strongest electric fields we can produce, is but How great is the supply the sun lavishes upon us be- a minute fraction of the æther in their neighbourhood. comes clear when we consider that the heat received This is proved by an experiment made by Sir Oliver the earth under a high sun and a clear sky is equivalent, Lodge in which light was made to travel through an according to the measurements of Langley, to about 7000 electric field in rapid motion. If the electric field had horse-power per acre. Though our engineers have not yet carried the whole of the æther with it, the velocity of discovered how to utilise this enormous supply of power, the light would have been increased by the velocity of they will, I have not the slightest doubt, ultimately succeed the electric field. As a matter of fact, no increase what. in doing so; and when coal is exhausted and our water- ever could be detected, though it would have been registered power inadequate, it may be that this is the source from if it had amounted to one-thousandth part of that of the which we shall derive the energy necessary for the world's field. work. When that comes about, our centres of industrial The æther carried along by a wave of light must be an activity may perhaps be transferred to the burning deserts exceedingly small part of the volume through which the of the Sahara, and the value of land determined by its wave is spread. Parts of this volume are in motion, but suitability for the reception of traps to catch sunbeams. by far the greater part is at rest; thus in the wave front

This energy, in the interval between its departure from there cannot be uniformity, at some parts the æther is the sun and its arrival at the earth, must be in the space moving, at others it is at rest-in other words, the wave between them. Thus this space must contain something front must be more analogous to bright specks on a dark which, like ordinary matter, can store up energy, which ground than to a uniformly illuminated surface. can carry at an enormous pace the energy associated with The place where the density of the æther carried along light and heat, and can, in addition, exert the enormous by an electric field rises to its highest value is close to a stresses necessary to keep the earth circling round the corpuscle, for round the corpuscles are by far the strongest sun and the moon round the earth.

electric fields of which we have any knowledge. We know The study of this all-pervading substance is perhaps the the mass of the corpuscle, we know from Kaufmann's most fascinating and important duty of the physicist. experiments that this arises entirely from the electric

On the electromagnetic theory of light, now universally charge, and is therefore due to the æther carried along accepted, the energy streaming to the earth travels through with the corpuscle by the lines of force attached to it. the æther in electric waves ; thus practically the whole of A simple calculation shows that one-half of this mass the energy at our disposal has at one time or another is contained in a volume seven times that of a corpuscle. been electrical energy. The æther must, then, be the Since we know the volume of the corpuscle as well as the seat of electrical and magnetic forces. We know, thanks mass, we can calculate the density of the æther attached to the genius of Clerk Maxwell, the founder and inspirer to the corpuscle; doing so, we find it amounts to the of modern electrical theory, the equations which express prodigious value of about 51010, or about 2000 million the relation between these forces, and although for some times that of lead. Sir Oliver Lodge, by somewhat purposes these are all we require, yet they do not tell different considerations, has arrived at a value of the same us very much about the nature of the æther.

order of magnitude. The interest inspired by equations, too, in some minds is Thus around the corpuscle æther must have an extravaapt to be somewhat Platonic; and something more grosslygant density : whether the density is as great as this in mechanical-a model, for example, is felt by many to be other places depends upon whether the æther is commore suggestive and manageable, and for them a more pressible or not. If it is compressible, then it may be powerful instrument of research, than a purely analytical condensed round the corpuscles, and there have theory.

abnormally great density ; if it is not compressible, then Is the æther dense or rare? Has it a structure? Is the density in free space cannot be less than the number it at rest or in motion ? are some of the questions which I have just mentioned. force themselves upon us.

With respect to this point we must remember that the Let us consider some of the facts known about the forces acting on the æther close to the corpuscle are proæther. When light falls on a body and is absorbed by digious. If the æther were, for example, an ideal gas it, the body is pushed forward in the direction in which the density of which increased in proportion to the pressure, the light is travelling, and if the body is free to move it however great the pressure might be, then if, when ex. is set in motion by the light. Now it is a fundamental posed to the pressures which exist in some directions close principle of dynamics that when a body is set moving in to the corpuscle, it had the density stated above, its density a certain direction, or, to use the language of dynamics, under atmospheric pressure would only be about 8x 10-16, acquires momentum in that direction, some other mass or a cubic kilometre would have a mass less than a gram; must lose the same amount of momentum; in other words, so that instead of being almost incomparably denser than the amount of momentum in the universe is constant. lead, it would be almost incomparably rarer than the Thus when the body is pushed forward by the light some lightest gas. other system must have lost the momentum the body I do not know at present of any effect which would acquires, and the only other system available is the wave enable us to determine whether æther is compressible or of light falling on the body; hence we conclude that there And although at first sight the idea that we must have been momentum in the wave in the direction in immersed in a medium almost infinitely denser than lead which it is travelling. Momentum, however, implies mass might seem inconceivable, it is not so if we remember in motion. We conclude, then, that in the æther through that in all probability matter is composed mainly of holes. which the wave is moving there is mass moving with We may, in fact, regard matter as possessing a bird-cage the velocity of light. The experiments made on the kind of structure in which the volume of the æther dispressure due to light enable us to calculate this mass, and turbed by the wires when the structure is moved is inwe find that in a cubic kilometre of æther carrying light finitesimal in comparison with the volume enclosed by as intense as sunlight is at the surface of the earth, the them. If we do this, no difficulty arises from the great mass moving is only about one-fifty-millionth of a milli- density of the æther; all we have to do is to increase the gram. We must be careful not to confuse this with the distance between the wires in proportion as we increase mass of a cubic kilometre of æther; it is only the mass the density of the æther. noved when the light passes through it; the vast majority Let us now consider how much æther is carried along of the æther is left undisturbed by the light. Now, on by ordinary matter, and what effects this might be exthe electromagnetic theory of light, a wave of light may pected to produce. be regarded as made up of groups of lines of electric The simplest electrical system we know, an electrified force moving with the velocity of light; and if we take sphere, has attached to it a mass of æther proportional to this point of view we can prove that the mass of æther its potential energy, and such that if the mass were to per cubic centimetre carried along is proportional to the

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equal the electrostatic potential energy of the particle. tion of energy the one form can be converted into the other This result can be extended to any electrified system, and at a fixed rate of exchange, so that when one unit of one it can be shown that such a system binds a mass of the kind disappears a unit of the other simultaneously appears. æther proportional to its potential energy. Thus a part

Now in many cases this rule is all that we require to of the mass of any system is proportional to the potential calculate the behaviour of the system, and the conception energy of the system.

of potential energy is of the utmost value in making the The question now arises, Does this part of the mass knowledge derived from experiment and observation availadd anything to the weight of the body? If the æther able for mathematical calculation. It must, however, I were not subject to gravitational attraction it certainly think, be admitted that from the purely philosophical point would not; and .even if the æther were ponderable, we of view it is open to serious objection. It violates, for might expect that as the mass is swimming in a sea of example, the principle of continuity. When a thing æther it would not increase the weight of the body to changes from a state A to a different state B, the prinwhich it is attached. But if it does not, then a body ciple of continuity requires that it must pass through a with a large amount of potential energy may have an number of states intermediate between A and B, so that appreciable amount of its mass in a form which does not the transition is made gradually, and not abruptly. Now, increase its weight, and thus the weight of a given mass when kinetic energy changes into potential, although there of it may be less than that of an equal mass of some is no discontinuity in the quantity of the energy, there substance with a smaller amount of potential energy. is in its quality, for we do not recognise any kind of Thus the weights of equal masses of these substances energy intermediate between that due to the motion and would be different. Now, experiments with pendulums, as that due to the position of the system, and some portions Newton pointed out, enable us to determine with great of energy are supposed to change per saltum from the accuracy the weights of equal masses of different sub kinetic to the potential form. In the case of the transition stances. Newton himself made experiments of this kind, of kinetic energy into heat energy in a gas, the disconand found that the weights of equal masses were the same tinuity has disappeared with a fuller knowledge of what for all the materials he tried. Bessel, in 1830, made some the heat cnergy in a gas is due to. When we experiments on this subject which are still the most ignorant of the nature of this energy, the transition from

we possess, and he showed that the weights of kinetic into thermal energy seemed discontinuous; but equal masses of lead, silver, iron, brass did not differ by now we know that this energy is the kinetic energy of as much as one part in 60,000.

the molecules of which the gas is composed, so that there The substances tried by Newton and Bessel did not, is no change in the type of energy when the kinetic however, include any of those substances which possess energy of visible motion is transformed into the thermal the marvellous power of radio-activity; the discovery of energy of a gas—it is just the transference of kinetic these came much later, and is one of the most striking energy from one body to another. achievements of modern physics.

If we regard potential energy as the kinetic energy of These radio-active substances are constantly giving out portions of the æther attached to the system, then all large quantities of heat, presumably at the expense of energy is kinetic energy, due to the motion of matter or their potential energy; thus when these substances reach of portions of æther attached to the matter. I showed. their final non-radio-active state their potential energy many years ago, in my Applications of Dynamics to must be less than · when they were radio-active. Prof. Physics and Chemistry, that we could imitate the effects Rutherford's measurements show that the energy emitted of the potential energy of a system by means of the kinetic by one gram of radium in the course of its degradation energy of invisible systems connected in an appropriate to non-radio-active forms is equal to the kinetic energy manner with the main system, and that the potential of a mass of 1/13th of a milligram moving with the energy of the visible universe may in reality be the kinetic velocity of light.

energy of an invisible one connected up with it. We This energy, according to the rule I have stated, corre- naturally suppose

that this invisible universe is the sponds to a mass of 1/13th of a milligram of the æther, luminiferous æther, that portions of the æther in rapid and thus a gram of radium in its radio-active state must motion are connected with the visible systems, and that have at least 1/13th of a milligram more of æther attached their kinetic energy is the potential energy of the systems. to it than when it has been degraded into the non-radio- We may thus regard the æther as a bank in which active forms. Thus if this æther does not increase the we may deposit energy and withdraw it at our convenience. weight of the radium, the ratio of mass to weight for

The mass of the æther attached to the system will change radium would be greater by about one part in 13,000 than as the potential energy changes, and thus the mass of a for its non-radio-active products.

system the potential energy of which is changing cannot I attempted several years ago to find the ratio of mass be constant; the fluctuations in mass under ordinary conto weight for radium by swinging a little pendulum, the ditions are, however, so small that they cannot be detected bob of which was made of radium. I had only a small by any means at present at our disposal. Inasmuch as quantity of radium, and was not, therefore, able to attain the various forms of potential energy are continually being any great accuracy. I found that the difference, if any changed into heat energy, which is the kinetic energy of in the ratio of the mass to weight between radium and the molecules of matter, there is a constant tendency for other substances was not more than one part in 2000. the mass of a system such as the earth or the sun to Lately we have been using at the Cavendish Laboratory diminish, and thus as time goes on for the mass of æther a pendulum the bob of which was filled with uranium gripped by the material universe to become smaller and oxide. We have got good reasons for supposing that smaller ; the rate at which it would diminish would, howuranium is a parent of radium, so that the great potential ever, get slower as time went on, and there is no reason energy and large æthereal mass possessed by the radium to think that it would ever get below a very large value. will be also in the uranium ; the experiments are not yet

Radiation of light and heat from an incandescent body completed. It is, perhaps, expecting almost too much to like the sun involves a constant loss of mass by the body. hope that the radio-active substances may add to the great Each unit of energy radiated carries off its quota of services they have already done to science by furnishing mass, but as the mass ejected from the sun per year is the first case in which there is some differentiation in the only one part in 20 billionths (1 in 2 x 10's) of the mass action of gravity.

of the sun, and as this diminution in mass is not necesThe mass of æther bound by any system is such that if sarily accompanied by any decrease in its gravitational it were to move with the velocity of light its kinetic attraction, we cannot expect to be able to get any evidence energy, would be equal to the potential energy of the

of this effect. system. This result suggests a new view of the nature As our knowledge of the properties of light has proof potential energy. Potential energy is usually regarded gressed, we have been driven to recognise that the æther, as essentially different from kinetic energy. Potential when transmitting light, possesses properties which, before energy depends on the configuration of the system, and the introduction of the electromagnetic theory, would have can be calculated from it when we have the requisite data; been thought to be peculiar to an emission theory of light kinetic energy, on the other hand, depends upon the velocity and to be fatal to the theory that light consists of unof the system. According to the principle of the conserva

dulations.

Take, for example, the pressure exerted by light. This Law of Thermodynamics is only true for radiation when would follow as a matter of course if we supposed light the radiation is produced by mechanism of a special type. to be small particles moving with great velocities, for Quite apart, however, from considerations of thermo these, if they struck against a body, would manifestly dynamics, we should expect that the light from a luminous tend to push it forward, while on the undulatory theory source should in many cases consist of parcels, possessthere seemned no reason why any effect of this kind should ing, at any rate to begin with, a definite amount of take place.

energy. Consider, for example, the case of a gas like Indeed, in 1792, this very point was regarded as a test sodium vapour, cmitting light of a definite wave-length; between the theories, and Bennet made experiments to see we may imagine that this light, consisting of electrical whether or not he could find any traces of this pressure. waves, is emitted by systems resembling Leyden jars. We now know that the pressure is there, and if Bennet's The energy originally possessed by such a system will be instrument had been more sensitive he must have observed the clectrostatic energy of the charged jar. When the it. It is perhaps fortunate that Bennet had not at his vibrations are started, this energy will be radiated away command more delicate apparatus. Had he discovered the into space, the radiation forming a complex system, conpressure of light, it would have shaken confidence in the taining, if the jar has no electrical resistance, the energy undulatory theory and checked that magnificent work at stored up in the jar. the beginning of the last century which so greatly in- The amount of this energy will depend on the size of creased our knowledge of optics.

the jar and the quantity of electricity with which it is As another example, take the question of the distribu-charged. With regard to the charge, we must remember tion of energy in a wave of light. On the emission theory that we are dealing with systems formed out of single the energy in the light is the kinetic energy of the light molecules, so that the charge will only consist of one or particles. Thus the energy of light is made up of distinct two natural units of electricity, or, at all events, some units, the unit being the energy of one of the particles. small multiple of that unit, while for geometrically similar

The idea that the energy has a structure of this kind | Leyden jars the energy for a given charge will be prohas lately received a good deal of support. Planck, in a portional to the frequency of the vibration; thus, the very remarkable series of investigations on the Thermo- energy in the bundle of radiation will be proportional to dynamics of Radiation, pointed out that the expressions the frequency of the vibration. for the cnergy and entropy of radiant energy were of We may picture to ourselves the radiation as consisting such a form as to suggest that the energy of radiation, of the lines of electric force which, before the vibrations like that of a gas on the molecular theory, was made up were started, were held bound by the charges on the jar, of distinct units, the magnitude of the unit depending on and which, when the vibrations begin, are thrown into the colour of the light; and on this assuinption he was rhythmic undulations, liberated from the jar and travel able to calculate the value of the unit, and from this through space with the velocity of light. deduce incidentally the value of Avogadro's constant- Now let us suppose that this system strikes against an the number of molecules in a cubic centimetre of gas at uncharged condenser and gives it a charge of electricity, standard temperature and pressure.

the charge on the plates of the condenser must be at least This result is most interesting and important, because if one unit of electricity, because fractions of this charge it were a legitimate deduction from the Second Law of do not exist, and each unit charge will anchor a unit Thermodynamics, it would appear that only a particular tube of force, which must come from the parcel of radiatype of mechanism for the vibrators which give out light tion falling upon it. Thus a tube in the incident light and the absorbers which absorb it could be in accordance will be anchored by the condenser, and the parcel formed with that law.

by this tube will be anchored and withdrawn as a whole If this were so, then, regarding the universe as froin the pencil of light incident on the condenser. If the collection of machines all obeying the laws of dynamics, energy required to charge up the condenser with a unit the Second Law of Thermodynamics would only be true of electricity is greater than the energy in the incident for a particular kind of machine.

parcel, the tube will not be anchored and the light will There seems, however, grave objection to this view, pass over the condenser and escape from it. These prinwhich I may illustrate by the case of the First Law of ciples that radiation is made up of units, and that it reThermodynamics, the principle of the Conservation of quires a unit possessing a definite amount of energy to Energy. This must be true whatever be the nature of excite radiation in a body on which it falls, perhaps receive the machines which make up the universe, provided they their best illustration in the 'remarkable laws' governing obey the laws of dynamics, any application of the prin- Secondary Röntgen radiation, recently discovered by Prof. ciple of the Conservation of Energy could not discriminate

Barkla. Prof. Barkla has found that each of the different between one type of machine and another.

chemical elements, when exposed to Röntgen rays, emits Now, the Second Law of Thermodynamics, though not a definite type of secondary radiation whatever may have a dynamical principle in as strict a sense as the law of been the type of primary; thus lead emits one type, copper the Conservation of Energy, is one that we should expect

another, and so on; but these radiations are not excited to hold for a collection of a large number of machines at all if the primary radiation is of a softer type than of any type, provided that we could not directly affect the the specific radiation emitted by the substance ; thus the individual machines, but could only observe the average

secondary radiation from lead being harder than that effects produced by an enormous number of them. On from copper, if copper is exposed to the secondary radiathis view, the Second Law, as well as the First, should tion from lead the copper will radiate, but lead will not he incapable of saying that the machines were of any

radiate when exposed to copper. Thus, if we suppose particular type : 'so that investigations founded on thermo- that the energy in a unit of hard Röntgen rays is greater dynamics, though the expressions they lead to may suggest

than that in one of soft, Barkla's results are strikingly -annot. I think, be regarded as proving—the unit struc

analogous to those which would follow on the unit theory ture of light energy.

of light. It would seem as if in the application of thermodynamics Though we have, I think, strong reasons for thinking 10 radiation some additional assumption has been implicitly that the energy in the light waves of definite wave-length introduced, for these applications lead to definite relations is done uo into bundles, and that these bundles, when hotween the energy of the light of any particular wave

emitted, all possess the same amount of energy, I do not length and the temperature of the luminous body.

think there is any reason for supposing that in any casual Now a possible way of accounting for the light emitted specimen of light of this wave-length, which may subhy hot bodies is 10 suppose that it arises from the collisions sequent to its emission have been many times refracted of corpuscles with the molecules of the hot body, but it or reflected, the bundles possess any definite amount of is only for one particular law of force between the cor- energy. For consider what must happen when a bundle puscles and the molecules that the distribution of energy is incident on a surface such as glass, when part of it is would be the same as that deduced by the Second Law

reflected and part transmitted. The bundle is divided into of Thermodynamics. so that in this case, as in the other.'

two portions, in each of which the energy is less than the results obtained by the application of thermodynamics the incident bundle, and since these portions diverge and to radiation would ramvia ivm

to suppose that the Second may ultimately be many thousands of miles apart, it

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wouid seem meaningless still to regard them as forming According to this theory, the radio-active elements are one unit. Thus the energy in the bundles of light, after not permanent, but are gradually breaking up inio they have suffered partial reflection, will not be the same elements of lower atomic weight; uranium, for example, -as in the bundles when they were emitted. The study of is slowly breaking up, one of the products being radiur., the dimensions of these bundles, for example, the angle while radium breaks up into a radio-active gas called they subtend at the luminous source, is an interesting sub- radium emanation, the emanation into another radio-active ject for investigation ; experiments on interference between substance, and so on, and that the radiations are a kind rays of light emerging in different directions from the of swan's song emitted by the atoms when they pasi luminous source would probably throw light on this point. from one form to another; that for example, it is when

I now pass to a very brief consideration of one of the a radium atom breaks up and an atom of the emanation most important and interesting advances ever made in appears that the rays which constitute the radio-activity physics, and in which Canada, as the place of the labours are produced. of Profs. Rutherford and Soddy, has taken a conspicuous Thus, on this view, the atoms of the radio-active part. I mean the discovery and investigation of radio- elements are not immortal; they perish after a life the activity. Radio-activity was brought to light by the average value of which ranges from thousands of millions Röntgen rays. One of the many remarkable properties of years in the case of uranium to a second or so in the of these rays is to excite phosphorescence in certain sub- case of the gaseous emanation from actinium. stances, including the salts of uranium, when they fall When the atoms pass from one state to another they upon them. Since Röntgen rays produce phosphorescence, give out large stores of energy; thus their descendants it occurred to Becquerel to try whether phosphorescence do not inherit the whole of their wealth of stored-up would produce Röntgen rays. He took some uranium energy; the estate becomes less and less wealthy with salts which had been made to phosphoresce by exposure, each generation; we find, in fact, that the politiciar., not to Röntgen rays, but to sunlight, tested them, and when he imposes death duties, is but imitating a proces found that they gave rays possessing properties which has been going on for ages in the case of these similar to Röntgen rays. Further investigation showed, radio-active substances. however, that to get these rays it was not necessary, to

Many points of interest arise when we consider the rate make the uranium phosphoresce, that the salts were just at which the atoms of radio-active substance disappear. as active if they had been kept in the dark. It thus Rutherford has shown that whatever be the age of these appeared that the property was due to the metal and atoms, the percentage of atoms which disappear in ore

to the phosphorescence, and that uranium and its second is always the same; another way of putting it is compounds possessed the power of giving out rays which, that the expectation of life of an atom is independent of like Röntgen rays;

affect a photographic plate, make its age-that an atom of radium one thousand years old certain minerals phosphoresce, and make gases through is just as likely to live for another thousand years as one which they pass conductors of electricity.

just sprung into existence. Niepce de Saint-Victor had observed some years before Now this would be the case if the death of the atorr this discovery that paper soaked in a solution of uranium were due to something from outside which struck old nitrate affected a photographic plate, but the observation and young indiscriminately; in a battle, for example, the excited but little interest. The ground had not then been chance of being shot is the same for old and young : So prepared, by the discovery of the Röntgen rays, for its that we are inclined at first to look to something coming reception, and it withered and was soon forgotten.

from outside as the cause why an atom of radium, fo: Shortly after Becquerel's discovery of uranium, Schmidt example, suddenly changes into an atom of the emanafound that thorium possessed similar properties. Then

tion. But here we are met with the difficulty that co M. and Mme. Curie, after a most difficult and laborious changes in the external conditions that we have as yet investigation, discovered two new substances, radium and been able to produce have had any effect on the life of polonium, possessing this property to enormously the atom; so far as we know at present, the life of a greater extent than either thorium or uranium, and this radium atom is the same at the temperature of a furnace was followed by the discovery of actinium by Debierne. as at that of liquid air-it is not altered by surrounding Now the researches of Rutherford and others have led the radium by thick screens of lead or other dense to the discovery of so many new radio-active substances materials to ward off radiation from outside, and, what that any attempts at christening seem to have been to my mind is especially significant, it is the same when abandoned, and they are denoted, like policemen, by the

the radium is in the most concentrated form, when its letters of the alphabet.

atoms are exposed to the vigorous bombardment from the Mr. Campbell has recently found that potassium, rays given off by the neighbouring atoms, as when it is though far inferior in this respect to any of the substances in the most dilute solution, when the rays are absorbed I have named, emits an appreciable amount of radiation, by the water which separates one atom from another. the amount depending only on the quantity of potassium, This last result seems to me to make it somewhat improband being the same whatever the source from which the able that we shall be able to split up the atoms of the potassium is obtained or whatever the elements with which non-radio-active elements by exposing them to the radiation it may be in combination.

from radium ; if this radiation is unable to affect the unThe radiation emitted by these substances is of three stable radio-active atoms, it is somewhat unlikely that it types, known a, B, and gravs. The a rays have will be able to affect the much more stable non-radio'been shown by Rutherford to be positively electrified

active elements. atoms of helium, moving with speeds which reach up to The evidence we have at present is against a disturbabout one-tenth of the velocity of light. The B rays are ance coming from outside breaking up the radio-active negatively electrified corpuscles, moving in some cases with

atoms, and we must therefore look to some process of very nearly the velocity of light itself, while the q rays decay in the atom itself; but if this is the case, how are are unelectrified, and are analogous to the Röntgen rays. we to reconcile it with the fact that the expectation of The radio-activity of uranium was shown by Crookes life of an atom does not diminish as the atom gets older:

arise from something mixed with the uranium, We can do this if we suppose that the atoms when they which differed sufficiently in properties from the uranium are first produced have not all the same strength of conitself to enable it to be separated by chemical analysis. știtution, that some are more robust than others, perhaps 'He took some uranium, and by chemical treatment because they contain more intrinsic energy to begin with, separated it into two portions, one of which was radio

and will therefore have a longer life. Now if when the active and the other not.

atoms are first produced there are some which will live Next Becquerel found that if these two portions were for one year, some for ten, some for a thousand, and kept for several months, the part which was not radio- so on; and if lives of all durations, from nothing to active to begin with regained radio-activity, while the infinity, are present in such proportion that the number

part which was radio-active to begin with had lost its of atoms which will live longer than a certain number radio-activity. These effects and many others receive a of years decreases in a constant proportion for each complete explanation by the theorv of radio-active change additional year of life, we can easily prove that the ex- which we owe to Rutherford and Soddy.

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its age may be. On this view the different atoms of a earthquake phenomena had previously come to the conradio-active substance are not, in all respects, identical. clusion that rocks similar to those at the earth's surface

The energy developed by radio-active substances is ex- only descend a short distance below the surface; he ceedingly large, one grain of radium developing nearly estimates this distance at about thirty miles, and conas much energy as would be produced by burning a ton cludes that at a depth greater than this the earth is fairly of coal. This energy is mainly in the a particles, the homogeneous. positively charged helium atoms which are emitted when

Though the discovery of radio-activity has taken away ihe change in the atom takes place; if this energy were one method of calculating the age of the earth it has. produced by electrical forces it would indicate that the supplied another. helium atom had moved through a potential difference of The gas helium is given out by radio-active bodies, and about two million volts on its way out of the atom of since, except in beryls, it is not found in minerals which radium. The source of this energy is a problem of the do not contain radio-active elements, it is probable that deepest interest; if it arises from the repulsion of similarly all the helium in these minerals has come from these electrified systems exerting forces varying inversely as the elements. In the case of a mineral containing uranium, square of the distance, then to get the requisite amount the parent of radium in radio-active equilibriumn, with of energy the systems, if their charges were comparable radium and its products, helium will be produced at a with the charge on the a particle, could not when they definite rate. Helium, however, unlike the radio-active. start be further apart than the radius of a corpuscle, elements, is permanent, and accumulates in the mineral; 10-13 cm. If we suppose that the particles do not acquire hence if we measure the amount of helium in a sample this energy at the explosion, but that before they are shot of rock and the amount produced by the sample in one out of the radium atom they move in circles inside this year we can find the length of time the helium has been atom with the speed with which they emerge, the forces accumulating, and hence the age of the rock. This required to prevent particles moving with this velocity method, which is due to Prof. Strutt, may lead to deterfrom flying oil at a tangent are so great that finite charges minations, not merely of the average age of the crust of electricity could only produce them at distances com- of the earth, but of the ages of particular rocks and the parable with the radius of a corpuscle.

date at which the various strata were deposited; he has, One method by which the requisite amount of energy for example, shown in this way that a specimen of the could be obtained is suggested by the view to which I mineral thorianite must be more than 240 million years: have already alluded-that in the atom we have electrified old. systems of very different types, one small, the other large ; The physiological and medical properties of the rays. the radius of one type is comparable with 10-13 cm., that emitted by radium is a field of research in which enough of the other is about 100,000 times greater. The electro- has already been done to justify the hope that it may lead static potential energy in the smaller bodies is enormously to considerable alleviation of human suffering. It seems greater than that in the larger ones; if one of these small quite definitely established that for some diseases, notably bodies were to explode and expand to the size of the rodent ulcer, treatment with these rays has produced relarger ones, we should have a liberation of energy large markable cures; it is imperative, lest we should be passenough to endow an a particle with the energy it possesses. ing over a means of saving life and health, that the Is it possible that the positive units of electricity were, to subject should be investigated in a much more systematic begin with, quite as small as the negative, but while in and extensive manner than there has yet been either time the course of ages most of these have passed from the or material for. Radium is, however, so costly that few smaller stage to the larger, there are some small ones hospitals could afford to undertake pioneering work of still lingering in radio-active substances, and it is the this kind; fortunately, however, through the generosity explosion of these which liberates the energy set free of Sir Ernest Cassel and Lord Iveagh a Radium Institute, during radio-active transformation ?

under the patronage of his Majesty the King, has been The properties of radium have consequences of enormous founded in London for the study of the medical properties importance to the geologist as well as to the physicist or of radium, and for the treatment of patients suffering chemist. In fact, the discovery of these properties has from diseases for which radium is beneficial. entirely altered the aspect of one of the most interesting The new discoveries made in physics in the last few. geological problems, that of the age of the earth. Before years, and the ideas and potentialities suggested by them,. the discovery of radium it was supposed that the supplies have had an effect upon the workers in that subject akin of heat furnished by chemical changes going on in the that produced in literature by the Renaissance. earth were quite insignificant, and that there was nothing Enthusiasm has been quickened, and there is a hopeful, to replace the heat which flows from the hot interior of youthful, perhaps exuberant, spirit abroad which leads the earth to the colder crust. Now when the earth first to make with confidence experiments which would solidified it only possessed a certain amount of capital in have been thought fantastic twenty years ago. It has the form of heat, and if it is continually spending this quite dispelled the pessimistic feeling, not uncommon at capital and not gaining any fresh heat it is evident that that time, that all the interesting things had been disthe process cannot have been going on for more than a covered, and all that was left was to alter a decimal or certain number of years; otherwise the earth would be two in some physical constant. There never was any colder than it is. Lord Kelvin in this way estimated the justification for this feeling, there never were any signs age of the earth to be less than 100 million years. Though of an approach to finality in science. The sum of knowthe quantity of radium in the earth is an exceedingly ledge is at present, at any rate, a diverging, not a consmall fraction of the mass of the earth, only amounting, verging, series. As we conquer peak after peak we see according to the determinations of Profs. Strutt and Joly, in front of us regions full of interest and beauty, but we to about five grams in a cube the side of which is 100 do not see our goal, we do not see the horizon; in the miles, yet the amount of heat given out by this small distance tower still higher peaks, which will yield to quantity of radium is so great that it is more than enough

those who ascend them still wider prospects, and deepen to replace the heat which flows from the inside to the the feeling, the truth of which is emphasised by every outside of the earth. This, as Rutherford has pointed advance in science, that “ Great are the Works of the out, entirely vitiates the previous method of determining Lord.” the age of the earth. The fact is that the radium gives out so much heat that we do not quite know what to do

SECTION A. with it, for if there was as much radium throughout the

MATHEMATICS AND PHYSICS. interior of the earth as there is in its crust, the tempera

OPENING AppreSS BY PROF. E. RUTHERFORD, M.A., D.Sc., ture of the earth would increase much more rapidly than

Prof. it does as we descend below the earth's surface.

F.R.S., PresideNT OF THE SECTION. Strutt has shown that if radium behaves in the interior It is a great privilege and pleasure to address the of the earth as it does at the surface, rocks similar to members of this Section on the occasion of the visit of those in the earth's crust cannot extend to a depth of the British Association to a country with which I have more than forty-five miles below the surface.

had such a long and pleasant connection. I feel myself It is remarkable that Prof. Milne from the study of in the presence of old friends, for the greater part of what

to

men

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