thus causing a discharge of the electrified body. If a sufficiently strong field is used, the ions are all swept to the electrodes before appreciable loss of their number can occur by recombination. The rate of discharge then reaches a steady maximum value which is not altered by a large increase in voltage. This maximum current through the gas is called the saturation current. The ions produced in gases by the rays from uranium and other radioactive substances are in general identical with those produced by X rays, and the mechanism of conductivity of the gas is very similar in both cases (see Conduction, Electric: § Through Gases).

In addition to radium and polonium, a number of other radioactive substances have been found in uranium minerals. With the exception of the radium emanation, none of these have yet been isolated in a pure state, although preparations of some of them have been obtained comparable in activity with radium itself. Debierne (9) found a radioactive substance which was separated from pitchblende with the rare earths and had chemical properties similar to those of thorium. This he called actinium. Giesel (10) independently noted the presence of a new radioactive substance which was usually separated with lanthanum and cerium from the minerals. It possessed the property of giving out a radioactive emanation or gas, the activity of which died away in a few seconds. For this reason he called it the emanating substance and afterwards emanium. Later work has shown that emanium is identical in chemical and radioactive properties with actinium, so that the former name will be retained.

We have already seen that Mme Curie gave the name polonium to a radioactive substance separated with bismuth. Later Marckwald found that a very radioactive substance was deposited from a solution of a radioactive mineral on a polished bismuth plate. The active matter was found to be deposited in the bismuth with tellurium, and he gave the name radiotellurium to this substance. In later work, he showed that the new substance could be chemically separated from tellurium. By treating the residues from 15 tons of Joachimsthal pitchblende, Marckwald (11) finally obtained 3 milligrams of intensely active material—far more active weight for weight than radium. It has been definitely settled that the active substance of Marckwald is identical with polonium. Both substances give out a type of easily absorbed α rays and both lose their activity at the same rate. The activity of polonium decays in a geometrical progression with the time and falls to half its initial value in 140 days. This law of decay, as we shall see, is characteristic of all radioactive products, although the period of decay is different in each case.

Mme Curie and Debierne (12) have described further experiments with polonium. The latter substance was extracted from several tons of pitchblende and purified until 2 milligrams of material were obtained containing about 1/10 milligram of pure polonium. From a knowledge of the relative periods of transformation of radium and polonium, it can be calculated that the amount of polonium in a radium mineral is 1/5000 of the amount of radium, while the activity of pure polonium measured by the α rays should be 5000 times greater than that of radium. As we have seen, polonium is rapidly transformed, and it is of great interest to determine the nature of the substance into which polonium changes. We shall see later that there is considerable evidence that polonium changes into lead.

Radiations from Radioactive Substances.—All the radioactive substances possess in common the property of emitting radiations which darken a photographic plate and cause a discharge of electrified bodies. Very active preparations of radium, actinium and polonium also possess the property of causing strong phosphorescence in some substances. Bodies which phosphoresce under X rays usually do so under the rays from radioactive matter. Barium platinocyanide, the mineral willemite (zinc silicate) and zinc sulphide are the best known examples.

There are in general three types of radiation emitted by the radioactive bodies, called the α, β and γ rays. Rutherford (2) in 1899 showed that the radiation from uranium was complex and consisted of (a) an easily absorbed radiation stopped by a sheet of paper or a few centimetres of air which he called the α rays and (b) a far more penetrating radiation capable of passing through several millimetres of aluminium, called the β rays. Later Villard found that radium emitted a very penetrating kind of radiation called the γ rays capable of passing before absorption through twenty centimetres of iron and several centimetres of lead.

Giesel and, later, Curie and Becquerel showed that the β rays of radium were deflected by a magnetic field. By the work of Becquerel and Kaufmann the β rays have been shown to consist of negatively charged particles projected with a velocity approaching that of light, and having the same small mass as the electrons set free in a vacuum tube. In fact the β rays are electrons spontaneously ejected from the radioactive matter at a speed on an average much greater than that observed in the electrons set free in a vacuum tube.

The very penetrating γ rays are not deflected in a magnetic or electric field and are believed to be a type of radiation similar to X rays. The γ rays are only observed in radioactive substances which emit β rays, and the penetrating power of the γ rays appears to be connected with the initial velocity of expulsion of the β rays. Two general theories have been advanced to account for the properties of these rays. On one view, the γ rays are to be regarded as electromagnetic pulses which have their origin in the expulsion of the β particle from the atom. On the other hand Bragg has collected evidence in support of the view that the γ rays are corpuscular and consist of uncharged particles or “ neutral doublets.” There is as yet no general consensus of opinion as to the true nature of the γ rays.

Under ordinary experimental conditions the greater part of the ionization observed in a gas is due to the α particles. This ionization due to the α rays does not extend in air at atmospheric pressure for more than 7 cms. from radium, and 8.6 cms. from thorium. If a screen of aluminium about .01 cms. thick is placed over the active material, the α rays are completely absorbed, and the ionization above the screen is then due to the β and γ rays alone. If a layer of lead about 2 mms. thick is placed over the active material, the β rays are stopped, and the ionization is then due almost entirely to the penetrating γ rays. By the use of screens of suitable thickness we are thus able to sift out the various types of rays. These three types of radiations all set up secondary radiations in passing through matter. A pencil of β rays falling on matter is widely scattered in all directions. This scattered radiation is sometimes called the secondary β rays. The γ rays give rise to secondary rays which consist in part of scattered γ rays and in part electrons moving with a high velocity. These secondary rays in turn produce tertiary rays and so on. The impact of the α rays on matter sets free a number of slow moving electrons which are very easily deflected by a magnetic or electric field. This type of radiation was first observed by J. J. Thomson, and has been called by him the δ rays.

Emanations or Radioactive Gases.—In addition to their power of emitting penetrating radiations, the substances thorium, actinium and radium possess another very striking and important property. Rutherford (15) in 1900 showed that thorium compounds (especially the oxide) continuously emitted a radioactive emanation or gas. This emanation can be carried away by a current of air and its properties tested apart from the substance which produces it. A little later Dorn showed that radium possesses a similar property, while Giesel and Debierne observed a similar effect with actinium. These emanations all possess the property of ionizing a gas and, if sufficiently intense, of producing marked photographic and phosphorescent action. The activity of the radioactive gases is not permanent but disappears according to a definite law with the time, viz. the activity falls off in a geometric progression with the time. The emanations are distinguished by the different rates at which they lose their activity. The emanation of actinium is very short lived, the time for the activity to fall to half value, i.e. the period of the emanation, being 3.7 seconds. The period of the thorium emanation is 54 seconds and of the radium emanation 3.9 days. This property of emitting an emanation is shown in a very striking manner by actinium. A compound of actinium is wrapped in a sheet of thin paper and laid on a screen of phosphorescent zinc sulphide. In a dark room the phosphorescence, marked by the characteristic scintillation, is seen to extend on all sides from the active body. A puff of air is seen to remove the emanation and with it the greater part of the phosphorescence. Fresh emanation immediately diffuses out and the experiment may be repeated indefinitely. The emanations have all the properties of radioactive gases. They can be transferred from point to point by currents of air. The emanations can be separated from the air or other gas with which they are mixed by the action of extreme cold. Rutherford and Soddy (16) showed that under ordinary conditions the temperature of condensation of the radium emanation mixed was -150° C.

The emanations are produced from the parent matter and escape into the air under some conditions. Rutherford and Soddy (17) made a systematic examination of the emanating power of thorium compounds under different conditions. The hydroxide emanates most freely, while in thorium nitrate, practically none of the emanation escapes into the air. Most of the compounds of actinium emanate very freely. Radium compounds, except in very thin films, retain most of the emanation in the compound. The occluded emanation can in all cases be released by solution or by heating. On account of its very slow period of decay, the emanation of radium can be collected like a gas and stored, when it retains its characteristic properties for a month or more.

Induced Activity.—Curie (18) showed that radium possessed another remarkable property. The surface of any body placed near radium, or still better, immersed in the emanation from it, acquires a new property. The surface after removal is found to be strongly active. Like the emanations, this induced activity in a body decays with the time, though at quite a different rate from the emanation itself. Rutherford (19) independently showed that thorium possessed a like property. He showed that the bodies made active behaved as if a thin film of intensely active matter were deposited on their surface. The active matter could be partly removed by rubbing, and could be dissolved off by strong acids. When the acid was evaporated the active matter remained behind. It was shown that induced activity was due to the emanations, and could not be produced if no emanation was present. We shall see that induced activity on bodies is due to a deposit of non-gaseous matter derived from the transformation of the emanations. Each emanation gives a distinctive active deposit which decays at different rates. The active deposits of radium, thorium and actinium are very complex, and consist of several types of matter. Several hours after removal from the emanation the active deposit from radium decays to half-value—26 minutes, for actinium half-value—34 minutes, for thorium half-value— 10.5 hours; The active deposits obtained on a platinum wire or plate are volatilized before a white heat, and are again deposited on the cooler bodies in the neighbourhood. Rutherford showed that the induced activity could be concentrated on the negative electrode in a strong electric field, indicating that the radioactive carriers had a positive charge. The distribution of the active deposit in a gas at low pressure has been investigated in detail by Makower and Russ.

This process of production and disappearance of active matter holds for all the radioactive bodies. We shall now consider some special cases of the variation of the amount of active matter with time which have proved of great importance in the analysis of radioactive changes.

(a) Suppose that initially the matter A is present, and this changes into B and B into C, it is required to find the number of atoms P, Q and R of A, B and C present at any subsequent time t. .

Let λ1, λ2, λ3 be the constants of transformation of A, B and C respectively. Suppose n be the number of atoms of A initially present. From the law of radioactive change it follows:

P=ne1t

(1)

dQ/dt=λ1P-λ2Q

(2)

dR/dt=λ2Q-λ3R

Substituting the value of P in terms of n in (1), dQ/dt = λ1ne1t2Q; the solution of which is of the form

Q=n(ae1t+be2t),

where a and b are constants. By substitution it is seen that a=λ1/(λ21). Since Q=0 when t=0, b= -λ1/(λ21)

(3)Thus Q = 1λ12(e2t-e1t)

Similarly it can be shown that

(4)

R=n(ae1t+be2t+ce3t)

where a=λ1λ212)(λ13), b=λ1λ221)(λ23), c=λ1λ231)(λ32)

It will be seen from (3), that the value of Q, initially zero, increases to a maximum and then decays; finally, according to an exponential law, with the period of the more slowly transformed product, whether A or B.

(b) A primary source supplies the matter A at a constant rate, and the process has continued so long that the amounts of the products A, B, C have reached a steady limiting value. The primary source is then suddenly removed. It is required to find the amounts of A, B and C remaining at any subsequent time t.

In this case of equilibrium, the number n0 of particles of A supplied per second from the source is equal to the number of particles which change into B per second, and also of B into C. This requires the relation

n01P0=y2Q03R0

where P0, Q0, R0 are the initial number of particles of A, B, C present, and λ1, λ2, λ3 are their constants of transformation. Using the same quotations as in case (1), but remembering the new initial conditions, it can easily be shown that the number of particles P, Q and R of the matter A, B and C existing at the time I after removal are given by

P=n0λ1e1t,

Q=n0λ12${\displaystyle {\big (}}$λ1λ2e2t-e1t${\displaystyle {\big )}}$,

R=n0(ae1t+be2t+ce3t),

where a=λ212)(λ13), b=112)(λ23), c=λ1λ2λ313)(λ23)

The curves expressing the rate of variation of P, Q, R with time are in these cases very different from case (1). (c) The matter A is supplied at a constant rate from a primary source. Required to find the number of particles of A, B and C present at any time t later, when initially A, B, and C were absent.

This is a converse case from case (2) and the solutions can be obtained from general considerations. Initially suppose A, B and C are in equilibrium with the primary source which supplied A at a constant rate. The source is then removed and the amounts of A, B and C vary according to the equation given in case (2). The source after removal continues to supply A at the same rate as before. Since initially the product A was in equilibrium with the source, and the radioactive processes are in no way changed by the removal of the source, it is clear that the amount of A present in the two parts in which the matter is distributed is unchanged. If P1, be the amount of A produced by the source in the time t, and P the amount remaining in the part removed, then P1+P=P0 where P, is the equilibrium value. Thus

P1/P0=1-P/P0.

The ratio P/P0 can be written down from the solution given in case (2). Similarly the corresponding values of Q1/Q0, R1/R0 may be at once derived. It is obvious in these cases that the curve plotted with P/P0 as ordinates and time as abscissae is complementary to the corresponding curve with P1/P0 as ordinates. This simple relation holds for all recovery and decay curves of radioactive products in general.

We have so far considered the variation in the number of atoms of successive products with time when the periods of the products are known. In practice, the variation of the number of atoms is deduced from measurements of activity, usually made by the electric method. Using the same notation as before, the activity of any product is proportional to its rate of breaking up, i.e. to λ1P where P is the number of atoms present. If two products are present, the activity is the sum of two corresponding terms λ1P and λ2Q. In practice, however, no two products emit α or β particles with the same velocity. The difference in ionizing power of a single a particle from the two products has thus to be taken into account. If, under the experimental conditions, the ionization produced by an α particle from the second product is K times that from the first product, the activity observed is proportional to λ1P+Kλ2Q. In this way, it is possible to compare the theoretical activity curves of a mixture of products with those deduced experimentally.

Analysis of Radioactive Changes.—The analysis of the successive changes occurring in uranium, thorium, radium and actinium has proved a very difficult matter. In order to establish the existence of a new product and to fix its position in the scheme of changes, it is necessary to show (a) that the new product has a distinctive period of decay and shows some distinctive physical or chemical properties; (b) that the product under consideration arises directly from the product preceding it in the scheme of changes, and is transformed into the product succeeding it.

While in the majority of cases the products break up either with the emission of α or β particles, some products have been observed which do not emit any characteristic radiation and have been called “ rayless products.” For example, radium D and thorium A are changing substances which break up without emitting either penetrating α or β rays. They appear to emit slow δ rays which can only be detected by special methods. The presence and properties of a rayless product can be easily inferred if it is transformed into a product emitting a radiation, for the variation in activity of the latter affords a method of determining the amount of the parent product present. The distinction between a “ ray ” and a “ rayless ” product is not clear. It may be that the atom of a rayless product undergoes a re-arrangement of its constituent parts giving rise to an atom of the same mass but of different properties. In the case of an α ray or β ray product, the expulsion of an α or β particle affords an obvious explanation of the appearance of a new product with distinctive physical properties.

In the table a list of the known products of transformation is given. In each case, the half period of transformation is given and the type of radiation emitted. If they product emits α rays, the range of ionization of the α particle in air is given.

In each of the groups under the heading uranium, thorium and actinium, each product is derived from the direct transformation of the product above it.

Radium Emanation.—The radium emanation is to be regarded as a typical radioactive product or transition element which exists in a gaseous form. It is produced from radium at a constant rate, and is transformed into radium A and helium. Its half-period of transformation is 3.86 days. The emanation from radium has been purified by condensing it in liquid air, and pumping out the residual gases. The volume (26) of the emanation at normal pressure and temperature to be derived from one gram of radium in equilibrium is about 0.6 cubic millimetres. This small quantity of gas contains initially more than three-quarters of the total activity of the radium before its separation. In a pure state, the emanation is 100,000 times as active weight for weight as pure radium. Pure emanation in a spectrum tube gives a characteristic spectrum of bright lines (27). The discharge in the gas is bluish in colour. With continued sparking, the emanation is driven into the walls of the tube and the electrodes. Notwithstanding the minute volume of emanation available, the boiling-point of the emanation has been determined at various pressures. At atmospheric pressure Rutherford (28) found the boiling-point to be -67° C., and Gray and Ramsay (29) 71° C. Liquid emanation appears colourless when first condensed; when the temperature is lowered, the liquid emanation freezes, and at the temperature of liquid air glows with a bright rose colour. The density of liquid emanation has been estimated at 5 or 6.

Approximate estimates of the molecular weight of the radium emanation were early made by diffusion methods. The molecular weight in most cases came out about 100. In a comparison by Perkins of the rate of diffusion of the emanation with that of a monatomic vapour of high molecular weight, viz. mercury, the value deduced was 234. Since the radium atom in breaking up gives rise to one atom of the emanation and one atom of helium, its atomic weight should be 226-4=222. The emanation appears to have no definite chemical properties, and in this respect belongs to the group of inert monatomic gases of which helium and argon are the best known examples. It is partially soluble in water, and readily absorbed by charcoal.

Actinium.—The transformations observed in actinium are very analogous to those in thorium. Actinium itself is a rayless product which changes into radioactinium, an α ray product of period 19.5 days, first separated by Hahn (32). This changes into actinium X, of period 10.2 days, first separated by Godlewski (33). Actinium X is transformed into the emanation which in turn gives rise to three further products, called actinium A, B and C. Although very active preparations of actinium have been prepared, it has so far not been found possible to separate the actinium from the rare earths with which it is mixed. We do not in consequence know its atomic weight or spectrum.

λ2Q=λ1P or Q/P=λ12=T2/T1

Since the direct parent of radium must be present in radioactive minerals, one of the constituents separated from the mineral must grow radium. This was shown to be the case by Boltwood (38), who found that actinium preparations produced radium at a fairly rapid rate. By the work of Rutherford and Boltwood, it was found that the growth of radium was not due to actinium itself, but to a new substance separated in some cases with the actinium. This new substance, which emits a rays, was separated by Boltwood (38), and called by him “ Ionium.” It has chemical properties-very similar to thorium. Soddy has shown that the period of ionium is probably not less than 20,000 years, indicating that ionium must exist in uranium minerals in not less than ten times the quantity of radium. It has not yet been directly shown that uranium produces ionium, but there can be no doubt that it does do so. Since ionium produces' radium, Boltwood (38) has determined, by direct experiment that radium is half transformed in 2000 years-a number in good agreement with other data on that subject. The constant relation between uranium and radium will only hold for old minerals where there has been no opportunity for chemical alteration or removal of its constituents by the action of percolating water or other agencies. It .is quite possible that altered minerals of no great age will not show this constant relation. It seems probable that this is the explanation of some results of Mlle Gleditsch, where the relation between uranium and radium has been found not to be constant for some mineral specimens.

Total activity mineral, 4.64 times uranium.

Taking into account the differences in the ionization due to an α particle from the various products, the results indicate that uranium expels two α particles for one from each of the other α ray products in the series of transformations. This indicates either that two particles are expelled during the transformation of the atom of uranium, or that another α ray product is present which has so far not been separated from the uranium.

Although thorium is nearly always present in old uranium minerals and uranium in thorium minerals, there does not appear to be any radioactive connexion between these two elements. Uranium and thorium are to be regarded as two distinct radioactive elements. With regard to actinium, there is still no definite information of its place in the scheme of transformations. Boltwood has shown that the amount of actinium in uranium minerals is proportional to the content of uranium. This indicates that actinium, like radium, is in genetic connexion with uranium. On the other hand, the activity of actinium with its series of α ray products is less than that of radium itself or uranium. In order to explain this anomaly, Rutherford has suggested that at a certain stage of disintegration of the uranium-radium series, the disintegration is complex, and two distinct kinds of matter appear, one in much larger quantity than the other. On this view, the smaller fraction is actinium, so that the latter is a branch. descendant of the main uranium-radium series.

End Products of Transformation.—It is now definitely established that the α particle expelled from any type of radioactive matter is an atom of helium, so that helium is a necessary accompaniment of radioactive changes involving the expulsion of a particles. After the radioactive transformations have come to an end, each of the elements uranium and thorium and actinium should give rise to an end or final product, which may be either a known element or some unknown element of very slow period of transformation. Supposing, as seems probable, that the expulsion of an α particle lowers the atomic weight of an element by four units-the atomic weight of helium-the atomic weights of each of the products in the uranium and radium series can be simply calculated. Since uranium expels two α particles, the atomic weight of the next ray product, ionium, is 238.5-8 or 230.5. The atomic weight of radium comes out to be 266.5, a number in good agreement with the experimental value. Similarly the atomic weight of polonium is 210.5, and that of the final product after the transformation of polonium should be 206.5. This value is very close to the atomic weight of lead, and indicates that this substance is the final product of the transformation of radium.

This suggestion was first put forward by Boltwood (40), who has collected a large amount of evidence bearing on this subject. Since in old minerals the transformations have been in progress for periods of time, in some cases measured by hundreds of millions of years, it is obvious that the end product, if a stable element, should be an invariable companion of the radio element and be present in considerable quantity. Boltwood has shown that lead always occurs in radioactive minerals, and in many cases in amount about that to be expected from their uranium. content and age. It is difficult to settle definitely this very important problem until it can be experimentally shown that radium is transformed into lead, or, what should prove simpler in practice, that polonium changes into helium and lead. Unfortunately for a solution of this problem within a reasonable time, a very large quantity of polonium would be necessary. Mme. Curie and Debierne have obtained a very active preparation of polonium containing about 110th milligram of pure polonium. Rutherford and Boltwood and Curie and Debierne have both independently shown that polonium produces helium -a result to be expected, since it emits α particles.

Production of Helium.—In 1902 Rutherford and Soddy suggested that the helium which is invariably found in radioactive minerals was derived from the disintegration of radioactive matter. In 1903 Ramsay and Soddy definitely showed that helium was produced by radium and also by its emanation. From the observed mass of the α particle, it seemed probable from the first that the α particle was an atom of helium. This conclusion was confirmed by the work of Rutherford and Geiger (41), who showed that the α particle was an atom of helium carrying two unit charges of electricity. In order to prove definitely this relation, it was necessary to show that the a particles, quite independently of the active matter from which they were expelled, gave rise to helium. This was done by Rutherford and Royds (42), who allowed the α particles from a large quantity of emanation to be fired through the very thin glass walls of the containing tube. The collected particle gave the spectrum of helium, showing, without doubt, that the α particle must be a helium atom.

Since the α particle is an atom of helium, all radioactive matter which expels α particles must give rise to helium. In agreement with this, Debierne and Giesel have shown that actinium as well as radium produces helium. Observations of the production of helium by radium have been made by Ramsay and Soddy, Curie and Dewar, Himstedt and others. The rate of production of helium per gram of radium was first definitely measured by Dewar (43). His preliminary measurements gave a value of 134 cubic mms. of helium per year per gram of radium and its products. Later observations extending over a larger interval give a rate of production about 168 cubic mms. per year. As a result of preliminary measurements, Boltwood and Rutherford (44) have found a growth of 163 cubic mms. per year. It is of interest to note that the rate of production of helium by radium is in excellent agreement with the value calculated theoretically. From their work of counting the particles and measuring their charge, Rutherford and Geiger showed that the rate of production of helium should be 158 cubic mms. per year.

Properties of the α Rays.—We have seen that the rays are positively charged atoms of helium projected at a high velocity, which are capable of penetrating through thin metal sheets and several centimetres of air. Early observations indicated that the ionization due to a layer of radioactive matter decreased approximately according to an exponential law with the thickness of the absorbing matter placed over the active matter. The true nature of the absorption of the α rays was first shown by Bragg and by Bragg and Kleeman (45). The active particles projected from a thin film of active matter of one kind have identical velocities, and are able to ionize the air for a definite distance, termed the “ range ” of the α particle. It was found that the ionization per centimetre of path due to a narrow pencil of α rays increases with the distance from the active matter, at first slowly, then more rapidly, near the end of the range. After passing through a maximum value the ionization falls off rapidly to zero. The range of an α particle in air has a definite value which can be accurately measured. If a uniform screen of matter is placed in the path of the pencil of rays the range is reduced by a definite amount proportional to the thickness of the screen. All the α particles have their velocity reduced by the same amount in their passage through the screen. The ranges in air of the α rays from the various products of the radioelements have been measured. The ranges for the different products vary between 2.8 cms. and 8.6 cms.

Bragg has shown that the range of an α particle in different elements is nearly proportional to the square roots of their atomic weights. Using the photographic method, Rutherford (46) showed that the velocity V of an α particle of range R cms. in air is given by V2=K(R+1.25), where K is a constant. In his experiments he was unable to detect particles which had a velocity lower than 8.8 X 108 cms. per second. Geiger (47), using the scintillation method, has recently found that a particles of still lower velocity can be detected under suitable conditions by the scintillations produced on a zinc sulphide screen. He has found that the connexion between velocity and range can be closely expressed by V3=KR, where K is a constant.

On account of the great energy of motion of the α particle, it was at first thought that it pursued a rectilinear path in the gas without appreciable defection due to its encounters with the molecules. Geiger (48) has, however, shown by the scintillation method that the α particles are scattered to a marked extent in passing through matter. The scattering increases with the atomic weight of the substance traversed, and becomes more marked with decreasing velocity of the α particle. A small fraction of the α particles falling on a thick screen are deflected through more than a right angle, and emerge again on the side of incidence.

XXII. 26

Rutherford and Geiger (49) have devised an electrical method of counting the α particles expelled from radioactive matter. The α particle enters through a small opening into a metal tube containing a gas at a reduced pressure. The ionization produced by the a particle in its passage through the gas is magnified several thousand times by the movement of the ions in a strong electric field. In this way, the entrance of an α particle into the detecting vessel is shown by a sudden and large deflection of the measuring instrument. By this method, they determined that 3.4 X 1010 α particles are ejected per second from one gram of radium itself and from each of its α ray products in equilibrium with it. By measuring the charge on a counted number of a particles, it was found that the α particle carries a positive charge of 9.3 X 10-10 electrostatic units. From other evidence, it is known that this must be twice the fundamental unit of charge carried by the hydrogen atom. It follows that this unit charge is 4.65 X 10-10 units. This value is in good agreement with numerous recent determinations of this fundamental quantity by other methods. With this data, it is possible to calculate directly the values of some important radioactive data. The calculated and observed values are given below:—

 Calculated. Observed. Volume of the emanation in cubic millimetres per gram of radium .585 .6 Volume of helium in cubic millimetres produced per year per gram of radium 158 169 Heating effect of radium per gram per hour in gram calories 113 118 Half-period of transformation of radium in year 1760 2000

The calculated values are in all cases in good agreement with the experimental numbers.

It is well known from the experiments of Sir William Crookes (50) that the α rays produce visible scintillations when they fall on a screen of phosphorescent zinc sulphide. This is shown in the instrument called the spinthariscope. By means of a suitable microscope, the number of these scintillations on a given area in a given time can be counted. The number so obtained is practically identical with the number of α particles incident on the screen, determined by the electrical method of counting. This shows that each α particle produces a visible flash of light when it falls on a suitable zinc sulphide screen. The scintillations produced by α rays are observed in certain diamonds, and their number has been counted by Regener (51) and the charge on each particle has been deduced. The latter was the first to employ the scintillation method for actual counting of α particles. Kinoshita has shown that the number of α particles can also be counted by the photographic method, and that each particle must produce a detectable effect.

Absorption of β Rays.—We have seen that the β particles, which are emitted from a number of radioactive products, carry a negative charge and have the same small mass as the particles constituting the cathode rays. The velocity of expulsion and penetrating power of the β rays varies widely for different products. For example, the rays from radium B are very easily absorbed, while some of the rays from radium C are of a very penetrating type. It has been found that for a single β ray product, the particles are absorbed according to an exponential law with the thickness of matter traversed, and Hahn has made use of this fact to isolate a number of new products. It has been generally assumed that the exponential law of absorption is a criterion that the β rays are all expelled at the same speed. In addition, it has been supposed that the β particles do not decrease much in velocity in passing through matter. Wilson has recently made experiments upon homogeneous β rays, and finds that the intensity of the radiation falls off in some cases according to a linear rather than to an exponential law, and that there is undoubted evidence that the β particles decrease in velocity in traversing matter. Experiments upon the absorption of β rays are greatly complicated by the scattering of the 5 rays in their encounters with the molecules. For example, if a pencil of β rays falls on a metal, a large fraction of the rays are scattered sufficiently to emerge on the side of incidence. This scattering of the β rays has been investigated by Eve, McLennan, Schmidt, Crowther and others. It has been found that the scattering for different chemical elements is connected with their atomic weight and their position in the periodic table. McCelland and Schmidt have given theories to accourf for the absorption of β rays by matter. The whole problem of absorption and scattering of particles by substances is very complicated, and the question is still under active examination and discussion. The negative charge carried by the β rays has been measured by a number of observers. It has been shown by Rutherford and Makower that the number of β particles expelled per second from one gram of radium in equilibrium is about that to be expected if each atom of the β ray products in breaking up emits one β particle.