Recent research on radioactivity E-Book

Pierre Curie

• Herausgeber: Edizioni Savine
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Veröffentlichungsjahr: 2023

Beschreibung

Since the discovery of strongly radioactive substances, research on radioactivity has been greatly developed. I propose in this article to give an account of the actual state of our knowledge relative to this subject, laying particular stress on the most recent work.
(Pierre Curie - 1904)

Leseprobe

RECENT RESEARCH ON RADIOACTIVITY.

By P. CURIE.

[Reprinted from the American Chemical Journal, Vol. XXXI, No. 4. April, 1904.]

RECENT RESEARCH ON RADIOACTIVITY.[1]

By P. Curie.

Since the discovery of strongly radioactive substances, research on radioactivity has been greatly developed. I propose in this article to give an account of the actual state of our knowledge relative to this subject, laying particular stress on the most recent work.[2]

[1]Translated from an article that appeared in Jour. d. Chim. Phys., I, 409 (1903) edited by Philippe A. Guye, Professor of Chemistry in the University of Geneva. See THIS JOURNAL, 31, 298 (1904).

[2]For more complete details of work done previous to May, 1903, see the thesis of Mme. Curie. It appeared in Ann. de Chim. et de Phys. in 1903 and 1904.

I. Radioactive Substances.

Becquerel Rays. Uranium and Thorium.—We call radioactive such substances as are capable of emitting spontaneously and continuously certain rays known as Becquerel rays. These rays act upon the photographic plate; they render the gases through which they pass conductors of electricity; they can pass through black paper and metals. The Becquerel rays cannot be reflected, refracted or polarized.

In 1896 Becquerel discovered that uranium and its compounds emit these new rays continuously. Schmidt and Mme. Curie then found almost simultaneously that thorium compounds are also radioactive. The radiations emitted by thorium compounds are comparable in intensity with those from the compounds of uranium. Radioactivity is an atomic property that accompanies the atoms of uranium and thorium wherever they are found; in a compound or a mixture its intensity is proportional to the amount of the metal present.

New Radioactive Substances.—Mme. Curie, in 1898, tried to learn whether there were among the elements then known any others possessing radioactivity; she could not find a single substance giving any considerable radiation, and concluded that the radioactive properties of the elements are at least 100 times more feeble than those of uranium and thorium. She found, on the other hand, that certain minerals containing uranium (pitchblende, chalcolite, and carnotite) are more active than metallic uranium; the activity of these minerals could not, then, be due solely to uranium or to other known elements. This discovery was fertile with new results. Mme. Curie and I showed, in an investigation carried on together, that pitchblende contains new radioactive substances, and we supposed that these substances contained new chemical elements.

There are known with certainty three new strongly radioactive substances: polonium, which was found in the bismuth obtained from the uranium minerals; radium, found with barium from the same source, and actinium, which was discovered by Debierne among the rare earths extracted from the same minerals. These three substances are present only in infinitesimal quantities in the uranium minerals, and all three possess a radioactivity about a million times greater than that of uranium or thorium.

Recently Giesel and Hofmann announced the presence of a fourth strongly radioactive substance in the uranium minerals, which had properties closely resembling those of lead; from the publications that have appeared up to this time I have not been able to form an opinion as to the nature of this substance.

It may be asked whether radioactivity is a general property of matter. This question cannot be regarded as actually settled. The investigations of Mme. Curie have proved that the different known substances do not possess an atomic radioactivity one-hundredth as great as that of uranium or thorium. On the other hand, certain chemical reactions may cause the formation of ions, conductors of electricity, without the active substance giving any evidence of atomic radioactivity. Thus white phosphorus by its oxidation renders the surrounding air a conductor of electricity, while red phosphorus and the phosphates are not at all radioactive.

Some old experiments by Russell, Colson and Tengyel showed that certain substances act upon the photographic plate at a distance. It is possible that this phenomenon is partly due to radioactivity, but it is not certain. Recent work by MacTennan and Burton, Strutt, and Lester Cooke, leads to the supposition that radioactivity belongs to all bodies to an extremely slight extent. The identity of these very feeble phenomena with atomic radioactivity can not yet be regarded as proved.

Radium.—Of all the strongly radioactive substances radium is the only one that has been proved to be a new element. Radium possesses a characteristic spectrum, the discovery and first study of which we owe to Demarçay, and which has since been studied by Runge and Precht and by Crookes. Radium is an element belonging after barium in the series of alkaline earths. Its atomic weight as determined by Mme. Curie is 225.

At present radium is obtained from a residue left on extracting uranium from pitchblende. This residue contains 2 or 3 decigrams of radium in a ton. At first 10 or 15 kilograms of radiferous barium salt are extracted from a ton of the residue, and from this the radium is prepared by fractional crystallization (with the chloride or bromide), the crystals that form from the solution being richer in radium than the salt that remains in the liquid.

The radiant activity of the radium salt can be measured at different times from the beginning of crystallization until it is finally dried in the air-bath. It is found that the activity has a certain initial value, then it increases as a function of the time, at first rapidly, then more and more slowly; it approaches asymptotically a limiting value which is about five times as great as the initial activity. The activity then remains invariable for some years if the salt be left as it is.

Polonium.—Polonium is, on the contrary, a substance that slowly loses its radioactivity from the moment when it is separated from the uranium mineral containing it. After some years the radioactivity of polonium almost completely disappears. Hence it acts like an unstable substance. It has not yet been shown that polonium is a new element distinct from ordinary bismuth.

Polonium can be concentrated by fractionally precipitating the subnitrate of polonium and bismuth by means of water. The part precipitated is the most active. One can also make a partial precipitation by means of hydrogen sulphide from a strongly acid solution in hydrochloric acid. These methods of fractionation are difficult because the precipitates are not easily redissolved. Marckwald concentrated the activity by plunging a rod of bismuth in a solution of bismuth and polonium. A layer of extremely active metal was deposited upon the rod.

Actinium.—The concentration of actinium is even more difficult than that of polonium. Solid salts containing actinium preserve their radioactivity unchanged for several years.

II. The Radiations from Radioactive Substances.

Complexity of the Radiation.—Radium is the substance whose radioactivity has been studied most completely. It is now known that it emits a number of rays of different nature, which can be placed in three groups. According to the notation adopted by Rutherford, I designate the three groups of rays by the letters α, β, and γ.

The action of the magnetic field serves to distinguish them. In an intense magnetic field the α-rays are slightly deviated from a straight path, and the deviation is of the same kind as that of the "canal" rays of Goldstein in a vacuum-tube. On the other hand, the β-rays are deviated like the cathode rays, and the γ-rays are not turned aside, but act like the Roentgen rays.

β-Rays.—The β-rays of radium, analogous to the cathode rays, form a heterogeneous group. They are distinguished from one another by their power of penetration and by the deviation caused by a magnetic field.

Certain β-rays are absorbed by aluminium foil a few hundreths of a millimeter in thickness, while others pass through several millimeters of lead.

Suppose we have a rectilinear pencil of Becquerel rays obtained by means of a particle of radium salt and a screen with a hole in it. If a uniform magnetic field is produced normal to the direction of the pencil the β-rays are incurved, and describe circular paths in a plane normal to the direction of the magnetic field. The rays of the described circumferences vary within wide limits. Becquerel has shown that the most penetrating rays are least deviated, and hence describe circumferences with the greatest radius of curvature. If the pencil of β-rays bent aside by the magnetic field is allowed to fall on the photographic plate, there is obtained an impression that is a true spectrum in which the different β-rays act separately.

Suppose the β-rays to be projectiles (electrons) negatively charged with electricity and shot from the radium with great velocity. Let be the mass of a projectile, its charge, its initial velocity, the radius of curvature of its path, the intensity of the magnetic field normal to the initial velocity, the magnetic permeability of the medium. We then have the easily established relation

The β-rays are also deviated in an electric field. Suppose we have a rectilinear pencil of these rays. If a uniform electric field be produced normal to the initial direction of the pencil, the rays are deviated in the opposite direction to the field, and describe parabolic paths. The experiment can be performed by passing the pencil of rays between two parallel metallic plates, between which there is established a difference of potential. The deviation is slight with the means one has at command, and it is convenient to work in a vacuum. The air is made a conductor of electricity, so that if we work in the air the insulation is imperfect, and it is difficult to maintain a constant and high difference of potential between the plates. The most penetrating β-rays are bent aside least.

The action of the electric field is in accord with the ballistic hypothesis previously stated. Accepting this hypothesis, suppose a uniform electric field with an intensity and a width , acts upon the charged projectile whose initial velocity is normal to the field. The deviation, , of the extremity of the trajectory at the departure from the field is given by formula (2), when the deviation is slight.

From the equations (1) and (2) can be deduced the velocity, , of the projectiles, and the ratio, /, of the electric charge to the corresponding mass.

The experiments of Becquerel have shown that for the β-rays of greatest intensity the ratio, /, approaches 107 electromagnetic units, and a value of 1.6 x 1010 cm/sec. These values are of the same order of magnitude as for the cathode rays.

Kaufmann has made exact experiments on the same subject. This physicist subjected a very narrow pencil of radium rays to the simultaneous action of a magnetic and an electric field, both fields being uniform and acting normal to the primary direction of the pencil. The pencil is received upon a photographic plate placed perpendicular to its initial direction. In the absence of the two fields the impression on the plate is a small circular disk, almost like a point. When the magnetic field acts alone the different β-rays are unequally deflected but remain in a plane normal to the field, producing upon the plate an impression in the form of a straight line. When the electric field acts alone the different β-rays are unequally deflected in a plane passing through the field and producing upon the plate an impression which is a straight line perpendicular to the one preceding. When both fields act simultaneously the impression on the plate is a curve. Each point of the curve corresponds to a different kind of β-ray. Taking as axes of coordinates on the plate the straight lines obtained when each of the two fields acts alone, the coordinates of each point of the curve represent the relative magnetic and electric deviations for each kind of ray.

The following are the figures for and / obtained by Kaufmann, referred to the most penetrating rays of radium. I give for comparison the values obtained by Simon for the cathode rays.

e/m

in electromagnetic units.

v cm/sec

1.865 x 10

7

0.7 x 10

10

(For the cathode rays. Simon.)

1.31 x 10

7

2.36 x 10

10

}

1.17 x 10

7

2.48 x 10

10

}

0.97 x 10

7

2.59 x 10

10

} (For the radium rays. Kaufmann.)

0.77 x 10

7

2.72 x 10

10

}

0.63 x 10

7

2.83 x 10

10

}

It is seen that certain β-rays have a velocity approaching that of light. One can understand why it is that particles having such velocity should be able to penetrate matter to such an extent, although they are so small.

The ratio /, seems to be the same for the least penetrating radium rays and the cathode rays. But this ratio diminishes as the velocity of the rays increases. J. J. Thomson and Townsend think that all charged electrons, when in motion, possess the same charge, which is equal to that carried by an atom of hydrogen when a solution is electrolyzed. If that be true, it must be admitted that the mass of the particles increases with their velocity, when the latter approaches that of light.

In the case of electrolysis the ratio, /, is equal to 9650, while this same ratio is equal to 1.865 x 107 for the cathode rays and the less penetrating β-rays. If it be admitted that the charge, , is the same in both cases, it can be deduced that the mass of an electron is about two thousand times less than that of an atom of hydrogen.

Theoretical considerations lead to the conception that the inertia of a particle is solely due to its being a moving charge, the velocity of a moving electric charge not being altered without some change of energy. In other words, the mass of the charged particle is, in part at least, an apparent mass or an electromagnetic mass. Abraham gave a formula for calculating the electromagnetic mass of a charged particle as a function of its velocity. According to this formula the mass due to electromagnetic reactions is a constant for low velocities; it increases with the velocity, and approaches infinity for velocities that approach that of light. The experiments of Kaufmann are in accord with this theory, and also lead to the conclusion that the mass of an electron is entirely of an electromagnetic nature. These results are of great theoretical importance. One can foresee the possibility of establishing mechanics upon the dynamics of small material centers charged and in motion.

α-Rays.—The α-rays of radium have very little power of penetration. A piece of aluminium foil a few hundredths of a millimeter in thickness absorbs them almost completely. They are also absorbed by the air, and cannot penetrate air at the atmospheric pressure to a greater distance than 10 cm. The α-rays form the most important part of the radiation of radium, provided we measure the radiation by the amount of ionization which it produces in the air.

The α-rays are very slightly deflected by the most intense magnetic and electric fields, and they were at first thought to be non-deviable under this influence. Nevertheless, independently of the action of the magnetic field, the laws of the absorption of the α-rays by superimposed screens is sufficient to distinguish them clearly from the Roentgen rays. In passing through successive screens the rays become less and less penetrating, while the penetrating power of the Roentgen rays increases. The ray is like a projectile whose energy diminishes on passing through each screen. A given screen also absorbs the α-rays to a much greater extent when it is placed at a distance than when it is placed quite near the radium.

Strutt suggested that the rays might be analogous to the canal rays of vacuum-tubes. Rutherford succeeded in showing the action of a magnetic field upon the rays and in making a preliminary measurement of the deviation. Becquerel confirmed the results obtained by Rutherford and gave a new measurement of the phenomenon. Des Coudres measured the electrical and magnetic deviation of the rays in a vacuum.

From these investigations it is seen that the α-rays act like projectiles having a great velocity and charged with electricity. The deviation in a magnetic field and in an electric field is the opposite of the deviation of the cathode rays.

The α-rays form an apparently homogeneous group, all being deflected to the same extent and, hence, not giving a spread-out spectrum as the rays do. The formulas (1) and (2) are applicable. According to the measurements of Des Coudres in a vacuum,

Hence the velocity of the particles is twenty times less than that of light. If we assume that the charge of a particle is the same as that of an atom of hydrogen, in electrolysis, it is found that its mass is of the same order of magnitude as the hydrogen atom (the ratio, /, is equal to 9650 for hydrogen in electrolysis). It may well be supposed that these particles, which are greater than the electrons and have a lower velocity, would be less capable of penetration.

According to the experiments of Becquerel, the curvature of the path of the α-rays moving in a uniform magnetic field is not constant when in air at the atmospheric pressure. At first the curvature is the same as that obtained in a vacuum, but it becomes less and less as the ray recedes from its source. This phenomenon can be explained by supposing that new particles attach themselves to the projectiles that make up the rays while the latter move through the air. This hypothesis explains why the absorbing power of a screen for the α-rays increases as it is removed farther from the radiant source.

The α-rays are the ones that are active in the very beautiful experiments with the spinthariscope of Crookes. In this apparatus a very small fragment of a salt of radium (a fraction of a milligram) is held by a metallic thread a short distance (0.5 mm.) from a phosphorescent screen of zinc sulphide. On examining, in the dark, the surface of the screen which is toward the radium by means of a magnifying glass, luminous points are seen sprinkled over the screen, reminding one of a starry sky. These luminous points appear and disappear continuously. From the ballistic theory it might be imagined that each luminous point that appeared and disappeared resulted from the shock of a projectile. This is the first instance of a phenomenon which enables one to distinguish the individual action of an atom.

γ-Rays.—The rays are exactly like the Roentgen rays. They seem to comprise but a small part of the total radiation. The γ-rays have a very extraordinary power of penetration, and they diffuse very little in passing through most substances.

Diffusion of the Radium Rays.—Suppose we have a pencil of Becquerel rays issuing from radium and limited by a screen of lead. If the pencil meets a thin screen the α-rays are absorbed, the β-rays are diffused in all directions, and the γ-rays go part way through the screen as a well-defined pencil with sharp edges. The γ-rays can also pass through a prism of thick glass as a well-defined straight pencil. The question has been raised whether the β-rays are always completely diffused when they penetrate a solid screen. The experiments of Becquerel show that a pencil of β-rays can propagate itself in a well defined way in paraffin. Becquerel made use of the action of the β-rays on the photographic plate to study the path of the rays when dispersed by a magnetic field. It can be seen from the prints that the most penetrating rays pass through 7 or 8 mm. of paraffin without marked diffusion, while the least penetrating rays are completely diffused after passing through 2 mm. The magnetic field deflects the β-rays in paraffin as in air.

Conductivity of Dielectric Liquids under the Influence of the Radium Rays.—Dielectric liquids become poor conductors under the influence of the radium rays. This can be shown with petroleum, ether, vaseline oil, benzene, amylene, carbon disulphide, and liquid air.

Radiation of Other Radioactive Substances.—Polonium emits only very slightly penetrating rays, which seem to be identical with the α-rays of radium. They possess almost the same power of penetration and are deflected in the same way by the magnetic field. Finally, the experiment of the spinthariscope can be performed with the α-rays of polonium. Hence, polonium is a source of the α-rays unmixed with other kinds of rays, which is of value in certain studies. But the source exhausts itself, and some years after it has been separated from the minerals containing it the polonium loses its activity.

Thorium, uranium, and actinium seem to emit the α- and β-rays; the deviability of the β-rays has been shown.

The Electric Charge of the Radium Rays.—According to the ballistic theory, the α-rays should carry positive electric charges and the β-rays negative electric charges. Mme. Curie and I have shown that, in conformity to the theory, the β-rays of radium charge positively the bodies that absorb them. To show this, a plate of lead is connected with an electrometer. The plate of lead is entirely covered with a layer of paraffin, which is in turn enveloped in fine aluminium foil connected with the earth. The radium, placed in a small dish, sends its rays upon the plate of lead, which is thus protected. The α-rays are stopped by the exterior layer of aluminium. Part of the β-rays pass through the aluminium and the paraffin and are absorbed by the lead, which becomes charged negatively. The paraffin is necessary to sufficiently insulate the plate of lead, which could be charged if it were surrounded by air, for it is made a conductor by Becquerel rays.

We have also shown that a salt of radium is charged positively when it is enveloped in an insulating layer, and that it emits β-rays from the exterior, while the α-rays cannot escape.

A sealed glass bulb containing a salt of radium becomes spontaneously charged like a Leyden jar. If after a sufficient time a mark is made on the wall of the flask with a glass-cutter, a spark is discharged, which pierces the glass at the point where it was made thin by the cutter. At the same time the experimenter feels a slight shock in his fingers from the passage of the discharge.

Phosphorescence of Substances under the Action of the Becquerel Rays. Light Emitted by Salts of Radium. Coloration of Substances by the Action of the Rays.—The radiation from radium causes phosphorescence in a great many substances: the salts of the alkalies and alkaline earths, uranyl-potassium sulphate, organic substances, cotton, paper, cinchonine sulphate, skin, glass, quartz, etc. The most sensitive substances are barium platinocyanide, willemite (silicate of zinc), sulphide of zinc, and diamond. With the penetrating β-rays willemite and the platinocyanide are the most sensitive, while with the α-rays it is better to use phosphorescent sulphide of zinc.

Phosphorescent substances are altered by the prolonged action of the radium rays. They become less excitable and are less luminous under the influence of the rays. At the same time they change their color or become colored. Glass becomes violet, black or brown. Salts of the alkalies turn yellow, green or blue. Transparent quartz becomes smoky quartz. Colorless topaz turns yellow, orange, etc. Glass colored by radium is thermoluminescent. On heating it to 500° it emits light. At the same time it becomes colorless and returns to its original condition. It is then capable of being colored anew by the action of the rays of radium.

The salts of radium are spontaneously luminous. It might be said that they make themselves luminous by the action of the Becquerel rays they emit. Anhydrous chloride and bromide of radium are the salts that give the most intense luminescence. They may be obtained so luminous that the light can be seen in full daylight. The light emitted by the salts of radium recalls in tint that from a fire-fly. The luminosity of the radium salt decreases with time without ever completely disappearing, and those that were colorless at first become gray, yellow or violet.

Physiological Effects of the Radium Rays.—The rays from radium cause different physiological actions.

A salt of radium placed in an opaque box made of cardboard or metal acts on the eye and produces the sensation of light. To obtain this result, the box containing the radium is placed before the closed eye or against the temple. In this experiment the center of the eye becomes luminous by phosphorescence under the influence of the radium rays, and the light one sees has its source in the eye itself.

The rays of radium act on the epidermis. If we place on the skin for a few minutes a bulb containing radium no particular sensation is felt. But fifteen or twenty days afterward it produces a reddening of the skin, then a slough in the place where the bulb was applied. If the action of the rays be long enough there is finally formed a sore that takes several months to heal. The action of the rays from radium is analogous to that produced by the Roentgen rays. The attempt has been made to utilize this action in the treatment of lupus and cancer.

The radium rays also act upon the nervous centers and cause paralysis and death. (Danysz.) They seem also to act with especial intensity upon growing tissues. (Bohn.)

The Employment of Radium in the Study of Atmospheric Electricity.—The radium rays have also been utilized in the study of atmospheric electricity (Paulsen, Witkowski, Moureaux). A small quantity of a salt of radium placed at the extremity of a metallic rod serves as a point of contact for the potential. By this very simple arrangement we can avoid the use of flames or of water-dropping apparatus to measure the potential at any point in the atmosphere.

III. The Heat Given Off by the Salts of Radium.

The salts of radium continually give off heat. Its amount is sufficient for it to be detected by means of a crude experiment with two ordinary mercury thermometers. Two identical heat-insulating vacuum-bulbs are used. In one of these is placed a glass bottle containing 0.7 gram of pure radium bromide; in the second is placed a glass bottle containing some inactive substance, such as barium chloride. The temperature of the contents of each is shown by a thermometer placed with its bulb near the bottle. The mouths of the flasks are closed with cotton. Under these conditions the thermometer in the same flask as the radium always has a temperature 3° higher than that indicated by the other thermometer.

The quantity of heat given off can be estimated by means of a Bunsen ice calorimeter. By placing in the calorimeter a glass bottle containing a salt of radium, it is found that there is a continued supply of heat that stops as soon as the radium is removed. A determination made with a salt of radium that had been prepared a long time before shows that each gram of radium gives off 80 small calories an hour. Hence radium gives off enough heat to melt its own weight of ice every hour. Nevertheless the radium salt seems to remain in the same condition, and besides, not a single ordinary chemical reaction can be called in to explain such a continuous liberation of heat.

It has been shown, also, that a recently prepared salt of radium sets free a relatively small amount of heat. The heat set free in a given time then increases continually, and tends towards a definite value which is not quite reached at the end of a month.

When a salt of radium is dissolved in water and the solution enclosed in a sealed tube, the amount of heat given off at first is slight. It then increases and tends to become constant at the end of a month. When the limit is reached, the amount of heat given off from the radium in solution is the same as if it were in the solid state.

The amount of heat set free by radium at different temperatures can also be estimated by causing it to boil a liquefied gas and measuring the volume of gas that is liberated. The experiment can be performed with methyl chloride, which boils at -21°.

Figure I

The experiment was also performed by Dewar and myself with liquid oxygen (-180°) and liquid hydrogen (-252°). The latter is especially suitable for the experiment. A tube, , closed at the lower end and surrounded by a Dewar vacuum-bulb, contains a little liquid hydrogen, . A delivery-tube, , allows the gas to be collected in a graduated tube filled with water. The tube, , and its insulator are plunged into a bath of liquid hydrogen, '. Under these conditions no gas is produced in the tube . When a bottle containing 0.7 gram of radium bromide is placed in the hydrogen in , there is a continuous liberation of hydrogen gas, and 73 cc. of the gas are collected per minute. (The radium bromide had been made only ten days before.)

IV. Induced Radioactivity and Radioactive Emanations.

Induced Radioactivity.—Radium, thorium, and actinium have the property of acting externally, apart from the Becquerel rays that they emit. They communicate little by little their radioactive properties to substances in their neighborhood, and the latter emit in turn Becquerel rays. The activity can be transmitted to gases, liquids, and solids. The phenomenon is known as induced radioactivity.

Induced radioactivity propagates itself in gases step by step by a sort of conduction, and is not at all due to direct radiation from the body which causes it.

When the activated substance is removed from the radioactive body, the induced radioactivity on it persists for a certain time. It diminishes, nevertheless, little by little and finally disappears.

Emanation.—To explain these phenomena, Rutherford supposes that radium and thorium are constantly giving off an unstable, radioactive, gaseous material, which he calls an emanation. The emanation spreads into the gas surrounding the radioactive substance. It destroys itself little by little by giving off Becquerel rays and by producing other unstable radioactive substances that are not volatile. These new substances attach themselves to the surface of solid bodies and render them radioactive.

Without stating so many hypotheses, we may adopt the name emanation to designate the radioactive energy in the form which it has when it spreads into the gas surrounding the radioactive substance. It may be supposed also, that this energy disappears in creating the energy of induced radioactivity in solids.

The Radioactivity Induced by Radium and the Emanation from Radium.—When a solid salt of radium is placed in a closed space filled with air, the interior walls of the closed space and all solids in it become radioactive. We may, for example, place inside the space a plate of any solid, leave it there a certain time, and then remove it to study its activity. It is found that the activity of the plate increases at first according to the length of its stay in the space, but that it reaches a limiting value after a sufficiently long time. When the excited plate is removed from the space it loses its activity according to an exponential law, the radiation falling to one-half its value in about half an hour. In a general way all solid substances under the same conditions acquire and lose their activity in the same manner.

The phenomena are much more intense (about twenty times), if instead of the solid salt of radium, we place a solution of it inside the closed space in an open vessel.

The nature and the pressure of the gas contained in the closed space have no influence upon the observed phenomena.

The induced activity in a closed space is proportional to the quantity of radium present.

When the space containing the radium communicates with a second by a tube, solids contained in the second become equally active after a sufficient time. The transmission of the activating property can even take place from one to the other through a capillary tube.

When the gas which has been made active by remaining in one space containing radium is passed into another, it retains for a considerable time the property of rendering radioactive solids that are brought in contact with it. The gas thus removed from the action of the radium gradually loses, nevertheless, its power of causing radioactivity. It disappears as a function of the time, according to an exponential law. It loses one-half of its value every four days.

To interpret this phenomenon it must be assumed that radium continuously gives off a constant radioactive emanation. This emanation spreads into the air in a closed space, and acting on solids, makes them radioactive. When the air is transferred to another space, the emanation is carried along with it. Finally, it is destroyed spontaneously with such velocity that the quantity of emanation in the gas diminishes by one-half every four days.

In a space containing radium there is established a state of equilibrium when the quantity of emanation in the space is such that the loss of emanation resulting from its spontaneous destruction is exactly compensated by the continued supply coming from the radium.

Figure II

The following experiment can be performed: The glass receiver, (Fig. II.), filled with air, communicates through the constricted part, , with the bulb, , which contains radium, . At the end of a certain time the emanation has spread into , and its interior walls have become active. The receiver, , is then separated from the radium by closing with a flame. The external radiation of may then be studied by placing it in the interior cylinder of a cylindrical condenser (Fig. III.). The interior cylinder is of aluminium, . It is brought to a potential of 500 volts. The external cylinder, , of the condenser is of copper. It is in connection with an electrometer and piezoelectric quartz. By means of the quartz the current passing through the condenser is measured. This current is caused by the Becquerel rays that escape from the tube , pass through the cylinder of aluminium , and make the air between the two cylinders a conductor of electricity. The apparatus is surrounded by a protective metallic envelope, , connected with the earth.

Figure III

It is found that the radiation from the tube diminishes with time according to an exact exponential law, expressed by the equation

In a second experiment the tube may be made active as before, and the interior can then be evacuated by pumping out the air containing the emanation. Under these conditions the radiation of the receiver, , diminishes much more rapidly, and becomes twice as feeble in about half an hour. This law of loss of activity is the same as that according to which excited bodies lose their activity when they are exposed to the free air. The result is the same if inactive air be admitted to the receiver, , after having been evacuated.

One is thus led to the conclusion that in the first experiment the activity of the receiver, , is caused by the air charged with the emanation contained in the receiver, and that the law of the diminution of the radiation in this experiment represents as well the law of the spontaneous disappearance of the emanation.

Figure IV

When a vacuum is produced in the receiver, , which contains air charged with the emanation, and when the radiation of the receiver is measured immediately after the extraction of the air, it is found that the radiation has not changed at the moment when the active air is withdrawn. The Becquerel radiation of air charged with the emanation does not, then, produce any action in this experiment. This radiation probably exists, but it is composed of very slightly penetrating rays, incapable of passing through the glass wall. In this connection the following experiment can be performed. One of the ends of the metallic tube, (Fig. IV.), communicates at by means of a rubber tube with a receiver, , in which is a solution of a radium salt. The other end of the tube, , is closed with an insulating stopper, . Through the stopper passes a metallic rod, , connected with an electrometer. The tube, , and the rod, , form a cylindrical condenser. The tube, , is brought to a potential of 500 volts. The metallic tube, , connected with the earth, serves as a guard-tube. When the tube, , is sufficiently active it is removed from the radium, and the intensity of the current passing through the condenser is measured. Then the active air which fills the condenser is rapidly driven out, inactive air is admitted, and a new measurement of the intensity of the current is made immediately. It is found that the current has become six times more feeble. Thus, during the second measurement the radiation of the excited walls acts solely to ionize the air in the condenser, while during the first measurement the emanation acts as well. We may, then, suppose that it also emits an emanation. This radiation is necessarily very slightly penetrating, for its action cannot be detected on the exterior.