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Appendix A. Properties of the ? Rays 543

Appendix B. Radio-active Minerals 554

Index 559

For the convenience of the reader, the sections and chapters which contain mostly new matter, or have been either partly or wholly rewritten, are appended below.

ABBREVIATIONS OF REFERENCES TO SOME OF THE JOURNALS.

A great impetus to the study of this subject was initially given by the experiments of Lenard on the cathode rays, and by R?ntgen's discovery of the X rays. An examination of the conductivity imparted to a gas by the X rays led to a clear view of the mechanism of the transport of electricity through gases by means of charged ions. This ionization theory of gases has been shown to afford a satisfactory explanation not only of the passage of electricity through flames and vapours, but also of the complicated phenomena observed when a discharge of electricity passes through a vacuum tube. At the same time, a further study of the cathode rays showed that they consisted of a stream of material particles, projected with great velocity, and possessing an apparent mass small compared with that of the hydrogen atom. The connection between the cathode and R?ntgen rays and the nature of the latter were also elucidated. Much of this admirable experimental work on the nature of the electric discharge has been done by Professor J. J. Thomson and his students in the Cavendish Laboratory, Cambridge.

An examination of natural substances, in order to see if they gave out dark radiations similar to X rays, led to the discovery of the radio-active bodies which possess the property of spontaneously emitting radiations, invisible to the eye, but readily detected by their action on photographic plates and their power of discharging electrified bodies. A detailed study of the radio-active bodies has revealed many new and surprising phenomena which have thrown much light, not only on the nature of the radiations themselves, but also on the processes occurring in those substances. Notwithstanding the complex nature of the phenomena, the knowledge of the subject has advanced with great rapidity, and a large amount of experimental data has now been accumulated.

In order to explain the phenomena of radio-activity, Rutherford and Soddy have advanced a theory which regards the atoms of the radio-active elements as suffering spontaneous disintegration, and giving rise to a series of radio-active substances which differ in chemical properties from the parent elements. The radiations accompany the breaking-up of the atoms, and afford a comparative measure of the rate at which the disintegration takes place. This theory is found to account in a satisfactory way for all the known facts of radio-activity, and welds a mass of disconnected facts into one homogeneous whole. On this view, the continuous emission of energy from the active bodies is derived from the internal energy inherent in the atom, and does not in any way contradict the law of the conservation of energy. At the same time, however, it indicates that an enormous store of latent energy is resident in the radio-atoms themselves. This store of energy has not been observed previously, on account of the impossibility of breaking up into simpler forms the atoms of the elements by the action of the chemical or physical forces at our command.

On this theory we are witnessing in the radio-active bodies a veritable transformation of matter. This process of disintegration was investigated, not by direct chemical methods, but by means of the property possessed by the radio-active bodies of giving out specific types of radiation. Except in the case of a very active element like radium, the process of disintegration takes place so slowly, that hundreds if not thousands of years would be required before the amount transformed would come within the range of detection of the balance or the spectroscope. In radium, however, the process of disintegration takes place at such a rate that it should be possible within a limited space of time to obtain definite chemical evidence on this question. The recent discovery that helium can be obtained from radium adds strong confirmation to the theory; for helium was indicated as a probable disintegration product of the radio-active elements before this experimental evidence was forthcoming. Several products of the transformation of the radio-active bodies have already been examined, and the further study of these substances promises to open up new and important fields of chemical enquiry.

In this book the experimental facts of radio-activity and the connection between them are interpreted on the disintegration theory. Many of the phenomena observed can be investigated in a quantitative manner, and prominence has been given to work of this character, for the agreement of any theory with the facts, which it attempts to explain, must ultimately depend upon the results of accurate measurement.

The value of any working theory depends upon the number of experimental facts it serves to correlate, and upon its power of suggesting new lines of work. In these respects the disintegration theory, whether or not it may ultimately be proved to be correct, has already been justified by its results.

The most remarkable property of the radio-active bodies is their power of radiating energy spontaneously and continuously at a constant rate, without, as far as is known, the action upon them of any external exciting cause. The phenomena at first sight appear to be in direct contradiction to the law of conservation of energy, since no obvious change with time occurs in the radiating material. The phenomena appear still more remarkable when it is considered that the radio-active bodies must have been steadily radiating energy since the time of their formation in the earth's crust.

Immediately after R?ntgen's discovery of the production of X rays, several physicists were led to examine if any natural bodies possessed the property of giving out radiations which could penetrate metals and other substances opaque to light. As the production of X rays seemed to be connected in some way with cathode rays, which cause strong fluorescent and phosphorescent effects on various bodies, the substances first examined were those that were phosphorescent when exposed to light. The first observation in this direction was made by Niewenglowski, who found that sulphide of calcium exposed to the sun's rays gave out some rays which were able to pass through black paper. A little later a similar result was recorded by H. Becquerel for a special calcium sulphide preparation, and by Troost for a specimen of hexagonal blend. These results were confirmed and extended in a later paper by Arnold. No satisfactory explanations of these somewhat doubtful results have yet been given, except on the view that the black paper was transparent to some of the light waves. At the same time Le Bon showed that, by the action of sunlight on certain bodies, a radiation was given out, invisible to the eye, but active with regard to a photographic plate. These results have been the subject of much discussion; but there seems to be little doubt that the effects are due to short ultra-violet light waves, capable of passing through certain substances opaque to ordinary light. These effects, while interesting in themselves, are quite distinct in character from those shown by the radio-active bodies which will now be considered.

Becquerel found later that all the compounds of uranium as well as the metal itself possessed the same property, and, although the amount of action varied slightly for the different compounds, the effects in all cases were comparable. It was at first natural to suppose that the emission of these rays was in some way connected with the power of phosphorescence, but later observations showed that there was no connection whatever between them. The uranic salts are phosphorescent, while the uranous salts are not. The uranic salts, when exposed to ultra-violet light in the phosphoroscope, give a phosphorescent light lasting about ?01 seconds. When the salts are dissolved in water, the duration is still less. The amount of action on the photographic plate does not depend on the particular compound of uranium employed, but only on the amount of uranium present in the compound. The non-phosphorescent are equally active with the phosphorescent compounds. The amount of radiation given out is unaltered if the active body be kept continuously in darkness. The rays are given out by solutions, and by crystals which have been deposited from solutions in the dark and never exposed to light. This shows that the radiation cannot be due in any way to the gradual emission of energy stored up in the crystal in consequence of exposure to a source of light.

Since the uranium is thus continuously radiating energy from itself, without any known source of excitation, the question arises whether any known agent is able to affect the rate of its emission. No alteration was observed when the body was exposed to ultra-violet light or to ultra-red light or to X rays. Becquerel states that the double sulphate of uranium and potassium showed a slight increase of action when exposed to the arc light and to sparks, but he considers that the feeble effect observed was another action superimposed on the constant radiation from uranium. The intensity of the uranium radiation is not affected by a variation of temperature between 200? C. and the temperature of liquid air. This question is discussed in more detail later.

The radiations from uranium are thus analogous, as regards their photographic and electrical actions, to R?ntgen rays, but, compared with the rays from an ordinary X ray tube, these actions are extremely feeble. While with R?ntgen rays a strong impression is produced on a photographic plate in a few minutes or even seconds, several days' exposure to the uranium rays is required to produce a well-marked action, even though the uranium compound, enveloped in black paper, is placed close to the plate. The discharging action, while very easily measurable by suitable methods, is also small compared with that produced by X rays from an ordinary tube.

If an active body like uranium be mixed with an inactive body, the resulting activity in the mixture is generally considerably less than that due to the active substance alone. This is due to the absorption of the radiation by the inactive matter present. The amount of decrease largely depends on the thickness of the layer from which the activity is determined.

Mme Curie made a detailed examination by the electrical method of the great majority of known substances, including the very rare elements, to see if they possessed any activity. In cases where it was possible, several compounds of the elements were examined. With the exception of thorium and phosphorus, none of the other substances possessed an activity even of the order of 1/100 of uranium.

The ionization of the gas by phosphorus does not, however, seem to be due to a penetrating radiation like that found in the case of uranium, but rather to a chemical action taking place at its surface. The compounds of phosphorus do not show any activity, and in this respect differ from uranium and the other active bodies.

Le Bon has also observed that quinine sulphate, if heated and then allowed to cool, possesses for a short time the property of discharging both positively and negatively electrified bodies. It is necessary, however, to draw a sharp line of distinction between phenomena of this kind and those exhibited by the naturally radio-active bodies. While both, under special conditions, possess the property of ionizing the gas, the laws controlling the phenomena are quite distinct in the two cases. For example, only one compound of quinine shows the property, and that compound only when it has been subjected to a preliminary heating. The action of phosphorus depends on the nature of the gas, and varies with temperature. On the other hand, the activity of the naturally radio-active bodies is spontaneous and permanent. It is exhibited by all compounds, and is not, as far as is yet known, altered by change in the chemical or physical conditions.

Pitchblende from Johanngeorgenstadt 8?3 x 10??? " Joachimsthal 7?0 " " Pzibran 6?5 " " Cornwall 1?6 " Clevite 1?4 " Chalcolite 5?2 " Autunite 2?7 " Thorite from 0?3 to 1?4 " Orangite 2?0 " Monazite 0?5 " Xenotine 0?03 " Aeschynite 0?7 " Fergusonite 0?4 " Samarskite 1?1 " Niobite 0?3 " Carnotite 6?2 "

Some activity is to be expected in these minerals, since they all contain either thorium or uranium or a mixture of both. An examination of the action of the uranium compounds with the same apparatus and under the same conditions led to the following results:

Uranium 2?3 x 10??? amperes Black oxide of uranium 2?6 " Green " " 1?8 " Acid uranic hydrate 0?6 " Uranate of sodium 1?2 " Uranate of potassium 1?2 " Uranate of ammonia 1?3 " Uranous sulphate 0?7 " Sulphate of uranium and potassium 0?7 " Acetate 0?7 " Phosphate of copper and uranium 0?9 " Oxysulphide of uranium 1?2 "

The interesting point in connection with these results is that some specimens of pitchblende have four times the activity of the metal uranium; chalcolite, the crystallized phosphate of copper and uranium, is twice as active as uranium; and autunite, a phosphate of calcium and uranium, is as active as uranium. From the previous considerations, none of the substances should have shown as much activity as uranium or thorium. In order to be sure that the large activity was not due to the particular chemical combination, Mme Curie prepared chalcolite artificially, starting with pure products. This artificial chalcolite had the activity to be expected from its composition, viz. about 0?4 of the activity of the uranium. The natural mineral chalcolite is thus five times as active as the artificial mineral.

It thus seemed probable that the large activity of some of these minerals, compared with uranium and thorium, was due to the presence of small quantities of some very active substance, which was different from the known bodies thorium and uranium.

This supposition was completely verified by the work of M. and Mme Curie, who were able to separate from pitchblende by purely chemical methods two active bodies, one of which in the pure state is over a million times more active than the metal uranium.

This important discovery was due entirely to the property of radio-activity possessed by the new bodies. The only guide in their separation was the activity of the products obtained. In this respect the discovery of these bodies is quite analogous to the discovery of rare elements by the methods of spectrum analysis. The method employed in the separation consisted in examining the relative activity of the products after chemical treatment. In this way it was seen whether the radio-activity was confined to one or another of the products, or divided between both, and in what ratio such division occurred.

The activity of the specimens thus served as a basis of rough qualitative and quantitative analysis, analogous in some respects to the indication of the spectroscope. To obtain comparative data it was necessary to test all the products in the dry state. The chief difficulty lay in the fact that pitchblende is a very complex mineral, and contains in varying quantities nearly all the known metals.

Radium is extracted from pitchblende by the process used to separate barium, to which radium is very closely allied in chemical properties. After the removal of other substances, the radium remains behind mixed with barium. It can, however, be partially separated from the latter by the difference in solubility of the chlorides in water, alcohol, or hydrochloric acid. The chloride of radium is less soluble than that of barium, and can be separated from it by the method of fractional crystallization. After a large number of precipitations, the radium can be freed almost completely from the barium.

Both polonium and radium exist in infinitesimal quantities in pitchblende. In order to obtain a few decigrammes of very active radium, it is necessary to use several tons of pitchblende, or the residues obtained from the treatment of uranium minerals. It is thus obvious that the expense and labour involved in preparation of a minute quantity of radium are very great.

M. and Mme Curie were indebted for their first working material to the Austrian government, who generously presented them with a ton of the treated residue of uranium materials from the State manufactory of Joachimsthal in Bohemia. With the assistance of the Academy of Science and other societies in France, funds were given to carry out the laborious work of separation. Later the Curies were presented with a ton of residues from the treatment of pitchblende by the Soci?t? Centrale de Produits Chimiques of Paris. The generous assistance afforded in this important work is a welcome sign of the active interest taken in these countries in the furthering of purely scientific research.

The rough concentration and separation of the residues was performed in the chemical works, and there followed a large amount of labour in purification and concentration. In this manner, the Curies were able to obtain a small quantity of radium which was enormously active compared with uranium. No definite results have yet been given on the activity of pure radium, but the Curies estimate that it is about one million times that of uranium, and may possibly be still higher. The difficulty of making a numerical estimate for such an intensely active body is very great. In the electric method, the activities are compared by noting the relative strength of the maximum or saturation current between two parallel plates, on one of which the active substance is spread. On account of the intense ionization of the gas between the plates, it is not possible to reach the saturation current unless very high voltages are applied. Approximate comparisons can be made by the use of metal screens to cut down the intensity of the radiations, if the proportion of the radiation transmitted by such a screen has been determined by direct experiment on impure material of easily measurable activity. The value of the activity of radium compared with that of uranium will however vary to some extent according to which of the three types of rays is taken as a basis of comparison.

It is thus difficult to control the final stages of the purification of radium by measurements of its activity alone. Moreover the activity of radium immediately after its preparation is only about one-fourth of its final value; it gradually rises to a maximum after the radium salt has been kept in the dry state for about a month. For control experiments in purification, it is advisable to measure the initial rather than the final activity.

Mme Curie has utilized the coloration of the crystals of radiferous barium as a means of controlling the final process of purification. The crystals of salts of radium and barium deposited from acid solutions are indistinguishable by the eye. The crystals of radiferous barium are at first colourless, but, in the course of a few hours, become yellow, passing to orange and sometimes to a beautiful rose colour. The rapidity of this coloration depends on the amount of barium present. Pure radium crystals do not colour, or at any rate not as rapidly as those containing barium. The coloration is a maximum for a definite proportion of radium, and this fact can be utilized as a means of testing the amount of barium present. When the crystals are dissolved in water the coloration disappears.

Giesel has observed that pure radium bromide gives a beautiful carmine colour to the Bunsen flame. If barium be present in any quantity, only the green colour due to barium is observed, and a spectroscopic examination shows only the barium lines. This carmine coloration of the Bunsen flame is a good indication of the purity of the radium.

Wave length Intensity Wave length Intensity 4826?3 10 4600?3 3 4726?9 5 4533?5 9 4699?6 3 4436?1 6 4692?1 7 4340?6 12 4683?0 14 3814?7 16 4641?9 4 3649?6 12

The lines are all sharply defined, and three or four of them have an intensity comparable with any known lines of other substances. There are also present in the spectrum two strong nebulous bands. In the visible part of the spectrum, which has not been photographed, the only noticeable ray has a wave length 5665, which is, however, very feeble compared with that of wave length 4826?3. The general aspect of the spectrum is similar to that of the alkaline earths; it is known that these metals have strong lines accompanied by nebulous bands.

Later observations on the spectrum of radium have been made by Runge, Exner and Haschek, with specimens of radium prepared by Giesel. Crookes has photographed the spectrum of radium in the ultra-violet, while Runge and Precht, using a highly purified sample of radium, observed a number of new lines in the spark spectrum. It has been mentioned already that the bromide of radium gives a characteristic pure carmine-red coloration to the Bunsen flame. The flame spectrum shows two broad bright bands in the orange-red, not observed in Demar?ay's spectrum. In addition there is a line in the blue-green and two feeble lines in the violet.

In these experiments about 0?1 gram of pure radium chloride was obtained by successive fractionations. The difficulty involved in preparing a quantity of pure radium chloride large enough to test the atomic weight may be gauged from the fact that only a few centigrams of fairly pure radium, or a few decigrams of less concentrated material, are obtained from the treatment of about 2 tons of the mineral from which it is derived.

Runge and Precht have examined the spectrum of radium in a magnetic field, and have shown the existence of series analogous to those observed for calcium, barium, and strontium. These series are connected with the atomic weights of the elements in question, and Runge and Precht have calculated by these means that the atomic weight of radium should be 258--a number considerably greater than the number 225 obtained by Mme Curie by means of chemical analysis. Marshall Watts, on the other hand, using another relation between the lines of the spectrum, deduced the value obtained by Mme Curie. Runge has criticised the method of deduction employed by Marshall Watts on the ground that the lines used for comparison in the different spectra were not homologous. Considering that the number found by Mme Curie agrees with that required by the periodic system, it is advisable in the present state of our knowledge to accept the experimental number rather than the one deduced by Runge and Precht from spectroscopic evidence.

There is no doubt that radium is a new element possessing remarkable physical properties. The detection and separation of this substance, existing in such minute proportions in pitchblende, has been due entirely to the characteristic property we are considering, and is the first notable triumph of the study of radio-activity. As we shall see later, the property of radio-activity can be used, not only as a means of chemical research, but also as an extraordinarily delicate method of detecting chemical changes of a very special kind.

As in the case of uranium or thorium, the photographic action is mainly due to the penetrating or cathodic rays. The radiographs obtained with radium are very similar to those obtained with X rays, but lack the sharpness and detail of the latter. The rays are unequally absorbed by different kinds of matter, the absorption varying approximately as the density. In photographs of the hand the bones do not stand out as in X ray photographs.

All the radium salts possess the property of causing rapid colorations of the glass vessel which contains them. For feebly active material the colour is usually violet, for more active material a yellowish-brown, and finally black.

Precipitation in hot solutions, slightly acidulated with hydrochloric acid, by excess of hyposulphite of soda. The active matter is present almost entirely in the precipitate.

Precipitation of neutral nitrate solutions by oxygenated water. The precipitate carries down the active body.

Precipitation of insoluble sulphates. If barium sulphate, for example, is precipitated in the solution containing the active body, the barium carries down the active matter. The thorium and actinium are freed from the barium by conversion of the sulphate into the chloride and precipitation by ammonia.

In this way Debierne has obtained a substance comparable in activity with radium. The separation, which is difficult and laborious, has not yet been carried far enough to bring out any new lines in the spectrum.

Giesel found that the activity of this substance was permanent and seemed to increase during the six months' interval after separation. In this respect it is similar to radium compounds, for the activity of radium, measured by the electric method, increases in the course of a month's interval to four times its initial value at separation.

There can be no doubt that the "actinium" of Debierne and the "emanium" of Giesel contain the same radio-active constituent, for recent work has shown that they exhibit identical radio-active properties. Each gives out easily absorbed and penetrating rays, and emits a characteristic emanation of which the rate of decay is the same for both substances. The rate of decay of the emanation is the simplest method of distinguishing actinium from thorium, which it resembles so closely in radio-active as well as in chemical properties. The emanation of actinium loses its radiating power far more rapidly than that of thorium, the time taken for the activity to fall to half value being in the two cases 3?7 seconds and 52 seconds respectively.

The rapid and continuous emission of this short-lived emanation is the most striking radio-active property possessed by actinium. In still air, the radio-active effects of this emanation are confined to a distance of a few centimetres from the active material, as it is only able to diffuse a short distance through the air before losing its radiating power. With very active preparations of actinium, the material appears to be surrounded by a luminous haze produced by the emanation. The radiations produce strong luminosity in some substances, for example, zinc sulphide, willemite and platino-cyanide of barium. The luminosity is especially marked on screens of zinc sulphide. Much of this effect is due to the emanation, for, on gently blowing a current of air over the substance, the luminosity is displaced at once in the direction of the current. With a zinc sulphide screen, actinium shows the phenomena of "scintillations" to an even more marked degree than radium itself.

The preparations of emanium are in some cases luminous, and a spectroscopic examination of this light has shown a number of bright lines.

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