where λ is the constant of decay of activity of the radium emanation and x the depth of the layer of water from the surface.
Putting
it was found that
The value of λ expressed in terms of a day as the unit of time is about ·17.
Thus the value of K for the diffusion of the radium emanation into water = ·066 cm.2 / day.
The value of K found by Stefan[260] for the diffusion of carbon dioxide into water was 1·36 cm.2/day. These results are thus in harmony with the conclusion drawn from the diffusion of the radium emanation into air, and show that the radium emanation behaves as a gas of high molecular weight.
Condensation of the Emanations.
165. Condensation of the emanations. During an investigation of the effect of physical and chemical agencies on the thorium emanation, Rutherford and Soddy[261] found that the emanation passed unchanged in amount through a white-hot platinum tube and through a tube cooled to the temperature of solid carbon dioxide. In later experiments the effects of still lower temperatures were examined, and it was then found that at the temperature of liquid air both emanations were condensed[262].
If either emanation is conveyed by a slow stream of hydrogen, oxygen, or air through a metal spiral immersed in liquid air, and placed in connection with a testing vessel as in Fig. 51, no trace of emanation escapes in the issuing gas. When the liquid air is removed and the spiral plunged into cotton-wool, several minutes elapse before any deflection of the electrometer needle is observed, and then the condensed emanation volatilizes rapidly, and the movement of the electrometer needle is very sudden, especially in the case of radium. With a fairly large amount of radium emanation, under the conditions mentioned, a very few seconds elapse after the first sign of movement before the electrometer needle indicates a deflection of several hundred divisions per second. It is not necessary in either case that the emanating compound should be retained in the gas stream. After the emanation is condensed in the spiral, the thorium or radium compound may be removed and the gas stream sent directly into the spiral. But in the case of thorium, under these conditions, the effects observed are naturally small owing to the rapid loss of the activity of the emanation with time, which proceeds at the same rate at the temperature of liquid air as at ordinary temperatures.
If a large amount of radium emanation is condensed in a glass U tube, the progress of the condensation can be followed by the eye, by means of the phosphorescence which the radiations excite in the glass. If the ends of the tube are sealed and the temperature allowed to rise, the glow diffuses uniformly throughout the tube, and can be concentrated at any point to some extent by local cooling of the tube with liquid air.
166. Experimental arrangements. A simple experimental arrangement to illustrate the condensation and volatilization of the emanation and some of its characteristic properties is shown in Fig. 58. The emanation obtained from a few milligrams of radium bromide by solution or heating is condensed in the glass U tube T immersed in liquid air. This U tube is then put into connection with a larger glass tube V, in the upper part of which is placed a piece of zinc sulphide screen Z, and in the lower part of the tube a piece of the mineral willemite. The stop-cock A is closed and the U tube and the vessel V are partially exhausted by a pump through the stop-cock B. This lowering of the pressure causes a more rapid diffusion of the emanation when released. The emanation does not escape if the tube T is kept immersed in liquid air. The stop-cock B is then closed, and the liquid air removed. No luminosity of the screen or the willemite in the tube V is observed for several minutes, until the temperature of T rises above the point of volatilization of the emanation. The emanation is then rapidly carried into the vessel V, partly by expansion of the gas in the tube T with rising temperature, and partly by the process of diffusion. The screen Z and the willemite W are caused to phosphoresce brilliantly under the influence of the rays from the emanation surrounding them.
Fig. 58.
If the end of the vessel V is then plunged into liquid air, the emanation is again condensed in the lower end of the tube, and the willemite phosphoresces much more brightly than before. This is not due to an increase of the phosphorescence of willemite at the temperature of the liquid air, but to the effect of the rays from the emanation condensed around it. At the same time the luminosity of the zinc sulphide gradually diminishes, and practically disappears after several hours if the end of the tube is kept in the liquid air. If the tube is removed from the liquid air, the emanation again volatilizes and lights up the screen Z. The luminosity of the willemite returns to its original value after the lapse of several hours. This slow change of the luminosity of the zinc sulphide screen and of the willemite is due to the gradual decay of the “excited activity” produced by the emanation on the surface of all bodies exposed to its action (chapter VIII). The luminosity of the screen is thus due partly to the radiation from the emanation and partly to the excited radiation caused by it. As soon as the emanation is removed from the upper to the lower part of the tube, the “excited” radiation gradually diminishes in the upper and increases in the lower part of the tube.
The luminosity of the screen gradually diminishes with the time as the enclosed emanation loses its activity, but is still appreciable after an interval of several weeks.
An apparatus of a similar character to illustrate the condensation of the radium emanation has been described by P. Curie[263].
Fig. 59.
167. Determination of the temperature of condensation. A detailed investigation was made by Rutherford and Soddy (loc. cit.) of the temperatures at which condensation and volatilization commenced for the two emanations. The experimental arrangement of the first method is shown clearly in Fig. 59. A slow constant stream of gas, entering at A, was passed through a copper spiral S, over 3 metres in length, immersed in a bath of liquid ethylene. The copper spiral was made to act as its own thermometer by determining its electrical resistance. The resistance temperature curve was obtained by observation of the resistances at 0°, the boiling point of liquid ethylene -103·5°, the solidification point of ethylene -169° and in liquid air. The temperature of the liquid air was deduced from the tables given by Baly for the boiling point of liquid air for different percentages of oxygen. The resistance-temperature curve, for the particular spiral employed, was found to be nearly a straight line between 0° and -192°C., cutting the temperature axis if produced nearly at the absolute zero. The resistance of the spiral, deduced from readings on an accurately calibrated Weston millivoltmeter, with a constant current through the spiral, was thus very approximately proportional to the absolute temperature. The liquid ethylene was kept vigorously stirred by an electric motor, and was cooled to any desired temperature by surrounding the vessel with liquid air.
The general method employed for the radium emanation was to pass a suitable amount of emanation, mixed with the gas to be used, from the gas holder B into the spiral, cooled below the temperature of condensation. After the emanation was condensed in the spiral, a current of electrolytic hydrogen or oxygen was passed through the spiral. The temperature was allowed to rise gradually, and was noted at the instant when a deflection of the electrometer, due to the presence of emanation in the testing vessel T, was observed. The resistance, subject to a slight correction due to the time taken for the emanation to be carried into the testing vessel, gave the temperature at which some of the emanation commenced to volatilize. The ionization current in the testing vessel rose rapidly to a maximum value, showing that, for a small increase of temperature, the whole of the radium emanation was volatilized. The following table gives an illustration of the results obtained for a current of hydrogen of 1·38 cubic centimetres per second.
| Temperature | Divisions per second of the electrometer |
|---|---|
| -160° | 0 |
| -156° | 0 |
| -154°·3 | 1 |
| -153°·8 | 21 |
| -152°·5 | 24 |
The following table shows the results obtained for different currents of hydrogen and oxygen.
| Current of Gas | T1 | T2 | |
|---|---|---|---|
| Hydrogen | ·25 c.c. per sec. | -151·3 | -150 |
| “ | ·32 „ „ | -153·7 | -151 |
| ” | ·92 „ „ | -152 | -151 |
| “ | 1·38 „ „ | -154 | -153 |
| ” | 2·3 „ „ | -162·5 | -162 |
| Oxygen | ·34 „ „ | -152·5 | -151·5 |
| “ | ·58 „ „ | -155 | -153 |
The temperature T1 in the above table gives the temperature of initial volatilization, T2 the temperature for which half of the condensed emanation had been released. For slow currents of hydrogen and oxygen, the values of T1 and T2 are in good agreement. For a stream of gas as rapid as 2·3 cubic centimetres per second the value of T1 is much lower. Such a result is to be expected; for, in too rapid a stream, the gas is not cooled to the temperature of the spiral, and, in consequence, the inside surface of the spiral is above the mean temperature, and some of the emanation escapes at a temperature apparently much lower. In the case of oxygen, this effect appears for a gas stream of 0·58 cubic centimetres per second.
In the experiments on the thorium emanation, on account of the rapid loss of activity, a slightly different method was necessary. The steady stream of gas was passed over the thorium compound, and the temperature was observed at the instant when an appreciable movement of the electrometer appeared. This gave the temperature at which a small fraction of the thorium emanation escaped condensation, and not the value T1 observed for the radium emanation, which gave the temperature for which a small fraction of the previously condensed emanation was volatilized.
The following table illustrates the results obtained.
| Current of Gas | Temperature | |
|---|---|---|
| Hydrogen | ·71 c.c. per sec. | -155° C. |
| “ | 1·38 „ „ | -159° C. |
| Oxygen | ·58 „ „ | -155° C. |
On comparing these results with the values obtained for the radium emanation, it will be observed that with equal gas streams the temperatures are nearly the same.
A closer examination of the thorium emanation showed, however, that this apparent agreement was only accidental, and that there was, in reality, a very marked difference in the effect of temperature on the two emanations. It was found experimentally that the radium emanation was condensed very near the temperature at which volatilization commenced, and that the points of condensation and volatilization were defined fairly sharply.
Fig. 60.
On the other hand, the thorium emanation required a range of over 30° C. after condensation had started in order to ensure complete condensation. Fig. 60 is an example of the results obtained with a steady gas stream of 1·38 c.c. per sec. of oxygen. The ordinates represent the percentage proportion of the emanation uncondensed at different temperatures. It will be observed that condensation commences about -120°, and that very little of the emanation escapes condensation at -155° C.
To investigate this difference of behaviour in the two emanations, a static method was employed, which allowed an examination of the two emanations to be made under comparable conditions. The emanation, mixed with a small amount of the gas to be used, was introduced into the cool spiral, which had been exhausted previously by means of a mercury pump. The amount of emanation remaining uncondensed after definite intervals was rapidly removed by means of the pump, and was carried with a constant auxiliary stream of gas into the testing vessel.
Tested in this way, it was found that the volatilization point of the radium emanation was very nearly the same as that obtained by the blowing method, viz. -150° C. With thorium, on the other hand, the condensation started at about -120° C., and, as in the blowing method, continued over a range of about 30° C. The proportion of the emanation condensed at any temperature was found to depend on a variety of conditions, although the point at which condensation commenced, viz. -120° C., was about the same in each case. It depended on the pressure and nature of the gas, on the concentration of the emanation, and on the time for which it was left in the spiral. For a given temperature a greater proportion of the emanation was condensed, the lower the pressure and the longer the time it was left in the spiral. Under the same conditions, the emanation was condensed more rapidly in hydrogen than in oxygen.
168. Thus there is no doubt that the thorium emanation begins to condense at a temperature higher than that at which the radium emanation condenses. The explanation of the peculiar behaviour of the thorium emanation is clear when the small number of emanation particles present in the gas are taken into consideration. It has been shown that both emanations give out only α rays. It is probable that the α particles from the two emanations are similar in character and produce about the same number of ions in their passage through the gas. The number of ions produced by each α particle before its energy is dissipated is probably about 70,000. (See section 252.)
Now, in the experiment, the electrometer readily measured a current of 10-3 electrostatic units. Taking the charge on an ion as 3·4 × 10-10 electrostatic units, this corresponds to a production in the testing vessel of about 3 × 106 ions per sec., which would be produced by about 40 expelled α particles per second. Each radiating particle cannot expel less than one α particle and may expel more, but it is likely that the number expelled by an atom of the thorium emanation is not greatly different from that expelled by an atom of the radium emanation.
In section 133 it has been shown that, according to the law of decay, λN particles change per second when N are present. Thus, to produce 40 α particles, λN cannot be greater than 40. Since for the thorium emanation λ is ¹⁄₈₇, it follows that N cannot be greater than 3500. The electrometer thus detected the presence of 3500 particles of the thorium emanation, and since in the static method the volume of the condensing spiral was about 15 c.c., this corresponded to a concentration of about 230 particles per c.c. An ordinary gas at atmospheric pressure and temperature probably contains about 3·6 × 1019 molecules per c.c. Thus the emanation would have been detected on the spiral if it had possessed a partial pressure of less than 10-17 of an atmosphere.
It is not surprising then that the condensation point of the thorium emanation is not sharply defined. It is rather a matter of remark that condensation should occur so readily with so sparse a distribution of emanation particles in the gas; for, in order that condensation may take place, it is probable that the particles must approach within one another’s sphere of influence.
Now in the case of the radium emanation, the rate of decay is about 5000 times slower than that of the thorium emanation, and consequently the actual number of particles that must be present to produce the same ionization per second in the two cases must be about 5000 times greater in the case of radium than in the case of thorium. This conclusion involves only the assumption that the same number of rays is produced by a particle of emanation in each case, and that the expelled particles produce in their passage through the gas the same number of ions. The number of particles present, in order to be detected by the electrometer, in this experiment, must therefore have been about 5000 × 3500, i.e. about 2 × 107. The difference of behaviour in the two cases is well explained by the view that, for equal electrical effects, the number of radium emanation particles must be far larger than the number of thorium emanation particles. The probability of the particles coming into each other’s sphere of influence will increase very rapidly as the concentration of the particles increases, and, in the case of the radium emanation, once the temperature of condensation is attained, all but a small proportion of the total number of particles present will condense in a very short time. In the case of the thorium emanation, however, the temperature might be far below that of condensation, and yet a considerable portion remain uncondensed for comparatively long intervals. On this view the experimental results obtained might reasonably be expected. A greater proportion of emanation condenses the longer the time allowed for condensation under the same conditions. The condensation occurs more rapidly in hydrogen than in oxygen, as the diffusion is greater in the former gas. For the same reason the condensation occurs faster the lower the pressure of the gas present. Finally, when the emanation is carried by a steady stream of gas, a smaller proportion condenses than in the other cases, because the concentration of emanation particles per unit volume of gas is less under these conditions.
It is possible that the condensation of the emanations may not occur in the gas itself but at the surface of the containing vessel. Accurate observations of the temperature of condensation have so far only been made in a copper spiral, but condensation certainly occurs in tubes of lead or glass at about the same temperature as in tubes of copper.
169. In experiments that were made by the static method with a very large quantity of radium emanation, a slight amount of escape of the condensed emanation was observed several degrees below the temperature at which most of the emanation was released. This is to be expected, since, under such conditions, the electrometer is able to detect a very minute proportion of the whole quantity of the emanation condensed.
Special experiments, with a large quantity of emanation, that were made with the spiral immersed in a bath of rapidly boiling nitric oxide, showed this effect very clearly. For example, the condensed emanation began to volatilize at -155° C. In 4 minutes the temperature had risen to -153·5°, and the amount volatilized was four times as great as at -155°. In the next 5-½ minutes the temperature had increased to -152·3° and practically the whole quantity, which was at least fifty times the amount at the temperature of -153·5°, had volatilized.
It thus seems probable that, if the temperature were kept steady at the point at which volatilization was first observed, and the released emanation removed at intervals, the whole of the emanation would in course of time be liberated at that temperature. Curie and Dewar and Ramsay have observed that the emanation condensed in a U tube, immersed in liquid air, slowly escapes if the pump is kept steadily working. These results point to the probability that the condensed emanation possesses a true vapour pressure, but great refinements in experimental methods would be necessary before such a conclusion could be definitely established.
The true temperature of condensation of the thorium emanation is probably about -120° C., and that of radium about -150° C. Thus there is no doubt that the two emanations are quite distinct from each other in this respect, and also with regard to their radio-activity, although they both possess the property of chemical inertness. These results on the temperatures of condensation do not allow us to make any comparison of the condensation points of the emanations with those of known gases, since the lowering of the condensation points of gases with diminution of pressure has not been studied at such extremely minute pressures.
170. It has been found[264] that the activity of the thorium emanation, when condensed in the spiral at the temperature of liquid air, decayed at the same rate as at ordinary temperatures. This is in accord with results of a similar kind obtained by P. Curie for the radium emanation (section 145), and shows that the value of the radio-active constant is unaffected by wide variations of temperature.
Amount of Emanation from Radium and Thorium.
171. It has been shown in section 93 from experimental data that 1 gram of radium bromide at its minimum activity emits about 3·6 × 1010 α particles per second. Since the activity due to the emanation stored up in radium, when in a state of radio-active equilibrium, is about one quarter of the whole and about equal to the minimum activity, the number of α particles projected per second by the emanation from 1 gram of radium bromide is about 3·6 × 1010. It has been shown in section 152 that 463,000 times the amount of emanation produced per second is stored up in the radium. But, in a state of radio-active equilibrium, the number of emanation particles breaking up per second is equal to the number produced per second. Assuming that each emanation particle in breaking up expels one α particle, it follows that the number of emanation particles present in 1 gram of radium bromide in radio-active equilibrium is 463,000 × 3·6 × 1010, i.e. 1·7 × 1016. Taking the number of hydrogen molecules in 1 c.c. of gas at atmospheric pressure and temperature as 3·6 × 1019 (section 39), the volume of the emanation from 1 gram of radium bromide is 4·6 × 10-4 cubic centimetres at atmospheric pressure and temperature. Assuming the composition of radium bromide as RaBr2, the amount from 1 gram of radium in radio-active equilibrium is 0·82 cubic millimetres. Quite independently of any method of calculation it was early evident that the volume of the emanation was very small, for all the earlier attempts made to detect its presence by its volume were unsuccessful. It will be seen, however, that, when larger quantities of radium were available for experiment, the emanation has been collected in volume sufficiently large to measure.
In the case of thorium, the maximum quantity of emanation to be obtained from 1 gram of the solid is very minute, both on account of the small activity of thorium and of the rapid break up of the emanation after its production. Since the amount of emanation, stored in a non-emanating thorium compound, is only 87 times the rate of production, while in radium it is 463,000 times, and the rate of production of the emanation by radium is about 1 million times faster than by thorium, it follows that the amount of emanation to be obtained from 1 gram of thorium is not greater than 10-10 of the amount from an equal weight of radium, i.e. its volume is not greater than 10-13 c.c. at the ordinary pressure and temperature. Even with large quantities of thorium, the amount of emanation is too small ever to be detected by its volume.
172. Volume of the emanation from radium. The evidence already considered points very strongly to the conclusion that the emanation possesses all the properties of a chemically inert gas of high molecular weight.
Since the emanation continuously breaks up, and is transformed into a solid type of matter, which is deposited on the surface of bodies, the volume of the emanation, when separated from radium, should contract at the same rate as it loses its activity, i.e. it should decrease to half value in about four days. The amount of emanation to be obtained from a given quantity of radium is a maximum when the rate of production of new emanation balances its rate of change. This condition is practically attained when the emanation has been allowed to collect for an interval of one month. The probable volume of the emanation to be obtained from 1 gram of radium was early calculated on certain assumptions, and from data then available the writer[265] deduced that the volume of the emanation from 1 gram of radium lay between ·06 and ·6 cubic millimetre at atmospheric pressure and temperature, and was probably nearer the latter value. The volume to be expected on the latest data has been discussed in the preceding section and shown to be about ·82 cubic mm. The volume of the emanation is thus very small, but not too small to be detected if several centigrams of radium are available. This has been proved to be the case by Ramsay and Soddy[266] who, by very careful experiment, finally succeeded in isolating a small quantity of the emanation and in determining its volume. The experimental method employed by them will now be briefly described.
Fig. 61.
The emanation from 60 milligrams of radium bromide in solution was allowed to collect for 8 days and then drawn off through the inverted siphon E (Fig. 61) into the explosion burette F. This gas consisted for the most part of hydrogen and oxygen, produced by the action of the radiations on the water of the solution. After explosion, the excess of hydrogen mixed with emanation was left some time in contact with caustic soda, placed in the upper part of the burette, in order to remove all trace of carbon dioxide. In the meantime the upper part of the apparatus had been completely evacuated. The connection C to the pump was closed, and the hydrogen and emanation were allowed to enter the apparatus, passing over a phosphorous pentoxide tube D. The emanation was condensed in the lower part of the capillary tube A, by surrounding it with the tube B filled with liquid air. The process of condensation was rendered manifest by the brilliant luminosity of the lower part of the tube. The mercury from the burette was then allowed to run to G, and the apparatus again completely evacuated. The connection of the pump was again closed, the liquid air was removed and the volatilized emanation forced into the fine capillary tube A. Observations were then made, from day to day, of the volume of the emanation. The results are given in the table below.
| Time | Volume | Time | Volume |
|---|---|---|---|
| Start | 0·124 cub. mm. | 7 days | 0·0050 cub. mm. |
| 1 day | 0·027 „ | 9 „ | 0·0041 „ |
| 3 „ | 0·011 „ | 11 „ | 0·0020 „ |
| 4 „ | 0·0095 „ | 12 „ | 0·0011 „ |
| 6 „ | 0·0063 „ | 28 „ | 0·0004 „ |
The volume contracted with the time, and was very small after a month’s interval, but the minute bubble of the emanation still retained its luminosity to the last. The tube became deep purple in colour, which rendered readings difficult except with a strong light. There was a sudden decrease in the first day, which may have been due to the mercury sticking in the capillary tube.
The experiments were repeated with another capillary tube and the volume of gas observed at normal pressure was 0·0254 c. mm. The gas obtained was found to obey Boyle’s law within the limit of experimental error over a considerable range of pressure. But, unlike in the first experiment, the gas did not contract but expanded rapidly during the first few hours, and then more slowly, finally reaching a volume after 23 days of 0·262 c. mm. or about 10 times the initial volume. The measurements were complicated by the appearance of bubbles of gas in the top of the mercury column. The differences observed in these two experiments are difficult to account for. We shall see, later, that the emanation always produces helium, and, in the first experiment, the decrease of the volume to zero indicates that the helium was buried or absorbed in the walls of the tube. In the second case, probably owing to some difference in the glass of the capillary tube, the helium may have been released. This suggestion is confirmed by the observation that the volume of gas, after the experiment ended, gave a brilliant spectrum of helium.
We shall see later that there is considerable evidence that the α particles expelled from radio-active substances consist of helium atoms. Since the particles are projected with great velocity, they will first be buried in the walls of the tube, and then may gradually diffuse out into the gas again under conditions probably depending on the kind of glass employed. Since α particles are projected from the emanation and also from two of the rapidly changing products which arise from it, the volume of helium should, on this view, be three times the initial volume of the emanation. If the helium produced escaped from the walls of the tube into the gas, the apparent volume of the gas in the capillary should increase to three times the initial volume in a month’s interval, for during that time the emanation itself has been transformed into a solid type of matter deposited on the walls of the tube.
Ramsay and Soddy concluded from their experiments that the maximum volume of emanation to be obtained from 1 gram of radium was about 1 cubic millimetre at standard pressure and temperature, and that the emanation was produced from 1 gram of radium at the rate of 3 × 10-6 c. mm. per second. This amount is in very good agreement with the calculated value, and is a strong indication of the general correctness of the theory on which the calculations are based.
173. Spectrum of the emanation. After the separation of the emanation and the determination of its volume, Ramsay and Soddy made numerous attempts to obtain its spectrum. In some of the earlier experiments several bright lines were seen for a short time, but these lines were soon masked by the appearance of the hydrogen lines. In later experiments Ramsay and Collie[267] succeeded in obtaining a spectrum of the emanation, which persisted for a short time, during which a rapid determination of the wave-lengths was made. They state that the spectrum was very brilliant, consisting of very bright lines, the spaces between being perfectly dark. The spectrum bore a striking resemblance in general character to the spectrum of the gases of the argon family.
The spectrum soon faded, and the spectrum of hydrogen began to appear. The following table shows the wave-length of the lines observed in the spectrum. The degree of coincidence of the lines of known wave-lengths shows that the error is probably less than five Ångström units.
| Wave-length | Remarks |
|---|---|
| 6567 | Hydrogen C; true wave-length, 6563; observed each time. |
| 6307 | Observed only at first; evanescent. |
| 5975 | „ „ „ |
| 5955 | „ „ „ |
| 5805 | Observed each time; persistent. |
| 5790 | Mercury; true wave-length, 5790. |
| 5768 | „ „ 5769. |
| 5725 | Observed only at first; evanescent. |
| 5595 | Observed each time; persistent and strong. |
| 5465 | Mercury; true wave-length, 5461. |
| 5105 | Not observed at first; appeared after some seconds; persisted and was visible during the second examination. |
| 4985 | Observed each time; persistent and strong. |
| 4865 | Hydrogen F; true wave-length, 4861. |
| 4690 | Observed only at first. |
| 4650 | Not observed when the emanation was examined again. |
| 4630 | „ „ „ |
| 4360 | Mercury: true wave-length, 4359. |
The experiments were repeated with a new supply of emanation, and some of the stronger lines were observed again, while some new lines made their appearance. Ramsay and Collie suggest that the strong line 5595 may be identical with a line which was observed by Pickering[268] in the spectrum of lightning, and was not identified with the spectrum of any known gas.
Until large quantities of radium are available for the experimenter it would appear difficult to make sure how many of these lines must be ascribed to the spectrum of the emanation or to measure the wave-lengths with accuracy.
The results are of great interest, as showing that the emanation has a definite and new spectrum of the same general character as the argon group of gases to which, as we have seen, it is chemically allied.
Summary of Results.
174. The investigations into the nature of the radio-active emanations have thus led to the following conclusions:—The radio-elements thorium, radium and actinium continuously produce from themselves radio-active emanations at a rate which is constant under all conditions. In some cases, the emanations continuously diffuse from the radio-active compounds into the surrounding gas; in other cases, the emanations are unable to escape from the material in which they are produced, but are occluded, and can only be released by solution or by the action of heat.
The emanations possess all the properties of radio-active gases. They diffuse through gases, liquids, and porous substances, and can be occluded in some solids. Under varying conditions of pressure, volume, and temperature, the emanations distribute themselves in the same way and according to the same laws as does a gas.
The emanations possess the important property of condensation under the influence of extreme cold, and by that means can be separated from the gases with which they are mixed. The radiation from the emanation is material in nature, and consists of a stream of positively charged particles projected with great velocity.
The emanations possess the property of chemical inertness, and in this respect resemble the gases of the argon family. The emanations are produced in minute amount; but a sufficient quantity of the radium emanation has been obtained to determine its volume and its spectrum. With regard to their rates of diffusion, the emanations of both thorium and radium behave like gases of high molecular weight.
These emanations have been detected and their properties investigated by the property they possess of emitting radiations of a special character. These radiations consist entirely of α rays, i.e. particles, projected with great velocity, which carry a positive charge and have a mass about twice that of the hydrogen atom. The emanations do not possess the property of permanently radiating, but the intensity of the radiations diminishes according to an exponential law with the time, falling to half value, from actinium in 4 seconds, from thorium in one minute, and from radium in about four days. The law of decay of activity does not seem to be influenced by any physical or chemical agency.
The emanation particles gradually break up, each particle as it breaks up expelling a charged body. The emanation after it has radiated ceases to exist as such, but is transformed into a new kind of matter, which is deposited on the surface of bodies and gives rise to the phenomena of excited activity. This last property, and the connection of the emanation with it, are discussed in detail in the next chapter.