Chapter XIV (1)

RADIO-ACTIVITY OF THE ATMOSPHERE AND OF ORDINARY MATERIALS.

=273. Radio-activity of the atmosphere.= The experiments of Geitel[384] and C. T. R. Wilson[385] in 1900 showed that a positively or negatively charged conductor placed inside a closed vessel gradually lost its charge. This loss of charge was shown to be due to a small ionization of the air inside the vessel. Elster and Geitel also found that a charged body exposed in the open air lost its charge rapidly, and that the rate of discharge was dependent on the locality and on atmospheric conditions. A more detailed description and discussion of these results will be given later in section 284.

In the course of these experiments, Geitel observed that the rate of discharge increased slightly for some time after the vessel had been closed. He considered that this might possibly be due to the existence of some radio-active substances in the air, which produced excited activity on the walls of the vessel and so increased the rate of dissipation of the charge. In 1901 Elster and Geitel[386] tried the bold experiment of seeing whether it were possible to extract a radio-active substance from the air. The experiments of the writer had shown that the excited radio-activity from the thorium emanation could be concentrated on the negative electrode in a strong electric field. This result indicated that the carriers of the radio-activity had a positive charge of electricity. Elster and Geitel therefore tried an experiment to see whether positively charged carriers, possessing a similar property, were present in the atmosphere. For this purpose a cylinder of wire-netting, charged negatively to 600 volts, was exposed for several hours in the open air. The cylinder was then removed, and quickly placed in a large bell-jar, inside which was placed an electroscope to detect the rate of discharge. It was found that the rate of discharge was increased to a slight extent. In order to multiply the effect a wire about 20 metres in length was exposed at some height from the ground, and was kept charged to a high potential by connecting it to the negative terminal of an influence machine. After exposure for some hours, this wire was removed and placed inside the dissipation vessel. The rate of discharge was found to be increased many times by the presence of the wire. No increase was observed when the wire was charged positively instead of negatively. The results also showed that the radio-active matter could be removed from the wire in the same way as from a wire made active by exposure in the presence of the thorium emanation. A piece of leather moistened with ammonia was rubbed over the active wire. On testing the leather, it was found to be strongly radio-active. When a long wire was used, the amount of activity obtained on the leather was comparable with that possessed by a gram of uranium oxide.

The activity produced on the wire was not permanent, but disappeared to a large extent in the course of a few hours. The amount of activity produced on a wire of given size, exposed under similar conditions, was independent of the material of the wire. Lead, iron and copper wires gave about equal effects.

The amount of activity obtained was greatly increased by exposing a negatively charged wire in a mass of air which had been undisturbed for a long time. Experiments were made in the great cave of Wolfenbüttel, and a very large amount of activity was observed. By transferring the activity to a piece of leather it was found that the rays could appreciably light up a screen of barium platinocyanide in the dark[387]. The rays also darkened a photographic plate through a piece of aluminium 0·1 mm. in thickness.

These remarkable experiments show that the excited radio-activity obtained from the atmosphere is very similar in character to the excited activity produced by the emanations of radium and thorium. No investigators have contributed more to our knowledge of the radio-activity and ionization of the atmosphere than Elster and Geitel. The experiments here described have been the starting-point of a series of researches by them and others on the radio-active properties of the atmosphere, which have led to a great extension of our knowledge of that important subject.

Rutherford and Allan[388] determined the rate of decay of the excited activity produced on a negatively charged wire exposed in the open air. A wire about 15 metres long was exposed in the open air, and kept charged by an influence machine to a potential of about −10,000 volts. An hour’s exposure was sufficient to obtain a large amount of excited activity on the wire. The wire was then rapidly removed and wound on a framework which formed the central electrode in a large cylindrical metal vessel. The ionization current for a saturation voltage was measured by means of a sensitive Dolezalek electrometer. The current, which is a measure of the activity of the wire, was found to diminish according to an exponential law with the time, falling to half value in about 45 minutes. The rate of decay was independent of the material of the wire, of the time of exposure, and of the potential of the wire.

An examination was also made of the nature of the rays emitted by the radio-active wire. For this purpose a lead wire was made radio-active in the manner described, and then rapidly wound into the form of a flat spiral. The penetrating power of the rays was tested in a vessel similar to that shown in Fig. 17. Most of the ionization was found to be due to some very easily absorbed rays, which were of a slightly more penetrating character than the α rays emitted from a wire made active by the radium or thorium emanations. The intensity of the rays was cut down to half value by about 0·001 cm. of aluminium. The photographic action observed by Elster and Geitel through 0·1 mm. of aluminium showed that some penetrating rays were also present. This was afterwards confirmed by Allan, who used the electric method. These penetrating rays are probably similar in character to the β rays from the radio-elements.

=274.= The excited activity produced on the negatively charged wire cannot be due to an action of the strong electric field on the surface of the wire; for very little excited activity is produced if the wire is charged to the same potential inside a closed cylinder.

We have seen that the excited activity produced on the wire can be partially removed by rubbing and by solution in acids, and, in this respect, it is similar to the excited activity produced in bodies by the emanations of radium and thorium. The very close similarity of the excited activity obtained from the atmosphere to that obtained from the radium and thorium emanations suggests the probability that a radio-active emanation exists in the atmosphere. This view is confirmed by a large amount of indirect evidence discussed in sections 276, 277 and 280.

Assuming the presence of a radio-active emanation in the atmosphere, the radio-active effects observed receive a simple explanation. The emanation in the air gradually breaks up, giving rise in some way to positively charged radio-active carriers. These are driven to the negative electrode in the electric field, and there undergo a further change, giving rise to the radiations observed at the surface of the wire. The matter which causes excited activity will thus be analogous to the active deposit of radium and thorium.

Since the earth is negatively electrified with regard to the upper atmosphere, these positive radio-active carriers produced in the air are continuously deposited on the surface of the earth. Everything on the surface of the earth, including the external surface of buildings, the grass, and leaves of trees, must be covered with an invisible deposit of radio-active material. A hill, or mountain peak, or any high mass of rock or land, concentrates the earth’s electric field at that point and consequently will receive more excited radio-activity per unit area than the plain. Elster and Geitel have pointed out that the greater ionization of the air observed in the neighbourhood of projecting peaks receives a satisfactory explanation on this view.

If the radio-active carriers are produced at a uniform rate in the atmosphere, the amount of excited activity _I_{t}_, produced on a wire exposed under given conditions, will, after exposure for a time _t_, be given by

$$ I_t = I₀ (1 − e^{–λt} ) $$,

where _I₀_ is the maximum activity on the wire and λ is the constant of decay of the excited activity. Since the activity of a wire after removal falls to half value in about 45 minutes, the value of λ is 0·92 (hour)⁻¹. Some experiments made by Allan[389] are in rough agreement with the above equation. Accurate comparative results are difficult to obtain on account of the inconstancy of the radio-activity of the open air. After an exposure of a wire for several hours, the activity reached a practical maximum, and was not much increased by continued exposure.

We have seen (section 191) that the carriers of the active deposit of radium and thorium move in an electric field with about the same velocity as the ions. We should expect therefore that a long wire charged to a high negative potential would abstract the active carriers from the atmosphere for a considerable distance. This does not appear to be the case, for Eve (see section 281) has found that the carriers are only abstracted from the air for a radius of less than one metre, for a potential of the wire of −10,000 volts. It seems probable that the carriers of the active matter are deposited on the numerous fine dust particles present in the air and thus move very slowly even in a strong electric field.

The amount of excited activity produced on a wire, supported some distance from the surface of the earth, should increase steadily with the voltage, for the greater the potential, the greater the volume of air from which the radio-active carriers are abstracted.

The presence of radio-active matter in the atmosphere will account for a considerable portion of the ionization of the air observed near the earth. This important question is discussed in more detail in section 281.

=275. Radio-activity of freshly fallen rain and snow.= C. T. R. Wilson[390] tried experiments to see if any of the radio-active material from the air was carried down by rain. For this purpose a quantity of freshly fallen rain was collected, rapidly evaporated to dryness in a platinum vessel, and the activity of the residue tested by placing the vessel in an electroscope. In all cases, the rate of discharge of the electroscope was considerably increased. From about 50 c.c. of rain water, an amount of activity was obtained sufficient to increase the rate of discharge of the electroscope four or five times, after the rays had traversed a thin layer of aluminium or gold-leaf. The activity disappeared in the course of a few hours, falling to half value in about 30 minutes. Rain water, which had stood for some hours, showed no trace of activity. Tap water, when evaporated, left no active residue.

The amounts of activity obtained from a given quantity of rain water were all of the same order of magnitude, whether the rain was precipitated in fine or in large drops, by night or by day, or whether the rain was tested at the beginning or at the end of a heavy rainfall lasting several hours.

The activity obtained from rain is not destroyed by heating the platinum vessel to a red heat. In this and other respects it resembles the excited activity obtained on negatively charged wires exposed in the open air.

C. T. R. Wilson[391] obtained a radio-active precipitate from rain water by adding a little barium chloride and precipitating the barium with sulphuric acid. An active precipitate was also obtained when alum was added to the water, and the aluminium precipitated by ammonia. The precipitates obtained in this way showed a large activity. The filtrate when boiled down was quite inactive, showing that the active matter had been completely removed by precipitation. This effect is quite analogous to the production of active precipitates from a solution containing the active deposit of thorium (see section 185).

The radio-activity of freshly fallen snow was independently observed by C. T. R. Wilson[392] in England, and Allan[393] and McLennan[394] in Canada. In order to obtain a large amount of activity, the surface layer of snow was removed, and evaporated to dryness in a metal vessel. An active residue was obtained with radio-active properties similar to those observed for freshly fallen rain. Both Wilson and Allan found that the activity of rain and snow decayed at about the same rate, the activity falling to half value in about 30 minutes. McLennan states that he found a smaller amount of radio-activity in the air after a prolonged fall of snow.

Schmauss[395] has observed that drops of water falling through air ionized by Röntgen rays acquire a negative charge. This effect is ascribed to the fact that the negative ions in air diffuse faster than the positive. On this view the drops of rain and flakes of snow would acquire a negative charge in falling through the air. They would in consequence act as collectors of the positive radio-active carriers from the air. On evaporation of the water the radio-active matter would be left behind.

=276. Radio-active emanations from the earth.= Elster and Geitel observed that the air in caves and cellars was, in most cases, abnormally radio-active, and showed very strong ionization. This action might possibly be due to an effect of stagnant air, by which it produced a radio-active emanation from itself, or to a diffusion of a radio-active emanation from the soil. To test whether this emanation was produced by the air itself, Elster and Geitel shut up the air for several weeks in a large boiler, but no appreciable increase of the activity or ionization was observed. To see whether the air imprisoned in the capillaries of the soil was radio-active, Elster and Geitel[396] put a pipe into the earth and sucked up the air into a testing vessel by means of a water pump.

The apparatus employed to test the ionization of the air is shown in Fig. 103. _C_ is an electroscope connected with a wire net, _Z_. The active air was introduced into a large bell-jar of 27 litres capacity, the inside of which was covered with wire netting, _MM´_. The bell-jar rested on an iron plate _AB_. The electroscope could be charged by the rod _S_. The rate of discharge of the electroscope, before the active air was introduced, was noted. On allowing the active air to enter, the rate of discharge increased rapidly, rising in the course of a few hours in one experiment to 30 times the original value. They found that the emanation produced excited activity on the walls of the containing vessel. The air sucked up from the earth was even more active than that observed in caves and cellars. There can thus be little doubt that the abnormal activity observed in caves and cellars is due to a radio-active emanation, present in the earth, which gradually diffuses to the surface and collects in places where the air is not disturbed.

Results similar to those obtained by Elster and Geitel for the air removed from the earth at Wolfenbüttel were also obtained later by Ebert and Ewers[397] at Munich. They found a strongly active emanation in the soil, and, in addition, examined the variation with time of the activity due to the emanation in a sealed vessel. After the introduction of the active air into the testing vessel, the activity was observed to increase for several hours, and then to decay, according to an exponential law, with the time, falling to half value in about 3·2 days. This rate of decay is more rapid than that observed for the radium emanation, which decays to half value in a little less than four days. The increase of activity with time is probably due to the production of excited activity on the walls of the vessel by the emanation. In this respect it is analogous to the increase of activity observed when the radium emanation is introduced into a closed vessel. No definite experiments were made by Ebert and Ewers on the rate of decay of this excited activity. In one experiment the active emanation, after standing in the vessel for 140 hours, was removed by sucking ordinary air of small activity through the apparatus. The activity rapidly fell to about half value, and this was followed by a very slow decrease of the activity with time. This result indicates that about half the rate of discharge observed was due to the radiation from the emanation and the other half to the excited activity produced by it.

The apparatus employed by Ebert and Ewers in these experiments was very similar to that employed by Elster and Geitel, shown in Fig. 103. Ebert and Ewers observed that, when the wire net attached to the electroscope was charged negatively, the rate of discharge observed was always greater than when it was charged positively. The differences observed between the two rates of discharge varied between 10 and 20 per cent. A similar effect has been observed by Sarasin, Tommasina and Micheli[398] for a wire made active by exposure to the open air. This difference in the rates of discharge for positive and negative electricity is probably connected with the presence of particles of dust or small water globules suspended in the gas. The experiments of Miss Brooks (section 181) have shown that the particles of dust present in the air containing the thorium emanation become radio-active. A large proportion of these dust particles acquire a positive charge and are carried to the negative electrode in an electric field. This effect would increase the rate of discharge of the electroscope when charged negatively. In later experiments, Ebert and Ewers noticed that, in some cases, when the air had been kept in the vessel for several days, the effect was reversed, and the electroscope showed a great rate of discharge when charged positively.

J. J. Thomson[399] has observed that the magnitude of the ionization current depends on the direction of the electric field, if fine water globules are suspended in the ionized gas.

In later experiments, Ebert[400] found that the radio-active emanation could be removed from the air by condensation in liquid air. This property of the emanation was independently discovered by Ebert before he was aware of the results of Rutherford and Soddy on the condensation of the emanations of radium and thorium. To increase the amount of radio-active emanation in a given volume of air, a quantity of the active air, obtained by sucking the air from the soil, was condensed by a liquid air machine. The air was then allowed partially to evaporate, but the process was stopped before the point of volatilization of the emanation was reached. This process was repeated with another quantity of air and the residues added together. Proceeding in this way, he was able to concentrate the emanation in a small volume of air. On allowing the air to evaporate, the ionization of the air in the testing vessel increased rapidly for a time and then slowly diminished. Ebert states that the maximum for the emanation which had been liquefied for some time was reached earlier than for fresh air. The rate of decay of activity of the emanation was not altered by keeping it at the temperature of liquid air for some time. In this respect it behaves like the emanations of radium and thorium.

J. J. Thomson[401] found that air bubbled through Cambridge tap water showed much greater conductivity than ordinary air. The air was drawn through the water by means of a water pump into a large gasometer, when the ionization current was tested with a sensitive electrometer. When a rod charged negatively was introduced into this conducting air it became active. After an exposure for a period of 15 to 30 minutes in the conducting gas, the rod, when introduced into a second testing vessel, increased the saturation current in the vessel to about five times the normal amount. Very little effect was produced when the rod was uncharged or charged positively for the same time. The activity of the rod decayed with the time, falling to half value in about 40 minutes. The amount of activity produced on a wire under constant conditions was independent of the material of the wire. The rays from the rod were readily absorbed in a few centimetres of air.

These effects were, at first, thought to be due to the action of the small water drops suspended in the gas, for it was well known that air rapidly drawn through water causes a temporary increase in its conductivity. Later results, however, showed that there was a radio-active emanation present in Cambridge tap water. This led to an examination of the waters from deep wells in various parts of England, and J. J. Thomson found that, in some cases, a large amount of emanation could be obtained from the well water. The emanation was released either by bubbling air through the water or by boiling the water. The gases obtained by boiling the water were found to be strongly active. A sample of air mixed with the radio-active emanation was condensed. The liquefied gas was allowed to evaporate, and the earlier and later portions of the gas were collected in separate vessels. The final portion was found to be about 30 times as active as the first portion.

An examination of the radio-active properties of the active gases so obtained has been made by Adams[402]. He found that the activity of the emanation decayed, according to an exponential law, with the time, falling to half value in about 3·4 days. This is not very different from the rate of decay of the activity of the radium emanation, which falls to half value in a little less than four days. The excited activity produced by the emanation decayed to half value in about 35 minutes. The decay of the excited activity from radium is at first irregular, but after some time falls off, according to an exponential law, diminishing to half value in 28 minutes. Taking into account the uncertainty attaching to measurements of the very small ionization observed in these experiments, the results indicate that the emanation obtained from well water in England is similar to, if not identical with, the radium emanation. Adams observed that the emanation was slightly soluble in water. After well water had been boiled for a while and then put aside, it was found to recover its power of giving off an emanation. The amount obtained after standing for some time was never more than 10 per cent. of the amount first obtained. Thus it is probable that the well water, in addition to the emanations mixed with it, has also a slight amount of a permanent radio-active substance dissolved in it. Ordinary rain water or distilled water does not give off an emanation.

Bumstead and Wheeler[403] have made a very careful examination of the radio-activity of the emanation obtained from the surface water and soil at New Haven, Connecticut. The emanation, obtained from the water by boiling, was passed into a large testing cylinder, and measurements of the current were made by means of a sensitive electrometer. The current gradually rose to a maximum, after the introduction of the emanation, in exactly the same way as the current increases in a vessel after the introduction of the radium emanation. The decay of activity of the emanations obtained from the water and soil was carefully measured, and, within the limits of experimental error, agreed with the rate of decay of activity observed for the radium emanation. The identity of the emanations from the water and soil with the radium emanation was still further established by experiments on the rate of diffusion of the emanation through a porous plate. By comparative tests it was found that the coefficient of diffusion of the emanations from the water and soil was the same as for the radium emanation. Also, by comparison of the rate of diffusion of carbonic acid, it was found that the density of the emanation was about four times that of carbonic acid, a result in good agreement with that found for the radium emanation (sections 161 and 162).

Bumstead[404] has found that a considerable amount of thorium as well as radium emanation exists in the air of New Haven. For a three hour exposure in the open air, 3 to 5 per cent. of the excited activity on the wire is due to thorium. For a twelve hour exposure, the thorium activity was sometimes 15 per cent. of the whole. On account of the comparatively slow decay of the excited activity of thorium, the activity on the wire after removal for three or four hours was due almost entirely to thorium. The rate of decay could then be measured accurately, and was found to be the same as for a wire exposed in the presence of the thorium emanation.

Dadourian[405] has made an examination of the underground air in New Haven, and has found that this too contains a large quantity of the thorium emanation. A circular hole about 50 cms. in diameter and 2 metres deep was dug in the ground. A number of wires were wound on an insulated frame and suspended in the hole, the top of the hole then being covered over. The wire was charged negatively by a Wimshurst machine. After a long exposure the excited activity on the wire diminished at a rate that showed it to be a mixture of the excited activities of thorium and radium.

A very large amount of work has been done in examining various hot and mineral springs for the presence of the radium emanation, and it is not possible here to refer more than briefly to a few of the very numerous papers that have been published on this subject both in Europe and America. H. S. Allen and Lord Blythswood[406] have observed that the hot springs at Bath and Buxton gave off a radio-active emanation. This was confirmed by Strutt[407], who found that the escaping gases contained the radium emanation, and also that the mud deposited from the springs contained a trace of radium salts. These results are of considerable interest, for Lord Rayleigh has observed that helium is contained among the gases evolved by the springs. It appears probable that the helium observed is produced from the radium or radio-active deposits through which the water flows. Many mineral and hot springs which are famous for their curative properties have been found to contain traces of radium and also considerable amounts of radium emanation. It has been suggested that the curative properties may be due to some extent to the presence of these minute quantities of radium.

Himstedt[408] found that the thermal springs at Baden Baden contained the radium emanation, while Elster and Geitel[409] examined the deposits formed by these springs and found them to contain small quantities of radium salts. Results of a similar character were obtained for a number of waters in Germany by Dorn[410], Schenck[411], and H. Mache[412].

Curie and Laborde[413] have tested the waters of a large number of mineral springs and found that the great majority contain the radium emanation. In this connection, it is of interest to note that Curie and Laborde found very little emanation in the waters of Salins-Moutiers, while Blanc[414] observed, on the other hand, that the sediment from the spring was very active. A closer examination of this deposit by Blanc revealed the fact that it contained a considerable quantity of thorium. This was proved by finding that it gave out an emanation, which lost half of its activity in one minute, and produced excited activity, which fell to half value in about 11 hours. Boltwood[415] has tested a number of samples of spring water from different sources in America and has found that many of them contain the radium emanation.

Most of the results upon the amount of radium emanation from different sources have been expressed in arbitrary units without, in many cases, any comparative standard being given. Boltwood (_loc. cit._) has described a satisfactory method for collecting and testing the emanation from different waters, and has suggested that the rate of discharge observed by the electroscope or the electrometer should be expressed in terms of the effect due to the emanation liberated on solution of a definite weight of the mineral uraninite. Since in every mineral so far examined, the amount of radium present is proportional to the amount of uranium, such a standard would be sufficiently definite for practical purposes. The emanation liberated from a few centigrams of the mineral is sufficient to give a convenient rate of discharge of an electroscope. Such a method is preferable to using a known quantity of a radium compound as a standard, since it is difficult to know with certainty the activity of the preparations of radium which may be in the possession of the different experimenters.

=277. Radio-activity of constituents of the earth.= Elster and Geitel[416] observed that, although in many cases the conductivity of the air was abnormally high in underground enclosures, the conductivity varied greatly in different places. In the Baumann Cave, for example, the conductivity of the air was nine times the normal, but in the Iberg Cave only three times the normal. In a cellar at Clausthal the conductivity was only slightly greater than the normal, but the excited radio-activity obtained on a negatively charged wire exposed in it was only ¹⁄₁₁ of the excited radio-activity obtained when the wire was exposed in the free air. They concluded from these experiments that the amount of radio-activity in the different places probably varied with the nature of the soil. Observations were then made on the conductivity of the air sucked up from the earth at different parts of the country. The clayey and limestone soils at Wolfenbüttel were found to be strongly active, the conductivity varying from four to sixteen times the normal amount. A sample of air from the shell limestone of Würzburg and from the basalt of Wilhelmshöhe showed very little activity.

Experiments were made to see whether any radio-active substance could be detected in the soil itself. For this purpose some earth was placed on a dish and introduced under a bell-jar, similar to that shown in Fig. 103. The conductivity of the air in the bell-jar increased with the time, rising to three times the normal value after several days. Little difference was observed whether the earth was dry or moist. The activity of the soil seemed to be permanent, for no change in the activity was observed after the earth had been laid aside for eight months.

Attempts were then made to separate the radio-active constituent from the soil by chemical treatment. For this purpose a sample of clay was tested. By extraction with hydrochloric acid all the calcium carbonate was removed. On drying the clay the activity was found to be reduced, but it spontaneously regained its original activity in the course of a few days. It seems probable, therefore, that an active product had been separated from the soil by the acid. Elster and Geitel consider that an active substance was present in the clay, which formed a product more readily soluble in hydrochloric acid than the active material itself. There seemed to be a process of separation analogous to that of Th X from thorium by precipitation with ammonia.

Experiments were also made to see whether substances placed in the earth acquired any radio-activity. For this purpose samples of potter’s clay, whitening, and heavy spar, wrapped in linen, were placed in the earth 50 cms. below the surface. After an interval of a month, these were dug up and their activity examined. The clay was the only substance which showed any activity. The activity of the clay diminished with the time, showing that activity had been excited in it by the emanations present in the soil.

Elster and Geitel[417] have found that a large quantity of the radio-active emanation can be obtained by sucking air through clay. In some cases, the conductivity of the air in the testing vessel was increased over 100 times. They have also found that the so-called “fango”—a fine mud obtained from hot springs in Battaglia, Northern Italy—gives off three or four times as much emanation as clay. By treating the fango with acid, the active substance present was dissolved. On adding some barium chloride to the solution, and precipitating the barium as sulphate, the active substance was removed, and in this way a precipitate was obtained over 100 times as active, weight for weight, as the original fango. Comparisons were made of the rate of decay of the excited activity, due to the emanation from fango, with that due to the radium emanation, and within the limits of error, the decay curves obtained were found to be identical. There can thus be no doubt that the activity observed in fango is due to the presence of a small quantity of radium. Elster and Geitel calculate that the amount of radium, contained in it, is only about one-thousandth of the amount to be obtained from an equal weight of pitchblende from Joachimsthal.

Vincenti and Levi Da Zara[418] have found that the waters and sediments of a number of hot springs in Northern Italy contain the radium emanation. Elster and Geitel observed that natural carbonic acid obtained from great depths of old volcanic soil was radio-active, while Burton[419] found that the petroleum from a deep well in Ontario, Canada, contained a large quantity of emanation, probably of radium, since its activity fell to half value in 3·1 days, while the excited activity produced by the emanation fell to half value in about 35 minutes. A permanently active deposit was left behind after volatilization of the oil, indicating that probably one or more of the radio-elements were present in minute quantity.

Elster and Geitel[420] have found that the active sediments obtained from springs at Nauheim and Baden Baden showed abnormal rates of decay of the excited activity. This was finally traced to the presence in the deposit of both thorium and radium. By suitable chemical methods, the two active substances were separated from each other and were then tested separately.

=278. Effect of meteorological conditions upon the radio-activity of the atmosphere.= The original experiments of Elster and Geitel on the excited radio-activity derived from the atmosphere were repeated by Rutherford and Allan[421] in Canada. It was found that a large amount of excited radio-activity could be derived from the air, and that the effects were similar to those observed by Elster and Geitel in Germany. This was the case even on the coldest day in winter, when the ground was covered deeply with snow and wind was blowing from the north over snow-covered lands. The results showed that the radio-activity present in the air was not much affected by the presence of moisture, for the air during a Canadian winter is extremely dry. The greatest amount of excited activity on a negatively charged wire was obtained in a strong wind. In some cases the amount produced for a given time of exposure was ten to twenty times the normal amount. A cold bright day of winter usually gave more effect than a warm dull day in summer.

Elster and Geitel[422] have made a detailed examination of the effect of meteorological conditions on the amount of excited radio-activity to be derived from the atmosphere. For this purpose a simple portable apparatus was devised by them and used for the whole series of experiments. A large number of observations were taken, extending over a period of twelve months. They found that the amount of excited activity obtained was subject to great variations. The extreme values obtained varied in the ratio of 16 to 1. No direct connection could be traced between the amount of ionization in the atmosphere and the amount of excited activity produced. They found that the greatest amount of excited activity was obtained during a fog, when the amount of ionization in the air was small. This result, however, is not necessarily contradictory to the view that the ionization and activity of the air are to a certain extent connected. From the experiments of Miss Brooks on the effect of dust in acting as carriers of excited activity, more excited activity should be obtained during a fog than in clear air. The particles of water become centres for the deposit of radio-active matter. The positive carriers are thus anchored and are not removed from the air by the earth’s field. In a strong electric field, these small drops will be carried to the negative electrode and manifest their activity on the surface of the wire. On the other hand, the distribution of water globules throughout the air causes the ions in the air to disappear rapidly in consequence of their diffusion to the surface of the drops (see section 31). For this reason the denser the fog, the smaller will be the conductivity observed in the air.

Lowering the temperature of the air had a decided influence. The average activity observed below 0° C. was 1·44 times the activity observed above 0° C. The height of the barometer was found to exert a marked influence on the amount of excited activity to be derived from the air. The lower the barometer the greater was the amount of excited activity in the air. The effect of variation of the height of the barometer is intelligible, when it is considered that probably a large proportion of the radio-activity observed in the air is due to the radio-active emanations which are continuously diffusing from the earth into the atmosphere. Elster and Geitel have suggested that a lowering of the pressure of the air would cause the air from the ground to be drawn up from the capillaries of the earth into the atmosphere. This, however, need not necessarily be the case if the conditions of the escape of the emanation into the atmosphere are altered by the variation of the position of underground water or by a heavy fall of rain.

The amount of excited activity to be derived from the air on the Baltic Coast was only one-third of that observed inland at Wolfenbüttel. Experiments on the radio-activity of the air in mid-ocean would be of great importance in order to settle whether the radio-activity observed in the air is due to the emanations from the soil alone. It is probable that the radio-activity of the air at different points of the earth may vary widely, and may largely depend on the nature of the soil.

Saake[423] has found that the amount of emanation present in the air at high altitudes in the valley of Arosa in Switzerland is much greater than the normal amount at lower levels. Elster and Geitel have observed that there is also a larger number of ions in the air at high altitudes, and suggest that the curative effect of thermal springs and the physiological actions of the air at high levels may be connected with the presence of an unusual amount of radio-active matter in the atmosphere. Simpson[424] made experiments on the amount of excited activity at Karasjoh, Norway, at a height of about 150 feet above sea level. The sun did not rise above the level of the horizon during the time the observations were taken. The average amount of excited activity obtained from the air was considerably greater than the normal amount observed by Elster and Geitel in Germany. This was the more surprising as the ground was frozen hard and covered with deep snow. Allan, working in Montreal, Canada, early observed that the amount of activity to be obtained from the air was about the same in summer as in winter, although, in the latter case, the whole earth was deeply frozen and covered with snow, and the winds blew from the north over snow-covered lands. Under such conditions, a diminution of the amount of activity is to be expected since the diffusion of the emanation must be retarded, if not altogether stopped, by the freezing of the soil. On the other hand, it appears difficult to escape from the conclusion of Elster and Geitel that the emanation present in the atmosphere is evolved from the earth itself.

Some interesting experiments have been made by McLennan[425] on the amount of excited radio-activity to be derived from the air when filled with fine spray. The experiments were made at the foot of the American Fall at Niagara. An insulated wire was suspended near the foot of the Fall, and the amount of excited activity on the wire compared with the amount to be obtained on the same wire for the same exposure in Toronto. The amount of activity obtained from the air at Toronto was generally five or six times that obtained from the air at the Falls. In these experiments it was not necessary to use an electric machine to charge the wire negatively, for the falling spray kept the insulated wire permanently charged to a potential of about −7500 volts. These results indicate that the falling spray had a negative charge and electrified the wire. The small amount of the excited radio-activity at the Falls was probably due to the fact that the negatively charged drops abstracted the positively charged radio-active carriers from the atmosphere, and in falling carried them to the river below. On collecting the spray and evaporating it, no active residue was obtained. Such a result is, however, to be expected on account of the minute proportion of the spray tested compared with that present in the air.

=279. A very penetrating radiation from the earth’s surface.= McLennan[426], and Rutherford and Cooke[427] independently, observed the presence of a very penetrating radiation inside buildings. McLennan measured the natural conductivity of the air in a large closed metal cylinder by means of a sensitive electrometer. The cylinder was then placed inside another and the space between filled with water. For a thickness of water between the cylinders of 25 cms. the conductivity of the air in the inner cylinder fell to about 63 per cent. of its initial value. This result shows that part of the ionization in the inner cylinder was due to a penetrating radiation from an external source, which radiation was partially or wholly absorbed in water.

Rutherford and Cooke observed that the rate of discharge of a sealed brass electroscope was diminished by placing a lead screen around the electroscope. A detailed investigation of the decrease of the rate of discharge in the electroscope, when surrounded by metal screens, was made later by Cooke[428]. A thickness of 5 cms. of lead round the electroscope decreased the rate of discharge about 30 per cent. Further increase of the thickness of the screen had no effect. When the apparatus was surrounded by 5 tons of pig-lead the rate of discharge was about the same as when it was surrounded by a plate about 3 cms. thick. An iron screen also diminished the rate of discharge to about the same extent as the lead. By suitably arranging lead screens it was found that the radiation came equally from all directions. It was of the same intensity by night as by day. In order to be sure that this penetrating radiation did not arise from the presence of radio-active substances in the laboratory, the experiments were repeated in buildings in which radio-active substances had never been introduced, and also on the open ground far removed from any building. In all cases a diminution of the rate of discharge of the electroscope, when surrounded by lead screens, was observed. These results show that a penetrating radiation is present at the surface of the earth, arising partly from the earth itself and partly from the atmosphere.

The result is not surprising when the radio-activity of the earth and atmosphere is taken into account. The writer has found that bodies made active by exposure to the emanations from thorium and radium give out γ rays. We may expect then that the very similar excited radio-activity which is present in the earth and atmosphere should also give rise to γ rays of a similar character. More recent work, however (section 286), indicates that this explanation is not sufficient to explain all the facts observed.

=280. Comparison of the radio-activity of the atmosphere with that produced by the radio-elements.= The radio-active phenomena observed in the earth and atmosphere are very similar in character to those produced by thorium and radium. Radio-active emanations are present in the air of caves and cellars, in natural carbonic acid, and in deep well water, and these emanations produce excited radio-activity on all bodies in contact with them. The question now arises whether these effects are due entirely to known radio-elements present in the earth or to unknown kinds of radio-active matter. The simplest method of testing this point is to compare the rate of decay of the radio-active product in the atmosphere with those of the known radio-active products of thorium and radium. A cursory examination of the facts at once shows that the radio-activity of the atmosphere is much more closely allied to effects produced by radium than to those due to thorium. The activity of the emanation released from well water, and also that sucked up from the earth, decays to half value in about 3·3 days, while the activity of the radium emanation decays to half value in an interval of 3·7 to 4 days. Considering the difficulty of making accurate determinations of these quantities, the rates of decay of the activity of the emanations from the earth and from radium agree within the limits of experimental error. A large number of observers have found that the radium emanation is present in the water of thermal springs and in the sediment deposited by them. Bumstead and Wheeler have shown that the emanation from the soil and surface water of New Haven is identical with that from radium. If the emanations from the earth and from radium are the same, the excited activities produced should have the same rate of decay. The emanation from well water in England approximately fulfils this condition (section 276), but an observation recorded by Ebert and Ewers (section 276) seems to show that the excited activity due to the emanation sucked up from the earth decays at a very slow rate compared with that due to radium.

Bumstead has given undoubted evidence that the thorium as well as the radium emanation is also present in the atmosphere at New Haven, while Dadourian has shown that it is emitted by New Haven soil. Blanc, and Elster and Geitel, have also found that thorium is present in the sediment from some thermal springs.

If the active matter in the atmosphere consists mainly of the radium emanation, the active deposit on a negatively charged wire, exposed in the open air, should initially consist of radium A, B and C. The curve of decay should be identical with the decay curve of the excited activity of radium, measured by the α rays, that is, there should be a rapid initial drop corresponding to the initial 3 minute change, then a slow rate of variation, the activity after several hours decaying to half value in about 28 minutes (see section 222). The rapid initial drop has been observed by Bumstead for the air at New Haven. Allan[429] did not observe this initial drop in Montreal, but found the activity fell to half value in about 45 minutes, reckoning from a time about 10 minutes after the removal of the active wire. This is about the rate of decay to be expected for the active deposit of radium over the same interval. Allan obtained evidence that there were several kinds of active matter deposited on the wire. For example, the activity transferred from the active wire to a piece of leather, moistened with ammonia, fell to half value in 38 minutes; for a piece of absorbent felt treated similarly, the activity fell to half value in 60 minutes, the normal time for the untreated wire being 45 minutes.

It is probable that this variation of the rate of decay is due to the fact that unequal proportions of radium B and C were transferred from the wire to the rubber. If a greater proportion of B than of C were removed, the decay would be slower and _vice versa_.

The fact that the activity of rain and snow falls to half value in about 30 minutes is a strong indication that the radium emanation is present in the atmosphere. The active matter with the rain and snow after standing some time would consist mainly of radium C and this should decay exponentially with the time, falling to half value in 28 minutes.

On account of the rapid decay of the thorium emanation—half value in one minute—it is not likely that much of the activity of the atmosphere can be ascribed to it. Its effect would be most marked near the surface of the soil.

There can be little doubt, that a large part of the radio-activity of the atmosphere is due to the radium emanation, which is continually diffusing into the atmosphere from the pores of the earth. Since radio-activity has been observed in the atmosphere at all points at which observations have, so far, been made, radio-active matter must be distributed in minute quantities throughout the soil of the earth. The volatile emanations escape into the atmosphere by diffusion, or are carried to the surface in spring water or by the escape of underground gases, and cause the radio-active phenomena observed in the atmosphere. The observation of Elster and Geitel that the radio-activity of the air is much less near the sea than inland is explained at once, if the radio-activity of the atmosphere is due mainly to the diffusion of emanations from the soil into the air above it.

The rare gases helium and xenon which exist in the atmosphere have been tested and found to be non-radio-active. The radio-activity of the air cannot be ascribed to a slight radio-activity possessed by either of these gases.

=281. Amount of the radium emanation in the atmosphere.= It is a matter of great interest to form an estimate of the amount of radium emanation present in the atmosphere, for since it comes from the earth, it indirectly serves as a means of estimating the amount of radium which is distributed over a thin crust of the earth.

Some experiments in this direction have been made by Eve in the laboratory of the writer. The experiments are not yet completed but the results so far obtained allow us to calculate the probable amount of emanation per cubic kilometre of the atmosphere near the earth.

Experiments were first made with a large iron tank 154 cms. square and 730 cms. deep, in a building in which no radium or other radio-active material had ever been introduced. The saturation ionization current for the air in the tank was first measured by means of an electroscope, connected with an insulated electrode passing up the centre of the closed tank. Assuming that the ionization in the tank was uniform, the number of ions produced per c.c. of the air in the tank was found to be 10. This is a considerably lower value than has usually been observed in a small closed vessel (see section 284). Cooke obtained the value 10 for a well cleaned brass electroscope, surrounded by lead, while Schuster obtained a value about 12 for the air in the laboratory of Owens College, Manchester.

In order to measure the amount of the excited activity from the tank, a central insulated wire was charged negatively to about 10,000 volts by a Wimshurst machine. After two hours, the wire was removed and wound on an insulated frame connected with a gold-leaf electroscope. The rate of decay of the activity on the wire was found to be about the same as for the excited activity produced by the radium emanation. In order to estimate the amount of radium emanation present in the large tank, special experiments were made with a smaller tank in which a known quantity of the radium emanation was introduced by employing a solution of pure radium bromide of known concentration. A central wire was made the negative electrode as before, and, after removal, it was wound on the frame and its activity tested. In this way it was found that the amount of radium emanation present in the large tank, in order to produce the excited activity observed, must have been equal to the equilibrium or maximum amount to be obtained from 9·5 × 10⁻⁹ grams of pure radium bromide. The volume of the large tank was 17 cubic metres, so that the amount of emanation present per cubic metre was equivalent to that liberated from 5·6 × 10⁻¹⁰ grams of radium bromide in radio-active equilibrium.

If the amount of the emanation in the tank is taken as the average amount existing in the outside air, _the amount of radium emanation present per cubic kilometre of the air is equivalent to that supplied by 0·56 grams of radium bromide_.