The existence of a very feeble radio-activity of ordinary matter has been deduced from the study of the conductivity of gases in closed vessels. The conductivity is extremely minute, and special methods are required to determine it with accuracy. A brief account will now be given of the gradual growth of our knowledge on this important question.

284. Conductivity of air in closed vessels. Since the time of Coulomb onwards several investigators have believed that a charged conductor placed inside a closed vessel lost its charge more rapidly than could be explained by the conduction leak across the insulating support. Matteucci, as early as 1850, observed that the rate of loss of charge was independent of the potential. Boys, by using quartz insulators of different lengths and diameters, arrived at the conclusion that the leakage must in part take place through the air. This loss of charge in a closed vessel was believed to be due in some way to the presence of dust particles in the air.

On the discovery that gases become temporary conductors of electricity under the influence of Röntgen rays and the rays from radio-active substances, attention was again drawn to this question. Geitel^[438] and C. T. R. Wilson^[439] independently attacked the problem, and both came to the conclusion that the loss of charge was due to a constant ionization of the air in the closed vessel. Geitel employed in his experiments an apparatus similar to that shown in Fig. 103. The loss of charge of an Exner electroscope, with the cylinder of wire netting Z attached, was observed in a closed vessel containing about 30 litres of air. The electroscope system was found to diminish in potential at the rate of about 40 volts per hour, and this leakage was shown not to be due to a want of insulation of the supports.

Wilson, on the other hand, used a vessel of very small volume, in order to work with air which could be completely freed from dust. In the first experiments a silvered glass vessel with a volume of only 163 c.c. was employed. The experimental arrangement is shown in Fig. 104.

The conductor, of which the loss of charge was to be measured, was placed near the centre of the vessel A. It consisted of a narrow strip of metal with a gold-leaf attached. The strip of metal was fixed to the upper rod by means of a small sulphur bead. The upper rod was connected with a sulphur condenser with an Exner electroscope B attached to indicate its potential. The gold-leaf system was initially charged to the same potential as the upper rod and condenser by means of a fine steel wire which was caused to touch the gold-leaf system by the attraction of a magnet brought near it. The rate of movement of the gold-leaf was measured by means of a microscope provided with a micrometer eye-piece. By keeping the upper rod at a slightly higher potential than the gold-leaf system, it was ensured that the loss of charge of the gold-leaf system should not be due in any way to a conduction leakage across the sulphur bead.

The method employed by Wilson in these experiments is very certain and convenient when an extremely small rate of discharge is to be observed. In this respect the electroscope measures with certainty a rate of loss of charge much smaller than can be measured by a sensitive electrometer.

Both Geitel and Wilson found that the leakage of the insulated system in dust-free air was the same for a positive as for a negative charge, and was independent of the potential over a considerable range. The leakage was the same in the dark as in diffuse daylight. The independence of leakage of the potential is strong evidence that the loss of charge is due to a constant ionization of the air. When the electric field acting on the gas exceeds a certain value, all the ions are carried to the electrodes before recombination occurs. A saturation current is reached, and it will be independent of further increase of the electric field, provided, of course, a potential sufficiently high to cause a spark to pass is not applied.

C. T. R. Wilson has recently devised a striking experiment to show the presence of ions in dust-free air which is not exposed to any external ionizing agency. Two large metal plates are placed in a glass vessel connected with an expansion apparatus similar to that described in section 34. On expanding the air, the presence of the ions is shown by the appearance of a slight cloud between the plates. These condensation nuclei carry an electric charge, and are apparently similar in all respects to the ions produced in gases by X rays, or by the rays from active substances.

Wilson found that the loss of charge of the insulated system was independent of the locality. The rate of discharge was unaltered when the apparatus was placed in a deep tunnel, so that it did not appear that the loss of charge was due to an external radiation. From experiments already described, however (section 279), it is probable that about 30 per cent. of the rate of discharge observed was due to a very penetrating radiation. This experiment of Wilson’s indicates that the intensity of the penetrating radiation was the same in the tunnel as at the earth’s surface. Wilson found that the ionization of the air was about the same in a brass vessel as in one of glass, and came to the conclusion that the air was spontaneously ionized.

Using a brass vessel of volume about 471 c.c., Wilson determined the number of ions that must be produced in air per unit volume per second, in order to account for the loss of charge of the insulated system. The leakage system was found to have a capacity of about 1·1 electrostatic units, and lost its charge at the rate of 4·1 volts per hour for a potential of 210 volts, and 4·0 volts per hour for a potential of 120 volts. Taking the charge on an ion as 3·4 × 10^-10 electrostatic units, this corresponds to a production of 26 ions per second.

Rutherford and Allan^[440] repeated the results of Geitel and Wilson, using an electrometer method. The saturation current was observed between two concentric zinc cylinders of diameter 25·5 and 7·5 cms. respectively and length 154 cms. It was found that the saturation current could practically be obtained with a potential of a few volts. Saturation was however obtained with a lower voltage after the air had remained undisturbed in the cylinders for several days. This was probably due to the gradual settling of the dust originally present in the air.

Later observations of the number of ions produced in air in sealed vessels have been made by Patterson^[441], Harms^[442], and Cooke^[443]. The results obtained by different observers are shown in the following table. The value of the charge on an ion is taken as 3·4 × 10^-10 electrostatic units:

It will be shown later that the differences in these results are probably due to differences in the radio-activity of the containing vessel.

285. Effect of pressure and nature of gas. C. T. R. Wilson (loc. cit.) found that the rate of leakage of a charged conductor varied approximately as the pressure of the air between the pressures examined, viz. 43 mms. and 743 mms. of mercury. These results point to the conclusion that, in a good vacuum, a charged body would lose its charge extremely slowly. This is in agreement with an observation of Crookes, who found that a pair of gold-leaves retained their charge for several months in a high vacuum.

Wilson^[444] at a later date investigated the leakage for different gases. The results are included in the following table, where the ionization produced in air is taken as unity:

With the exception of hydrogen, the ionization produced in different gases is approximately proportional to their density. The relative ionization is very similar to that observed by Strutt (section 45) for gases exposed to the influence of the α and β rays from radio-active substances, and points to the conclusion that the ionization observed may be due either to a radiation from the walls of the vessel or from external sources.

Jaffé^[445] has made a careful examination of the natural ionization in the very heavy gas nickel-carbonyl, Ni(CO)₄, in a small silvered glass vessel. The ionization of this gas was 5·1 times that of air at normal pressure while its density is 5·9 times that of air. The leak of the electroscope was nearly proportional to the pressures except at low pressure, when the leak was somewhat greater than would be expected if the pressure law held. The fact that a gas of such high density and complicated structure behaves like the simpler and lighter gases is a strong indication that the ionization itself is due to a radiation from the walls of the vessel and not to a spontaneous ionization of the gas.

Patterson^[446] examined the variation of the ionization of air with pressure in a large iron vessel of diameter 30 cms. and length 20 cms. The current between a central electrode and the cylinder was measured by means of a sensitive Dolezalek electrometer. He found that the saturation current was practically independent of the pressure for pressures greater than 300 mms. of mercury. Below a pressure of 80 mms. the current varied directly as the pressure. For air at atmospheric pressure, the current was independent of the temperature up to 450° C. With further increase of temperature, the current began to increase, and the increase was more rapid when the central electrode was charged negatively than when it was charged positively. This difference was ascribed to the production of positive ions at the surface of the iron vessel. The results obtained by Patterson render it very improbable that the ionization observed in air is due to a spontaneous ionization of the enclosed air: for we should expect the amount of this ionization to depend on the temperature of the gas. On the other hand, these results are to be expected if the ionization of the enclosed air is mainly due to an easily absorbed radiation from the walls of the vessel. If this radiation had a penetrating power about equal to that observed for the α rays of the radio-elements, the radiation would be absorbed in a few centimetres of air. With diminution of pressure, the radiations would traverse a greater distance of air before complete absorption, but the total ionization produced by the rays would still remain about the same, until the pressure was reduced sufficiently to allow the radiation to traverse the air space in the vessel without complete absorption. With still further diminution of pressure, the total ionization produced by the radiation, and in consequence the current observed, would vary directly as the pressure.

286. Examination of ordinary matter for radio-activity. Strutt^[447], McLennan and Burton^[448], and Cooke^[449], independently observed about the same time that ordinary matter is radio-active to a slight degree. Strutt, by means of an electroscope, observed that the ionization produced in a closed vessel varied with the material of the vessel. A glass vessel with a removable base was employed and the vessel was lined with the material to be examined. The following table shows the relative results obtained. The amount of leakage observed is expressed in terms of the number of scale divisions of the eye-piece passed over per hour by the gold-leaf:

There are thus marked differences in the leakage observed for different materials and also considerable differences in different samples of the same metal. For example, one specimen of platinum caused nearly twice the leakage of another sample from a different stock.

McLennan and Burton, on the other hand, measured by means of a sensitive electrometer the ionization current produced in the air in a closed iron cylinder 25 cms. in diameter and 130 cms. in length, in which an insulated central electrode was placed. The open cylinder was first exposed for some time at the open window of the laboratory. It was then removed, the top and bottom closed, and the saturation current through the gas determined as soon as possible. In all cases it was observed that the current diminished for two or three hours to a minimum and then very slowly increased again. In one experiment, for example, the initial current observed corresponded to 30 on an arbitrary scale. In the course of four hours the current fell to a minimum of 6·6, and 44 hours later had risen to a practical maximum of 24. The initial decrease observed is probably due to a radio-activity of the enclosed air or walls of the vessel, which decayed rapidly with the time. The decay of the excited activity produced on the interior surface of the cylinder when exposed to the air was probably responsible for a part of the decrease observed. McLennan ascribes the increase of current with time to a radio-active emanation which is given off from the cylinder, and ionizes the enclosed air. On placing linings of lead, tin, and zinc in the iron cylinder, considerable differences were observed both for the minimum current and also for the final maximum. Lead gave about twice the current due to zinc, while tin gave an intermediate value. These results are similar in character to those obtained by Strutt.

McLennan and Burton also investigated the effect of diminution of pressure on the current. The cylinder was filled with air to a pressure of 7 atmospheres, and allowed to stand until the current reached a constant value. The air was then allowed to escape and the pressure reduced to 44 mms. of mercury. The current was found to vary approximately as the pressure over the whole range. These results are not in agreement with the results of Patterson already described, nor with some later experiments of Strutt. McLennan’s results however point to the conclusion that the ionization was mainly due to an emanation emitted from the metal. Since the air was rapidly removed, a proportionate amount of the emanation would be removed also, and it might thus be expected that the current would vary directly as the pressure. If this is the case the current through the gas at low pressures should increase again to a maximum if time is allowed for a fresh emanation to form.

H. L. Cooke, using an electroscopic method, obtained results very similar to those given by Strutt. Cooke observed that a penetrating radiation was given out from brick. When a brass vessel containing the gold-leaf system was surrounded by brick, the discharge of the electroscope was increased by 40 to 50 per cent. This radiation was of about the same penetrating power as the rays from radio-active substances. The rays were completely absorbed by surrounding the electroscope with a sheet of lead 2 mms. in thickness. This result is in agreement with the observation of Elster and Geitel, already mentioned, that radio-active matter was present in clay freshly dug up from the earth.

Cooke also observed that the ionization of the air in a brass electroscope could be reduced to about one-third of its usual value if the interior surface of the brass was carefully cleaned. By removing the surface of the brass he was able to reduce the ionization of the enclosed air from 30 to 10 ions per c.c. per second. This is an important observation, and indicates that a large proportion of the radio-activity observed in ordinary matter is due to a deposit of radio-active matter on its surface. It has already been shown that bodies which have been exposed in the presence of the radium emanation retain a residual activity which decays extremely slowly. There can be no doubt that the radium emanation is present in the atmosphere, and the exposed surface of matter, in consequence, will become coated with an invisible film of radio-active matter, deposited from the atmosphere. On account of the slow decay of this activity it is probable that the activity of matter exposed in the open air would steadily increase for a long interval. Metals, even if they are originally inactive, would thus acquire a fairly permanent activity, but it should be possible to get rid of this by removing the surface of the metal or by chemical treatment. The rapid increase of activity of all matter left in a laboratory in which a large quantity of emanation has been released has been drawn attention to by Eve^[450]. This superficial activity, due to the products radium D, E, and F, was mainly removed by placing the metal in strong acid.

A number of experiments have been made by J. J. Thomson, N. R. Campbell, and A. Wood in the Cavendish laboratory to examine whether the radio-activity observed in ordinary matter is a specific property of such matter or is due to the presence of some radio-active impurity. An account of these experiments was given by Professor J. J. Thomson in a discussion on the Radio-activity of Ordinary Matter at the British Association meeting at Cambridge, 1904. The results^[451], as a whole, support the view that each substance gives out a characteristic type or types of radiation and that the radiation is a specific property of the substance. J. J. Thomson^[452] has made experiments to observe the action of different substances in cutting off the external very penetrating radiation (section 279) observed by Cooke and McLennan. He found that some substances cut off this external radiation, while others had little if any effect. For example, the ionization in a closed vessel was reduced 17 per cent. by surrounding it with a thick lead envelope; but, on surrounding it with an equivalent absorbing thickness of water, or water mixed with sand, no sensible diminution was observed. In other experiments Wood^[453] found that the diminution of the ionization by a given screen depended upon the material of the vessel. For example, the ionization in a lead vessel, surrounded by a lead screen, was reduced 10 per cent., while in an iron vessel it was reduced 24 per cent. He concludes from his experiments that the ionization observed in a closed vessel has a threefold origin. Part of it is due to an external penetrating radiation, part to a secondary radiation set up by it, while the remainder is due to an intrinsic radiation from the walls, altogether independent of the external radiation.

In some experiments of Campbell^[454], the variation of the ionization current between two parallel plates was observed for a progressive increase of the distance between them. The effects observed are shown in Fig. 105. The curves at first rise rapidly, then bend over and finally become a straight line. The knee of the curve is at a different distance for the different substances. The shape of these curves indicates that two types of radiation are present, one of which is readily absorbed in the gas while the other, a more penetrating type of radiation, extends over the whole distance between the plates. In another series of experiments, one side of the testing vessel was of thin aluminium, and the ionization current was observed when an exterior screen was brought up to it. Lead gave a considerable increase, but the radiation from it was readily absorbed by an interposed screen. The radiation emitted by carbon and zinc was more than twice as penetrating as from lead.

Attempts were made to see whether a radio-active emanation was given off by dissolving solid substances and then keeping the solutions in a closed vessel and afterwards testing the activity of the air drawn from them. In some cases an emanation was observed, but the amount varied with different specimens of the same material; in others no effect was detected.

When linings of different substances were placed in a closed testing vessel, the ionization current in most cases fell at first, passed through a minimum, and then slowly increased to a maximum. For lead the maximum was reached in 9 hours, for tin in 14 and for zinc in 18 hours. These results indicate that an emanation is given off from the metal, and that the amount reaches a maximum value at different intervals in the various cases. This was confirmed by an examination of a piece of lead which was left in radium-free nitric acid. Twenty times the normal effect was observed after this treatment. This is probably due to the increase of porosity of the lead which allows a greater fraction of the emanation produced in the metal to diffuse out with the gas.

The activity observed in ordinary matter is extremely small. The lowest rate of production of ions yet observed is 10 per cubic centimetre per second in a brass vessel. Suppose a spherical brass vessel is taken of capacity 1 litre. The area of the interior surface would be about 480 sq. cms. and the total number of ions produced per second would be about 10⁴. Now it has been shown, in section 252, that an α particle projected from radium itself gives rise to 8·6 × 10⁴ ions before it is absorbed in the gas. An expulsion of one α particle every 8 seconds from the whole vessel, or of one α particle from each square centimetre of surface per hour would thus account for the minute conductivity observed. Even if it were supposed that this activity is the result of a breaking up of the matter composing the vessel, the disintegration of one atom per second per gram, provided it was accompanied by the expulsion of an α particle, would fully account for the conductivity observed.

While the experiments, already referred to, afford strong evidence that ordinary matter does possess the property of radio-activity to a feeble degree, it must not be forgotten that the activity observed is excessively minute, compared even with a weak radio-active substance like uranium or thorium. The interpretation of the results is complicated, too, by the presence of the radium emanation in the atmosphere, for we have seen that the surface of every body exposed to the open air must become coated with the slowly changing transformation products of the radium emanation. The distribution of radio-active matter throughout the constituents of the earth renders it difficult to be certain that any substance, however carefully prepared, is freed from radio-active impurities. If matter in general is radio-active, it must be undergoing transformation at an excessively slow rate, unless it be supposed (see Appendix A) that changes of a similar character to those observed in the radio-elements may occur without the appearance of their characteristic radiations.