Chapter XIV (2)

For the purpose of calculation, suppose the emanation is uniformly distributed over the land portion of the earth (¼ of the total surface), and to extend to an average height of 5 kilometres. The air over the sea is not taken into account as its radio-activity has not been examined. The total amount of emanation present in the atmosphere under these conditions corresponds to that supplied by about 400 tons of radium bromide. In order to maintain this amount of emanation in the atmosphere, it must be supplied at a constant rate from the earth’s surface. Since the greater amount of the emanation probably escapes into the air by transpiration and diffusion through the soil, the emanation cannot reach the surface except from a very thin layer of the earth. The probable thickness of this layer can be estimated if it is assumed that the present loss of heat from the earth is supplied from the radio-active matter contained in it. We have seen (section 271) that, on this hypothesis, there must be an amount of active matter in the earth corresponding to about 300 million tons of radium. If this is supposed to be uniformly distributed, a thickness of layer of about 13 metres will suffice to maintain the calculated amount of emanation in the atmosphere. This thickness of layer is about the order of magnitude to be expected from general considerations.

These results lead indirectly to the conclusion that a large amount of emanation does undoubtedly exist in the surface crust of the earth.

Experiments were also made by Eve with a large zinc cylinder exposed in the open air. Volume for volume, the average amount of excited activity derived from it was only about one-third of that obtained from the large iron tank. This would reduce the amount of emanation, previously deduced, to about one-third.

Before such calculations can be considered at all definite, it will be necessary to make comparative measurements of the amount of emanation in the atmosphere at various parts of the earth. The air at Montreal is not abnormally active, so that the calculations probably give the right order of magnitude of the quantities.

Eve also observed that the amount of activity to be obtained per unit length of the wire in the zinc cylinder of about 70 cms. in diameter was about the same as for a wire ·5 mms. in diameter charged to 10,000 volts in the open air, supported 20 feet from the ground. This shows that such a potential does not draw in the carriers of excited activity which are more than half a metre away, and probably the range is even less.

It is of great importance to find how large a proportion of the number of ions produced in the atmosphere is due to the radio-active matter distributed throughout it. The results of Eve with the large iron tank, already referred to, indicate that a large proportion of the ionization in the tank was due to the radio-active matter contained in it, for the ratio of the excited activity on the central electrode to the total ionization current in the tank was about ⁷⁄₁₀ of the corresponding ratio for a smaller tank into which a supply of the radium emanation had been introduced.

This result requires confirmation by experiments at other parts of the earth, but the results point to the conclusion that a large part, if not all, of the ionization at the earth’s surface is due to radio-active matter distributed in the atmosphere. A constant rate of production of 30 ions per second per c.c. of air, which has been observed in the open air at the surface of the earth in various localities, would be produced by the presence in each c.c. of the air of the amount of emanation liberated from 2·4 × 10⁻¹⁵ grams of radium bromide in radio-active equilibrium. It is not likely, however, that the ionization of the upper part of the atmosphere is due to this cause alone. In order to explain the maintenance of the large positive charge, which generally exists in the upper atmosphere, there must be a strong ionization of the upper air, which may possibly be due to ionizing radiations emitted by the sun.

=282. Ionization of atmospheric air.= A large number of measurements have been made during the last few years to determine the relative amount of ionization in the atmosphere in different localities and at different altitudes. Measurements of this character were first undertaken by Elster and Geitel with a special type of electroscope. A charged body exposed to the air was attached to a portable electroscope, and the rate of loss of charge was observed by the movement of the gold or aluminium leaf. The rates of discharge of the electroscope for positive and negative electricity were generally different, the ratio depending on the locality and the altitude, and on the meteorological conditions. This apparatus is not suitable for quantitative measurements and the deductions to be drawn from the observations are of necessity somewhat indefinite.

Ebert[430] has designed a portable apparatus in which the number of ions per c.c. of the air can be determined easily. A constant current of air is drawn between two concentric cylinders by means of a fan actuated by a falling weight. The inner cylinder is insulated and connected with an electroscope. Knowing the capacity of the apparatus, and the velocity of the current of air, the rate of movement of the gold-leaf affords a measure of the number of ions present in unit volume of the air drawn between the cylinders.

In this way Ebert found that the number of ions in the air was somewhat variable, but on an average corresponded to about 2600 per c.c. in the particular locality where the measurements were made.

This is the equilibrium number of ions present per c.c. when the rate of production balances the rate of recombination. If _q_ is the number of ions produced per second per unit volume of the air and _n_ is the equilibrium number, then _q_ = α_n²_ where α is the constant of recombination (section 30).

By a slight addition to the apparatus of Ebert, Schuster[431] has shown that the constant of recombination for the particular sample of air under investigation can be determined. The value so obtained for air in the neighbourhood of Manchester was variable, and two or three times as great as for dust-free air. The results of some preliminary measurements showed that the number of ions present per c.c. of the air in different localities varied from 2370 to 3660, while the value of _q_, the number of ions produced per c.c. per second, varied between 12 and 38·5.

Rutherford and Allan and Eberts showed that the ions in the air had about the same mobility as the ions produced in air by Röntgen rays and radio-active substances. In some recent determinations by Mache and Von Schweidler[432], the velocity of the positive ion was found to be about 1·02 cms. per second, and that of the negative 1·25 cms., for a potential gradient of one volt per cm.

Langevin[433] has recently shown that in addition to these swift moving ions, there are also present in the atmosphere some ions which travel extremely slowly in an electric field. The number of these slowly moving ions in the air in Paris is about 40 times as great as the number of the swifter ions. This result is of great importance, for in the apparatus of Ebert these ions escape detection, since the electric field is not strong enough to carry them to the electrodes during the time of their passage between the cylinders.

=283. Radio-activity of ordinary materials.= It has been shown that radio-active matter seems to be distributed fairly uniformly over the surface of the earth and in the atmosphere. The very important question arises whether the small radio-activity observed is due to known or unknown radio-elements present in the earth and atmosphere, or to a feeble radio-activity of matter in general, which is only readily detectable when large quantities of matter are present. The experimental evidence is not yet sufficient to answer this question, but undoubted proof has been obtained that many of the metals show a very feeble radio-activity. Whether this radio-activity is due to the presence of a slight trace of the radio-elements or is an actual property of the metals themselves will be discussed in more detail in section 286.

Schuster[434] has pointed out that every physical property hitherto discovered for one element has been found to be shared by all the others in varying degrees. For example, the property of magnetism is most strongly marked in iron, nickel, and cobalt, but all other substances are found to be either feebly magnetic or diamagnetic. It might thus be expected on general principles that all matter should exhibit the property of radio-activity in varying degrees. On the view developed in chapter X., the presence of this property is an indication that the matter is undergoing change accompanied by the expulsion of charged particles. It does not, however, by any means follow that because the atom of one element in the course of time becomes unstable and breaks up, that, therefore, the atoms of all the other elements pass through similar phases of instability.

It has already been mentioned (section 8), that Mme Curie made a very extensive examination of most of the elements and their compounds for radio-activity. The electric method was used, and any substance possessing an activity of ¹⁄₁₀₀ of that of uranium would certainly have been detected. With the exception of the known radio-elements and the minerals containing uranium and thorium, no other substances were found to be radio-active even to that degree.

Certain substances like phosphorus[435] possess the property of ionizing a gas under special conditions. The air which is drawn over the phosphorus is conducting, but it has not yet been settled whether this conductivity is due merely to ions formed at the surface of the phosphorus or to ions produced by the phosphorus nuclei or emanations, as they have been termed, which are carried along with the current of air. It does not however appear that the ionization of the gas is in any way due to the presence of a penetrating type of radiation such as is emitted by the radio-active bodies. Le Bon (section 8) observed that quinine sulphate, after being heated to a temperature below the melting point and then allowed to cool, showed for a time strong phosphorescence and was able rapidly to discharge an electroscope. The discharging action of quinine sulphate under varying conditions has been very carefully examined by Miss Gates[436]. The ionization could not be observed through thin aluminium foil or gold-leaf, but appeared to be confined to the surface of the sulphate. The current observed by an electrometer was found to vary with the direction of the electric field, indicating that the positive and negative ions had very different mobilities. The discharging action appears to be due either to an ionization of the gas very close to the surface by some short ultra-violet light waves, accompanying the phosphorescence, or to a chemical action taking place at the surface.

Thus, neither phosphorus nor quinine sulphate can be considered to be radio-active, even under the special conditions when they are able to discharge an electrified body. No evidence in either case has been found that the ionization is due to the emission of a penetrating radiation.

No certain evidence has yet been obtained that any body can be made radio-active by exposure to Röntgen rays or cathode rays. A metal exposed to the action of Röntgen rays gives rise to a secondary radiation which is very readily absorbed in a few centimetres of air. It is possible that this secondary radiation may prove to be analogous in some respects to the α rays from the radio-elements. The secondary radiation, however, ceases immediately the Röntgen rays are cut off. Villard[437] stated that a piece of bismuth produced a feeble photographic action after it had been exposed for some time to the action of the cathode rays in a vacuum. It has not however been shown that the bismuth gives out rays of a character similar to those of the radio-active bodies. The experiments of Ramsay and Cooke on the production of apparent activity in inactive matter by the radiations from radium have already been discussed in section 264.

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:

Material of Number of ions Observer
vessel produced per
c.c. per second

Silvered 36 C. T. R. Wilson
glass

Brass 26 „ „

Zinc 27 Rutherford and
Allan

Glass 53 to 63 Harms

Iron 61 Patterson

Cleaned 10 Cooke
brass

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:

Gas Relative (Relative
ionization ionization) /
(density)

Air 1·00 1·00

Hydrogen 0·184 2·7

Carbon dioxide 1·69 1·10

Sulphur 2·64 1·21
dioxide

Chloroform 4·7 1·09

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:

Material of Leakage in
lining of scale divisions
vessel per hour

Tinfoil 3·3

„ another 2·3
sample

Glass coated 1·3
with phosphoric
acid

Silver 1·6
chemically
deposited on
glass

Zinc 1·2

Lead 2·2

Copper (clean) 2·3

„ (oxidized) 1·7

Platinum 2·0, 2·9, 3·9
(various
samples)

Aluminium 1·4

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.

Footnote 384:

Geitel, _Phys. Zeit._ 2, p. 116, 1900.

Footnote 385:

C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 32, 1900. _Proc. Roy.
Soc._ 68, p. 151, 1901.

Footnote 386:

Elster and Geitel, _Phys. Zeit._ 2, p. 590, 1901.

Footnote 387:

Elster and Geitel, _Phys. Zeit._ 3, p. 76, 1901.

Footnote 388:

Rutherford and Allan, _Phil. Mag._ Dec. 1902.

Footnote 389:

Allan, _Phil. Mag._ Feb. 1904.

Footnote 390:

C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 428, 1902.

Footnote 391:

C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 428, 1902; 12, p. 17,
1903.

Footnote 392:

C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 12, p. 85, 1903.

Footnote 393:

Allan, _Phys. Rev._ 16, p. 106, 1903.

Footnote 394:

McLennan, _Phys. Rev._ 16, p. 184, 1903.

Footnote 395:

Schmauss, _Annal. d. Phys._ 9, p. 224, 1902.

Footnote 396:

Elster and Geitel, _Phys. Zeit_. 3, p. 574, 1902.

Footnote 397:

Ebert and Ewers, _Phys. Zeit._ 4, p. 162, 1902.

Footnote 398:

Sarasin, Tommasina and Micheli, _C. R._ 139, p. 917, 1905.

Footnote 399:

J. J. Thomson, _Phil. Mag._ Sept. 1902.

Footnote 400:

Ebert, _Sitz. Akad. d. Wiss. Munich_, 33, p. 133, 1903.

Footnote 401:

J. J. Thomson, _Phil. Mag._ Sept. 1902.

Footnote 402:

Adams, _Phil. Mag._ Nov. 1903.

Footnote 403:

Bumstead and Wheeler, _Amer. Journ. Science_, 17, p. 97, Feb. 1904.

Footnote 404:

Bumstead, _Amer. Journ. Science_, 18, July, 1904.

Footnote 405:

Dadourian, _Amer. Journ. Science_, 19, Jan. 1905.

Footnote 406:

H. S. Allen and Lord Blythswood, _Nature_, 68, p. 343, 1903; 69, p.
247, 1904.

Footnote 407:

Strutt, _Proc. Roy. Soc._ 73, p. 191, 1904.

Footnote 408:

Himstedt, _Ann. d. Phys._ 13, p. 573, 1904.

Footnote 409:

Elster and Geitel, _Phys. Zeit._ 5, No. 12, p. 321, 1904.

Footnote 410:

Dorn, _Abhandl. d. Natur. Ges. Halle_, 25, p. 107, 1904.

Footnote 411:

Schenck, Thesis Univ. Halle, 1904.

Footnote 412:

Mache, _Wien. Ber._ 113, p. 1329, 1904.

Footnote 413:

Curie and Laborde, _C. R._ 138, p. 1150, 1904.

Footnote 414:

Blanc, _Phil. Mag._ Jan. 1905.

Footnote 415:

Boltwood, _Amer. Journ. Science_, 18, Nov. 1904.

Footnote 416:

Elster and Geitel, _Phys. Zeit._ 4, p. 522, 1903.

Footnote 417:

Elster and Geitel, _Phys. Zeit._ 5, No. 1, p. 11, 1903.

Footnote 418:

Vincenti and Levi Da Zara, _Atti d. R. Instit. Veneto d. Scienze_, 54,
p. 95, 1905.

Footnote 419:

Burton, _Phil. Mag._ Oct. 1904.

Footnote 420:

Elster and Geitel, _Phys. Zeit._ 6, No. 3, p. 67, 1905.

Footnote 421:

Rutherford and Allan, _Phil. Mag._ Dec. 1902.

Footnote 422:

Elster and Geitel, _Phys. Zeit._ 4, p. 138, 1902; 4, p. 522, 1903.

Footnote 423:

Saake, _Phys. Zeit._ 4, p. 626, 1903.

Footnote 424:

Simpson, _Proc. Roy. Soc._ 73, p. 209, 1904.

Footnote 425:

McLennan, _Phys. Rev._ 16, p. 184, 1903, and _Phil. Mag._ 5, p. 419,
1903.

Footnote 426:

McLennan, _Phys. Rev._ No. 4, 1903.

Footnote 427:

Rutherford and Cooke, _Americ. Phys. Soc._ Dec. 1902.

Footnote 428:

Cooke, _Phil. Mag._ Oct. 1903.

Footnote 429:

Allan, _Phil. Mag._ Feb. 1904.

Footnote 430:

Ebert, _Phys. Zeit._ 2, p. 622, 1901. _Zeitschr. f. Luftschiffahrt_,
4, Oct. 1902.

Footnote 431:

Schuster, _Proc. Manchester Phil. Soc._ p. 488, No. 12, 1904.

Footnote 432:

Mache and Von Schweidler, _Phys. Zeit._ 6, No. 3, p. 71, 1905.

Footnote 433:

Langevin, _C. R._ 140, p. 232, 1905.

Footnote 434:

Schuster, British Assoc. 1903.

Footnote 435:

J. J. Thomson, _Conduction of Electricity through Gases_, p. 324,
1903.

Footnote 436:

Miss Gates, _Phys. Rev._ 17, p. 499, 1903.

Footnote 437:

Villard, _Société de Physique_, July, 1900.

Footnote 438:

Geitel, _Phys. Zeit._ 2, p. 116, 1900.

Footnote 439:

C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 52, 1900. _Proc. Roy.
Soc._ 68, p. 152, 1901.

Footnote 440:

Rutherford and Allan, _Phil. Mag._ Dec. 1902.

Footnote 441:

Patterson, _Phil. Mag._ August, 1903.

Footnote 442:

Harms, _Phys. Zeit._ 4, No. 1, p. 11, 1902.

Footnote 443:

Cooke, _Phil. Mag._ Oct. 1903.

Footnote 444:

Wilson, _Proc. Roy. Soc._ 69, p. 277, 1901.

Footnote 445:

Jaffé, _Phil. Mag._ Oct. 1904.

Footnote 446:

Patterson, _Phil. Mag._ Aug. 1903.

Footnote 447:

Strutt, _Phil. Mag._ June, 1903. _Nature_, Feb. 19, 1903.

Footnote 448:

McLennan and Burton, _Phys. Rev._ No. 4, 1903. J. J. Thomson,
_Nature_, Feb. 26, 1903.

Footnote 449:

Cooke, _Phil. Mag._ Aug. 6, 1903. Rutherford, _Nature_, April 2, 1903.

Footnote 450:

Eve, _Nature_, March 16, 1905.

Footnote 451:

See article in _Le Radium_, No. 3, p. 81, Sept. 15, 1904.

Footnote 452:

J. J. Thomson, _Proc. Camb. Phil. Soc._ 12, p. 391, 1904.

Footnote 453:

Wood, _Phil. Mag._ April, 1905.

Footnote 454:

Campbell, _Nature_, p. 511, March 31, 1904. _Phil. Mag._ April, 1905.