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.
[Illustration: Fig. 103.]
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_.
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.
[Illustration: 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.
[Illustration: Fig. 105.]
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.
APPENDIX A.
PROPERTIES OF THE α RAYS.
A brief account is given here of some investigations made by the writer
on the properties of the α rays from radium—investigations which were
not completed in time for the results to be incorporated in the text.
The experiments were undertaken primarily with a view of determining
accurately the value of _e_/_m_ of the α particle from radium, in order
to settle definitely whether or not it is an atom of helium. In the
previous experiments of the writer, Becquerel, and Des Coudres, on this
subject (sections 89, 90, and 91), a thick layer of radium in
radio-active equilibrium has been used as a source of α rays. Bragg
(section 103) has shown that the rays emitted from radium under such
conditions are complex, and consist of particles projected over a
considerable range of velocity. In order to obtain a homogeneous pencil
of rays it is necessary to use a very thin layer of a simple
radio-active substance as a source of rays. In the experiments that
follow, this condition was fulfilled by using a fine wire which was made
active by exposure for several hours in the presence of a large quantity
of radium emanation. By charging the wire negatively the active deposit
was concentrated upon the wire, which was made intensely active. The
active deposit initially contains radium A, B, and C. The activity of
radium A practically disappears in about fifteen minutes, and the α
radiation is then due entirely to the single product radium C, since
radium B is a rayless product. The activity of radium C decreases to
about 15 per cent. of its initial value after two hours.
=Magnetic deflection of the α rays.= The photographic method was
employed to determine the deviation of the pencil of rays in a magnetic
field. The experimental arrangement is shown in Fig. 106. The rays from
the active wire, which was placed in a slot, passed through a narrow
slit and fell normally on a photographic plate, placed at a known
distance above the slit. The apparatus was enclosed in a brass tube
which could be exhausted rapidly to a low pressure by means of a Fleuss
pump. The apparatus was placed in a strong uniform magnetic field
parallel to the plane of the slit. The magnetic field was reversed every
ten minutes, so that on developing the plate two narrow bands were
observed, the distance between which represented twice the deviation
from the normal of the pencil of rays by the magnetic field. The width
of the band was found to be the same whether the magnetic field was
applied or not, showing that the pencil of rays was homogeneous and
consisted of α particles projected with the same velocity.
[Illustration: Fig. 106.]
By placing the photographic plate at different distances from the slit
it was found that the rays, after entering the magnetic field, described
the arc of a circle of radius ρ equal to 42·0 cms. The strength of field
_H_ was 9470 C.G.S. units, so that the value of _H_ρ for the α particles
expelled from radium C is 398,000. This is in good agreement with the
maximum values of _H_ρ, previously found for radium rays (see section
92).
The electric deviation of the rays from radium C has not yet been
accurately measured, but an approximate determination of _e_/_m_ for the
α particles can be obtained by assuming that the heating effect of
radium C is a measure of the kinetic energy of the α particles expelled
from it. We have seen in section 246 that the heating effect of the
radium C present in one gram of radium in radio-active equilibrium is 31
gram calories per hour, which corresponds to an emission of energy of
3·6 × 10⁵ ergs per second. Now when radio-active equilibrium is reached,
the number of α particles expelled from radium C per second is equal to
the number of α particles expelled per second from radium at its minimum
activity. This number, _n_, is 6·2 × 10¹⁰ (section 93).
Then ½ _mnv²_ = 3·6 × 10⁵,
or (_m_/_e_)_v²_ = 1·03 × 10¹⁶,
substituting the value of _n_, and the value of the ionic charge _e_.
The value of _e_ in this case has not been assumed, since _n_ = _i_/_e_,
where _i_ was the measured current due to the charge carried by the α
rays.
From the magnetic deflection, it is known that
(_m_/_e_)_v_ = 3·98 × 10⁵.
From these two equations we obtain
_v_ = 2·6 × 10⁹ cms. per second.
_e_/_m_ = 6·5 × 10³ electromagnetic units.
These values are in surprisingly good agreement with the previous values
of the writer and Des Coudres (section 91). On account of the
uncertainty attaching to the value of _n_, not much weight can be
attached to the determination by this method of the constants of the α
particles.
=Decrease of velocity of the α particles in passing through matter.=
Some experiments were made to determine the velocity of the α particles
from radium C after passing through known thicknesses of aluminium. The
previous apparatus was employed, and the distance between the
photographic bands was observed for successive layers of aluminium foil,
each ·00031 cms. thick, placed over the active wire. The photographic
plate was placed 2 cms. above the slit, and the magnetic field extended
1 cm. below the slit. The amount of deviation of the rays is inversely
proportional to their velocity after traversing the aluminium screens.
The impressions on the plate were clear and distinct, and about the same
in all cases, showing that the rays were still homogeneous after passing
through the aluminium.
A clear photographic impression was obtained for 12 layers of foil, but
it was not found possible to obtain any effect through 13 layers. This
result shows that the photographic action of the rays, like the ionizing
action, ceases very abruptly.
The results obtained are shown in the following table. Assuming that the
value of _e_/_m_ is constant, the third column gives the velocity of the
α particles after traversing the aluminium. This is expressed in terms
of _V₀_, the velocity of the α particle when the screens are removed.
Number of Distance Velocity
layers of between of α
aluminum bands on the particles
foil plate
0 1·46 mms. 1·00
_V₀_
5 1·71 „ ·85 „
8 1·91 „ ·76 „
10 2·01 „ ·73 „
12 2·29 „ ·64 „
13 No
photographic
effect
The velocity of the α particle is thus reduced only about 36 per cent.
of its initial value when it fails to produce any action on the
photographic plate.
Now Bragg has shown (section 104) that the α particle produces
approximately the same number of ions per cm. of path in air over its
whole range. Consequently, the simplest assumption to make is that the
energy of the α particle is diminished by a constant amount in
traversing each layer of foil. After passing through 12 layers the
kinetic energy is reduced to 41 per cent. of the maximum. Each layer of
foil thus absorbs 4·9 per cent. of the maximum energy. The observed
kinetic energy of the α particle after passing through successive layers
of foil, and the value calculated on the above assumptions, are shown in
the following table.
Number of Observed Calculated
layers of energy energy
aluminum
foil
0 100 100
5 73 75
8 58 61
10 53 51
12 41 41
The experimental and theoretical values agree within the limits of
experimental error. We may thus conclude, as a first approximation, that
the same proportion of the total energy is abstracted from the α
particles in passing through equal distances of the absorbing screen.
=Range of ionization and photographic action in air.= The abrupt falling
off of the photographic impression after the rays had passed through 12
layers of foil suggested that it might be directly connected with the
corresponding abrupt falling off of the ionization in air, so clearly
brought out by Bragg. This was found to be the case. It was found
experimentally that the absorption in each layer of aluminium foil was
equivalent to that produced by a distance of ·54 cms. of air. Twelve
layers of foil thus corresponded to 6·5 cms. of air. Now Bragg found
that the α rays from radium C ionize the air for a distance 6·7 cms.,
and that the ionization then falls off very rapidly. We may thus
conclude that the α rays cease to affect the photographic plate at the
same velocity as that at which they cease to ionize the gas. This is a
very important result, and, as we shall see later, suggests that the
action on the photographic plate is due to an ionization of the
photographic salts.
The velocity of the α particles from the different radio-active products
can at once be calculated, knowing the maximum range in air of the α
rays from each product. The latter have been experimentally determined
by Bragg. The velocity is expressed in terms of _V₀_, the initial
velocity of the α particles from radium C. The rays from radium C are
projected with a greater velocity than the rays from the other products
of radium.
Product Maximum Velocity
range of α of α
particles particles
in air
Radium 3 cms. ·82 _V₀_
Emanation 3·8 or 4·4 ·87 or ·90
cms. _V₀_
Rad. A 4·4 or 3·8 ·90 or ·87
„ _V₀_
Rad. C 6·7 1·00 _V₀_
„
It is difficult to determine from the experiments whether the range 3·8
cms. belongs to the rays from the emanation or from radium A. The mean
velocity of the α particles is thus ·90 _V₀_, and the maximum variation
for the individual products does not vary more than 10 per cent. from
the mean value.
The results of Becquerel, discussed in section 92, at once receive an
explanation on the above results. The α particles, expelled from radium
in radio-active equilibrium, have all ranges lying between 0 and 6·7
cms. of air. The velocity of the α particles which are able to produce a
photographic impression varies between ·64 _V₀_ and _V₀_. The particles
which have only a short range in air are projected with a smaller
velocity than those which have a greater range. The former are in
consequence more bent by a magnetic field. It is thus to be expected
that the apparent curvature of the path of rays in a uniform magnetic
field will be greater close to the radium than at some distance away.
=Range of phosphorescent action in air.= Some experiments were also made
to see whether the action of the α rays in producing luminosity in
substances like zinc sulphide, barium platinocyanide, and willemite,
ceased at the same distance as the ionizing action.
A very active wire was placed on a movable plate, the distance of which
from a fixed screen of phosphorescent substance could be varied. The
distance at which the phosphorescent action ceased could be determined
fairly accurately. Different thicknesses of aluminium foil were then
placed over the active wire, and the corresponding distance at which the
luminosity disappeared was measured. The results are shown graphically
in Fig. 107, where the ordinates represent the distance of the
phosphorescent screen from the active wire, and the abscissae the number
of layers of aluminium foil, each ·00031 cms. thick.
[Illustration: Fig. 107.]
It is seen that the curve joining the points is a straight line. 12·5
thicknesses of foil absorbed the rays to the same extent as 6·8 cms. of
air, so that each thickness of aluminium corresponded in absorbing power
to ·54 cms. of air. For a screen of zinc sulphide, the phosphorescent
action ceased at a distance of air of 6·8 cms., showing that the
photographic and phosphorescent ranges of the α rays in air were
practically identical.
The experiments with barium platinocyanide and willemite were more
difficult, as the β and γ rays from the active wire produced a
luminosity comparable with that produced by the α rays. Fairly
concordant results, however, were obtained by introducing a thin sheet
of black paper between the active wire and the screen. If the luminosity
was sensibly changed, it was concluded that the α rays still produced an
effect, and in this way the point of cessation of phosphorescent action
could be approximately determined. For example, with eight thicknesses
of foil over the active wire the additional thickness of air required to
cut off the phosphorescent effect of the a rays was 2·5 cms. for
willemite, and 2·1 cms. for barium platinocyanide.
The corresponding distance for zinc sulphide was 2·40 cms., a value
intermediate between the other two.
Since eight layers of foil are equivalent to 4·3 cms. of air, the ranges
in air of phosphorescent action for zinc sulphide, barium
platinocyanide, and willemite correspond to 6·7, 6·8, and 6·4 cms.
respectively. The differences observed are quite likely to be due to
experimental error.
=Discussion of results.= We have seen that the ionizing, phosphorescent,
and photographic actions of the α rays emitted from radium C cease after
traversing very nearly the same distance of air. This is a surprising
result when it is remembered that the α particle, after passing through
this depth of air, still possesses a velocity of at least 60 per cent.
of its initial value. Taking the probable value of the initial velocity
of the α particle from radium C as 2·5 × 10⁹ cms. per sec., the
ionizing, phosphorescent, and photographic actions cease when the
velocity of the α particle falls below 1·5 × 10⁹ cms. per second, that
is, a velocity of about ¹⁄₂₀ of that of light. The particle still
possesses nearly 40 per cent. of its initial energy of projection at
this stage.
These results show that the property of the α rays of producing
ionization in gases, of producing luminosity in some substances, and of
affecting a photographic plate, ceases when the velocity of the α
particle falls below a certain fixed value which is the same in each
case. It seems reasonable, therefore, to suppose that these three
properties of the α rays must be ascribed to a common cause. Now the
absorption of the α rays in gases is mainly a consequence of the energy
absorbed in the production of ions in the gas. When the α particles are
completely absorbed in the gas, the same total amount of ionization is
produced, showing that the energy required to produce an ion is the same
for all gases. On the other hand, for a constant source of radiation,
the ionization per unit volume of the gas is approximately proportional
to its density. Since the absorption of the α rays in solid matter is
approximately proportional to the density of the absorbing medium
compared with air, it is probable that this absorption is also a result
of the energy used up in producing ions in the solid matter traversed,
and that about the same amount of energy is required to produce an ion
in matter whether solid, liquid, or gaseous.
It is probable, therefore, that the production of ions in the
phosphorescent material and in the photographic film would cease at
about the same velocity for which the α particle is unable to ionize the
gas. On this view, then, the experimental results receive a simple
explanation. The action of the α rays in producing photographic and
phosphorescent actions is primarily a result of ionization. This
ionization may possibly give rise to secondary actions which influence
the effects observed.
This point of view is of interest in connection with the origin of the
“scintillations” observed in zinc sulphide and other substances when
exposed to the action of the α rays. This effect is ascribed by
Becquerel to the cleavage of the crystals under the bombardment of the α
particles. These results, however, show that we must look deeper for the
explanation of this phenomenon. The effect is primarily due to the
production of ions in the phosphorescent material and not to direct
bombardment, for we have seen that the α particle produces no
scintillations when it still possesses a large amount of kinetic energy.
It seems not unlikely that the scintillations produced by the α rays
must be ascribed to the recombination of the ions which are produced by
the α particle in the crystalline mass. It is difficult to see how this
ionization could result in a cleavage of the crystals.
This close connection of the photographic and phosphorescent actions of
the α rays with their property of producing ions, raises the question
whether photographic and phosphorescent actions in general may not, in
the first place, be due to a production of ions in the substance.
=Ionization curve for the α rays from radium C.= Mr McClung, working in
the laboratory of the writer, has recently determined the relative
ionization per unit path of the α particles projected from radium C,
using the method first employed by Bragg and discussed in section 104.
An active wire, exposed for several hours to the emanation from radium,
was used as a source of rays. The α particles were homogeneous, since
the film of radio-active matter was extremely thin.
The relation between the ionization observed over the cross section of
the narrow cone of rays and the distance from the source of rays is
shown in Fig. 108.
[Illustration: Fig. 108.]
The curve exhibits the same peculiarities as those given by Bragg for a
thin film of matter of one kind. The ionization of the α particle per
unit path increases slowly for about 4 cms. There is then a more rapid
increase just before the α particle ceases to ionize the gas, and then a
rapid falling off. The ionization does not appear to end so abruptly as
is really the case, since there is a correction to be applied for the
angle subtended by the cone of rays. The maximum range of the α rays in
air was 6·7 cms., a number in agreement with that obtained by Bragg by
measurements on the range of the rays from radium.
These results show that the ionization per unit path of the α particle
increases at first slowly and then rapidly with decrease of velocity
until the rays cease to ionize the gas.
=Energy required to produce an ion.= From the above results the energy
required to produce an ion by collision of the α particle with the gas
molecules can readily be deduced. The α particles, emitted from radium
itself, are initially projected with a velocity ·88_V₀_ where _V₀_ is
the initial velocity of projection of the α particles from radium C. The
α particles cease to ionize the gas at a velocity ·64_V₀_. From this it
can at once be deduced that ·48 of the total energy of the α particle,
shot out by radium itself, is absorbed when it ceases to ionize the gas.
Assuming that the heating effect of radium at its minimum activity—25
gram calories per hour per gram—is a measure of the kinetic energy of
the expelled α particles, it can be calculated that the kinetic energy
of each α particle is 4·7 × 10⁻⁶ ergs. The amount of energy absorbed
when the α particle just ceases to ionize the gas is 2·3 × 10⁻⁶ ergs.
Assuming that this energy is used up in ionization, and remembering that
the α particle from radium itself produces 86000 ions in its path
(section 252), the average energy required to produce an ion is 2·7 ×
10⁻¹¹ ergs. This is equivalent to the energy acquired by an ion moving
freely between two points differing in potential by 24 volts.
Townsend found that fresh ions were produced by an electron for a
corresponding difference of potential of 10 volts. Stark, from other
data, obtained a value 45 volts, while Langevin considers that 60 volts
is an average value. The value obtained by Rutherford and McClung for
ionization by X-rays was 175 volts, and is probably too high.
=Rayless changes.= We have seen that the α particles from the
radio-active substances are projected with an average velocity not more
than 30 per cent. greater than the minimum velocity, below which the α
particles are unable to produce any ionizing, photographic, or
phosphorescent action. Such a conclusion suggests that the property of
the radio-active substances of emitting α particles has been detected
because the α particles were projected slightly above this minimum
velocity. A similar disintegration of matter may be taking place in
other substances at a rate much greater than in uranium without
producing much electrical effect, provided the α particles are projected
below the critical velocity.
The α particle, on an average, produces about 100,000 ions in the gas
before it is absorbed, so that the electrical effect observed is about
100,000 times as great as that due to the charge carried by the α
particles alone.
It is not unlikely that the numerous rayless products which have been
observed may undergo disintegration of a similar character to the
products which obviously emit α rays. In the rayless product the α
particle may be expelled with a velocity less than 1·5 × 10⁹ cms. per
second and so fail to produce much electrical effect.
These considerations have an important bearing on the question whether
matter in general is radio-active. The property of emitting α particles
above the critical velocity may well be a property only of a special
class of substances, and need not be exhibited by matter in general. At
the same time the results suggest that ordinary matter may be undergoing
transformation accompanied by the expulsion of α particles at a rate
much greater than that shown by uranium, without producing appreciable
electrical or photographic action.
APPENDIX B.
RADIO-ACTIVE MINERALS.
Those natural mineral substances which possess marked radio-active
properties have been found to contain either uranium or thorium, one of
these elements being always present in sufficient proportion readily to
permit its chemical separation and identification by the ordinary
analytical methods[455].
A large number of uranium and thorium minerals are known at the present
time, but they are for the most part found very sparingly, and some of
them have been observed to occur only in a single locality. The chief
commercial sources of uranium are uraninite, gummite, and carnotite,
while thorium is obtained almost exclusively from monazite.
Rutherford and Soddy (_Phil. Mag._ 65, 561 (1903)), were the first to
call attention to the important fact that the relations between the
various radio-active substances and the other elements could best be
determined from the study of the natural minerals in which these bodies
occur, since these minerals represent mixtures of extreme antiquity,
which have remained more or less undisturbed for almost countless ages.
In dealing with these matters, however, it is highly important that we
bring to our aid the data furnished by geology and mineralogy, from
which it is often possible to determine the relative ages of the
different substances with at least a rough degree of approximation.
Thus, for example, if a certain mineral occurs as a primary constituent
of a rock of remote geological period, it can safely be assumed that its
age is greater than that of a similar or different mineral occurring in
a later formation. It is, moreover, quite evident that those minerals
which are obviously produced by the decomposition and alteration of the
primary minerals, through the action of percolating water and other
agencies acting from the surface downward, are of less antiquity than
the primary minerals from which they originated. Through the application
of these considerations it should, in general, be possible to arrange
the various minerals roughly in the order of their probable ages.
The most familiar and widely known uranium mineral is uraninite,
commonly called pitchblende, which consists essentially of uranium
dioxide (UO₂), uranium trioxide (UO₃), and lead oxide (PbO), present in
varying proportions. The uraninites can be distinguished as primary,
namely, those which occur as a primary constituent of pegmatitic dikes
and coarse granites, and secondary, when they occur in metalliferous
veins associated with the sulphides of silver, lead, copper, nickel,
iron, and zinc. The former varieties are quite frequently crystalline in
character, contain a larger proportion of the rare earths and helium,
and have a higher specific gravity than the latter, which are always
massive and botryoidal.
The following are the most prominent localities in which primary
uraninites occur:
1. North Carolina, U.S.A. (especially in Mitchell and Yancey counties).
The uraninite is found in a coarse pegmatitic dike which is mined for
the mica constituent. The associated feldspar of the dike is
considerably decomposed through the action of meteoric waters and gases,
and the uraninite itself is largely altered into the secondary minerals
gummite and uranophane through the same agencies. Among the associated
primary minerals are allanite, zircon, columbite, samarskite,
fergusonite and monazite, while the secondary minerals include gummite,
thorogummite, uranophane, autunite, phosphuranylite, hatchettolite, and
cyrtolite. The geological period of this formation is difficult to
establish with certainty, but is stated to be perhaps Archean, or
possibly to correspond with the close of the Ordovician or with the
Permian.
2. Connecticut, U.S.A. The best known localities are Glastonbury, where
the uraninite is found in the feldspar quarries, and Branchville, where
it occurs in an albitic granite. Both of these localities have furnished
fine crystals. The geological period probably corresponds with the close
of the Ordovician or Carboniferous eras, and is stated to be certainly
Post-Cambrian and Pre-Triassic. Among the associated minerals are
(primary) columbite, (secondary) torbernite and autunite.
3. Southern Norway, particularly in the neighbourhood of Moss. Here
uraninite occurs in the augite-syenite and pegmatite. The varieties
found are known as cleveite and bröggerite, and among the primary
associated minerals are orthite, fergusonite, monazite, and thorite. The
period is stated to be Post-Devonian.
4. Llano County, Texas. The variety of uraninite known as nivenite is
found here in a quartzose pegmatite, associated with the primary
minerals gadolinite, allanite and fergusonite, and the secondary
minerals cyrtolite, yttrialite, gummite, and thorogummite.
Secondary uraninite is found at Johanngeorgenstadt, Marienberg and
Schneeberg in Saxony, at Joachimsthal and Pribam in Bohemia, at
Cornwall in England, and at Black Hawk, Colorado, and in the Black
Hills, South Dakota, in the United States. The exact geological period
of most of these secondary occurrences is somewhat uncertain, but they
are undoubtedly very much later than the primary occurrences mentioned
above.
As a matter of general interest the analysis of a typical primary
uraninite (No. 1) and of a typical secondary uraninite (No. 2) is given
below[456]:
No. 1 No. 2
Glastonbury, Johanngeorgenstadt,
Conn. Saxony
Sp. Gr. 9·59 6·89
UO₃ 26·48 60·05
UO₂ 57·43 22·33
ThO₂ 9·79 ...
CeO₂ 0·25 ...
La₂O 0·13 ...
Y₂O₃ 0·20 ...
PbO 3·26 6·39
CaO 0·08 1·00
He und. und.
H₂O 0·61 3·17
Fe₂O₃ 0·40 0·21
SiO₂ 0·25 0·50
Al₂O₃ ... 0·20
Bi₂O₃ ... 0·75
CuO ... 0·17
MnO ... 0·09
MgO ... 0·17
Na₂O ... 0·31
P₂O₅ ... 0·06
SO₃ ... 0·19
As₂O₃ ... 2·34
Insoluble 0·70 ...
The following list comprises the more important radio-active minerals,
with their approximate chemical composition and some notes on their
occurrence and probable origin.
Name Composition Remarks
Uraninite, Oxides of Occurs primary
Cleveite, uranium and as a
Bröggerite, lead. Usually constituent of
Nivenite, contains rocks and
Pitchblende thorium, secondary in
other rare veins with
earths and metalliferous
helium. sulphides
Uranium
50-80%.
Thorium 0-10%
Gummite (Pb, Ca) An alteration
U₃SiO₁₂ product of
. 6H₂O? uraninite.
Uranium Formed by the
50-65% action of
percolating
waters
Uranophane, CaO . 2UO₃ An alteration
Uranotil . 2SiO₂ . product of
6H₂O uraninite
Uranium through
44-56% gummite
Carnotite A vanadate of Occurs as a
uranium and secondary
potassium. mineral
Uranium impregnating a
42-51% porous,
sedimentary
sandstone.
Found in
Colorado and
Utah
Uranosphaerite Bi₂O₃ . Alteration
2UO₃ . product of
3H₂O. other uranium
Uranium 41% minerals
Torbernite, CuO . 2UO₃ „ „
Cuprouranite . P₂O₅
. 8H₂O.
Uranium
44-51%
Autunite, CaO . 2UO₃ „ „
Calciouranite . P₂O₅
. 8H₂O.
Uranium
45-51%
Uranocircite BaO . 2UO₃ „ „
. P₂O₅
. 8H₂O.
Uranium 46%
Phosphuranylite 3UO₃ . „ „
P₂O₅ .
6H₂O.
Uranium
58-64%
Zunerite CuO . 2UO₃ „ „
. As₂O₅
. 8H₂O.
Uranium 46%
Uranospinite CaO . 2UO₃ „ „
. As₂O₅
. 8H₂O.
Uranium 49%
Walpurgite 5Bi₂O₃ „ „
. 3UO₃ .
As₂O₅ .
12H₂O.
Uranium 16%
Thorogummite UO₃ . A variety of
3ThO₂ . gummite
3SiO₂ .
6H₂O?
Uranium 41%
Thorite, ThSiO₄. A primary
Orangite, Uranium constituent of
Uranothorite 1-10%. pegmatite
Thorium oxide dikes
48-71%
Thorianite Oxide of Occurs as a
thorium, primary
uranium, the constituent of
rare earths a pegmatite
and lead. dike in
Contains a Ceylon.
relatively Geological age
large probably
proportion of Archean
helium.
Uranium
9-10%.
Thorium oxide
73-77%
Samarskite Niobate and Primary
tantalate of constituent of
rare earths. pegmatite
Uranium 8-10% dikes
Fergusonite Metaniobate „ „
and tantalate
of rare
earths.
Uranium 1-6%
Euxenite Niobate and „ „
titanate of
rare earths.
Uranium 3-10%
Monazite Phosphate of „ „
the rare
earths,
chiefly
cerium.
Uranium
0·3-0·4%
Footnote 455:
An apparent exception has been observed by Danne in the case of
certain lead minerals which occur under peculiar conditions at
d’Issy-l’Évêque, France. See p. 465.
Footnote 456:
Hillebrand, _Am. J. Sci._ 40, 384 (1890); _ibid._ 42, 390 (1891).
INDEX.
_The numbers refer to the pages._