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No. 22: THE RADIOACTIVITY OF LEAD, BY J. C. MCLENNAN
(Reprinted froM THE PHILOSOPHICAL MAGAZIne, December, 1907)
THE UNIVERSITY LIBRARY: PUBLISHED BY
THE LIBRARIAN, 1908

University of Toronto Studies
COMMITTEE OF MANAGEMENT

Chairman: ROBERT ALEXANDER FALCONER, M.A., Litt. D., LL.D., D.D.
President of the University
PROFESSOR W. J. ALEXANDER, PH.D.
PROFESSOR W. H. ELLIS, M.A., M. B.
PROFESSOR A. KIRSCHMANN, PH.D.
PROFESSOR J. J. MACKENZIE, B.A.
PROFESSOR R. RAMSAY WRIGHT, M. A., B.Sc., LL.D..
PROFESSOR GEORGE M. WRONG, M.A.
General Editor: H. H. LANGTON, M.A.
Librarian of the University

1
From the PHILOSOPHICAL MAGAZINE for December 1907.
On the Radioactivity of Lead and other Metals. By J. C.
MCLENNAN, Ph.D., Professor of Physics, University of
Toronto *.
IN
I. The Relative Activities of Different Metals.
N a paper in the Phil. Mag. of September 1906, Eve states
that while investigating the natural ionization of air
confined in vessels made of different metals, he found that
24 ions per c.c. were generated per sec. when the receivers
were made of copper, zinc, iron, and tinned iron, while 96
ions per c.c. were regularly produced in air per second
when the confining vessels were made of lead.
The high conductivity of air contained in lead vessels has
been frequently noted by other observers; and from Eve's
results it would appear that lead either contains some active
impurity from which other metals are entirely free or else
it possesses an intrinsic radiation very much stronger than
that exhibited by other metals.
The view that lead contains an active impurity is supported
by a description in the Phys. Zeit. of November 1906, of
some experiments by Elster and Geitel, in which they suc-
ceeded in extracting from lead oxide small quantities of an
active substance which from its characteristics they were
inclined to think was Radium F. In this paper they state
that they were unable to obtain any active emanation from
the materials treated, and on this account they suggest that
possibly the source of the Radium F can be traced to the
presence of Radium D in the lead.
Since the decay period for Radium D is forty years it
would follow, if the high activity of lead is due to the pre-
sence of this radium product, that very old lead should
* Communicated by Prof. J. J. Thomson, F.R.S.
761
Prof. J. C. McLennan on the
!
exhibit an activity less intense than that which it emitted
when freshly mined.
Eve does not appear to have tested many different samples
of lead, but if the explanation offered by Elster and Geitel
of the high activity of lead be correct, one should expect
to find that samples of lead selected at random from
different localities would exhibit widely differing degrees of
activity.
Such a difference in the radioactivity of lead obtained from
different sources was recently observed by the writer while
making some measurements on the conductivity of air
contained in metal vessels.
In these experiments the metals examined were made up
into cylinders 60 cm. long and 24 cm. in diameter, and from
measurements with a sensitive quadrant electrometer on the
saturation current through the air which they contained, their
activities were deduced.
The experiments were conducted in a room free from any
artificial contamination; and in carrying them out, the
cylinders were first carefully cleaned with glass paper and
then thoroughly washed out with hydrochloric acid, water,
ammonia, and ethyl alcohol, and finally, before making the
measurements, air filtered through glass and cotton-wool
was blown through each of them for fifteen or twenty minutes.
The results obtained with the different metals examined are
contained in Table I. (p. 762).
From this table it will be seen that the values of "q" for
aluminium and zinc are somewhat less than those found by
Eve for this constant with the same metals. They are, how-
ever, in good agreement with H. L. Cooke's corrected value
"q"=136 given by Eve for air confined in a well-cleaned
brass vessel.
The values found for "6 9 "in the experiments with lead
cylinders, as will be seen from the table, range from 23 to
160 ions per c.c. per second. The lowest value, 23, was
obtained with the lead which had been in the laboratory
between twenty-five and thirty years, and had probably been
a very much longer time away from the mine. With the
cylinder No. 4, which was made from an old drain-pipe, the
value of " q was found to be 78, a somewhat higher value
than than obtained with No. 1. Although both of these.
cylinders were made of comparatively old lead, it is highly
probable that No. 4, from the nature of its use, had become
contaminated with some active substance. It may possibly
too have possessed a higher activity than No. 1 when
originally mined.
""
Radioactivity of Lead and other Metals.
762
Material of
cylinder.
TABLE I.

Thickness
of sheet
"q"
Average No.
of ions per c.c.
Remarks.
No.
Length 60 cm.
Diameter 24 cm.
in mm.
1...... Lead
generated
per second.
1.85
23
О
2...... Lead
2.25
160
3...... Lead
1:45
37
4.... Lead
1.85
78
5...... Lead
1.80
34
6.....
Lead
1.80
55
7......
Lead
1.80
61
8...... Zinc
1.62
15
9......
Aluminium.
41
15
This sample was taken
from a sheet of lead
which had been used as
a lining in a case in-
stalled in the Univer-
sity over twenty-eight
years ago,
Commercial
English
sheet lead obtained
from the lead works at
Toronto.
Commercial
English
sheet lead selected
from a different ship-
ment from No. 2.
This sample was ob-
tained from a sheet
rolled from an old pipe
which had been used
as a drain for 25 or
30 years, and was after-
wards melted down.
Rolled from a pig of
lead recently received
from the smelter at
Trail, B.C. Canada.
Rolled from English
pig lead; Quirk and
Bartons.
Rolled from English
pig lead. Cookson's.
Commercial sheet zinc.
Commercial sheet alu-
minium.
With cylinder No. 5 the value obtained for "q" was 34
ions per c.c. per second. This lead, we have reason to believe,
was mined not more than two or three years ago, and under
the circumstances might have been expected to show a much
higher activity. Its activity, however, was practically the
same as that of No. 3, which was selected at random from a
commercial sheet of lead which probably had been on the
market for some years.
Cylinders No. 6 and No. 7 possessed a moderate activity
compared with the others of the same metal. The number
of ions per c.c. generated in air per second with them being
55 and 61 respectively.
With cylinder No. 2 the greatest ionization was obtained,
the value of "q" in this case being 160 ions per c.c. per second.
763
Prof. J. C. McLennan on the
This cylinder was treated precisely the same as the others,
but on account of its high activity special measurements were
made with it in order to investigate more fully the character
of the radiation which it emitted.
Measurements on the radiation from this cylinder showed
it to be in great measure an easily absorbed one. When
aluminium linings 0.73 mm. thick were inserted in cylinders
No. 1, No. 2, and No. 3, and measurements made on their
saturation currents, the values of "q" were found to be
12·0, 13·3, and 14.4 respectively. These numbers, it will be
seen, are slightly lower than those found for aluminium
alone, which is exactly as one would expect owing to the
absorption of the penetrating rays from the earth by the lead.
The value for “ 'q "13.3 found for No. 2 is slightly greater
than that "q"12 given by No. 1, although this lead
cylinder was 2.25 mm. thick, while No. 1 was only 185.
This would seem to indicate the existence of a penetrating
type of radiation issuing from No. 2 which was absent from
cylinder No. 1.
A second series of measurements was made with cylinder
No. 2 to investigate the distribution of the substance which
was the cause of its high activity. Readings were taken on
the saturation current first with the lead cylinder entirely
unscreened, then with one half of the cylindrical surface
screened internally with aluminium 0.73 mm. thick, and
finally with the whole of the inner cylindrical surface covered
with the aluminium.
The values are given in Table II., and from them it will
TABLE II.
Decrease in
Ionization.
Experiment
number.
Cylinder No. 2. (Arbitrary
Ionization.
Scale.)
(Arbitrary
Scale.)
1.......
Completely
screened.
un-
54.6
22.2
2.......
One-half
inner
32.4
cylindrical sur-
face screened.
22.57
3.......
All inner cylin-
drical surface
screened.
9.87
be seen that the decrease in conductivity was the same for
each half of the cylindrical surface. This goes to show
that the radioactive impurity in the lead was uniformly
Radioactivity of Lead and other Metals.
764
distributed over its surface. It was also very probably
distributed in a uniform manner throughout the mass
of the cylinder, as repeated scourings with glass-paper
failed to remove it. In this connexion it is of interest to
note that, during the last six months, measurements have
been repeatedly made on the conductivity of air confined in
this cylinder, but during that period no indication of a
falling off in the intensity of the radiation from it has been
observed.
II. On the Ionization produced in Metallic Receivers by the
Secondary Rays excited by the Gamma Rays from Radium.
From the foregoing results it is abundantly evident that
the high activity of lead, which has from time to time been
recorded by a number of observers, cannot be ascribed to
any intrinsic property of the metal, but must be connected
with the existence in it, in amounts varying with different
specimens, of some foreign body of considerable activity.
It is known nat part of the ionization in a gas confined in
metallic vessels must be due to the penetrating radiation
emitted by the earth, and part to the secondary rays excited
in the substance of the metallic receivers by these penetrating
rays. From the results given above, part must also be due,
in some cases at least, to active impurities present in the
metal.
The extremely low value found for the ionization with the
lead in cylinder No. 1, coupled with the value for "q
obtained with zinc and aluminium receivers, suggests the
possibility that the materials out of which these vessels were
inade were entirely or very largely free from active impurities,
and that the differences observed in the ionizations were due
to differences in the intensities of the secondary radiations
from the different metals. It is known that the secondary
radiation increases with the atomic weight of the metal
composing the radiating surface, and it seemed to the writer
possible that the difference in the values of "q" found for
lead and aluminium, namely 23 and 15 ions per c.c. per
second, might be accounted for entirely on this ground.
With the object of investigating this point an aluminium
cylinder was prepared from a thin sheet of the metal 0·41 mm.
in thickness, and a series of accurate measurements made on
the saturation current through the air which it contained.
A small quantity of radium bromide was enclosed in a block
of lead about 3 cms. in thickness, and placed at a distance
of about one metre from the aluminium cylinder. The
saturation current in the aluminium cylinder was again
765
Prof. J. C. McLennan on the
measured, and the difference between its value and that due
to the natural conductivity of the air was noted, and recorded
as being due to the gamma rays from the radium bromide
together with the secondary rays produced by this radiation.
Each of the first eight cylinders referred to in Table I.
was then used in turn as a screen between the lead block
containing the radium and the testing cylinder, and the cor-
responding saturation current measured. The differences.
between these readings and that taken with the radium
before the screens were inserted, were taken as a measure
of the absorption of the gamma rays by the respective
cylinders. From these differences, combined with the
ionization produced by the gamma rays impinging directly
on the testing cylinder, the absorptive power of each of the
cylinders was calculated as a percentage of the intensity of
the penetrating rays issuing from the lead block, and these
are given in Table III. The absorption of the gamma rays
by two sheets of aluminium 0.73 mm. and 1·46 mm. in thick-
ness was also determined in the same manner, and these
are recorded, together with the others, as Nos. 9 and 10 in
Table III.
TABLE III.
Cylinder Material.
No.
Percentage
in mm.
gamma rays.
Percentage
absorption by
No. of ions pro-
duced by natural
ionization.
(See Table I.)
1
2
Lead
1.85
15.36
23
....
2.25
16.29
160
""
3
1.45
9.36
37
...
""
4
1.85
11.2
78
""
5
1.80
12 92
34
""
6
1.80
10.12
55
""
7
1.80
13.23
61
8
Zinc
1.62
4 62
15
9
Aluminium
1.43
.92
10
.73
*46
""
A set of measurements was next made on the saturation
currents in each of the first three cylinders given in Tables I.
and III. These were taken (a) with the air under natural
ionization; (b) with the cylinder lined with aluminium, but
otherwise the same as in (a); (c) with the radium bromide
in the lead block mentioned above at a distance of 1 metre
from the unlined cylinders; and (d) with all the conditions
the same as in (b) excepting that the cylinders were lined
with the sheet aluminium.
Radioactivity of Lead and other Metals.
766
Assuming that this lining completely absorbed the secon-
dary radiation from the lead walls of the vessels, which is
probable as the secondary rays from lead are easily absorbed,
and neglecting the absorption of the gamma rays by the
aluminium lining, since from the numbers given in Table III.
it must necessarily have been less than one half of one per
cent., it follows that the difference between the readings
"c" and "a" represents the ionization produced in the
unlined lead cylinder by the radium; while the difference
between the readings "d" and "b" represents the ionization
produced in the lined cylinder by the same cause. The
excess of this first difference over the second may then be
taken without appreciable error as a measure of the excess of
the ionization produced by the secondary rays in the respec-
tive cylinders when unlined over that produced by the
secondary rays with the lining inserted.
with the lining inserted. In other words, it
may be taken as proportional to the difference between the
ionizing powers, in so far as the air in the cylinders is con-
cerned, of the secondary rays excited in lead and aluminium
by the penetrating rays which entered the cylinders.
Or taking I and Ias as proportional to the ionizations
produced in one of the cylinders by the secondary rays
excited in lead and aluminium respectively by the gamma
rays which entered it, we have
"a")
Its - Ias=(Reading "c" - Reading “a”)
- (Reading “d”-Reading "b").
(i.)
Further, it is known from an investigation by Eve* that
the ionizing power of the secondary rays excited in aluminium
by a gamma radiation is 28.6 per cent. of that possessed by
the secondary rays excited in lead by the same rays.
We have then this equation.
Its
100
28.6
.
Ias.
(ii.)
Again, denoting by Ip the ionization produced in the lead
cylinder under examination by the gamma rays from the
radium alone, we have
Itp + Its=The difference between readings "c" and "a"
with this cylinder.
(iii.)
From equations (i.), (ii.), and (iii.) it is possible then to
Eve, Phil. Mag. Dec. 1904.
767
Prof. J. C. McLennan on the
calculate I, Is, and Ias, and so deduce a relation between
the ionizations produced in a given lead cylinder by the
gamma rays which enter it and by the secondary rays
excited in the walls of the vessel by these penetrating rays.
The averages of a great many measurements made in
this way with the cylinders Nos. 1, 2, and 3 are given in
TABLE IV.

Combined ioni-
Combined ioniza-
Natural ioni- Natural ioni- zations in un-
No.
Cylinder zation (Arbi-zation (Arbi-
trary Scale). trary Scale).
tions in aluminium-
lined lead cylin-
der (Arbitrary
Scale).
lined lead cylinder
(Arbitrary Scale).
Reading "a." Reading "b."
Reading "c."
Reading "d."
1
7.7
4.02
97.75
53.52
2
53.94
4.48
142.44
48.87
3
12
4.67
113
57.97
Table IV.; and in Table V. the numbers corresponding to
the reduced values of these observations are recorded.
TABLE V.
Column 1. Column 2. Column 3. Column 4.
Column 5. Column 6. Column 7.
Cylinder
No.
from radium and
Ionization due to gamma
secondary rays excited in
lead by these rays. Ilp +Ils.
(Arbitrary Scale.)
rays
Ionization due to gamma
rays from radium and
secondary rays excited in
aluminium by these rays.
Ilp +Ias. (Arbitrary Scale.)
Ratio
Ratio
1ls
Ias
Ilp
(calculated)
Ilp
(calculated)
Value of ionization in un-
from
rays
lined lead cylinder due to
penetrating
radium
Ilp (calculated).
Value of Ip corrected for
absorption (calculated).
1
90'05
49.5
1.74
•49
33.05
39.0
100
X
(84 64 × 33·05)=39-0
2
88.50
44.39
2.31
*66
26.7
32.0
3
101.00
53.3
1.95
.56
34.2
38.0
+
Mean
2:00
...
•57
31.32
36.3
Applying equations (i.), (ii.), and (iii.) to the measurements
with cylinder No. 1, as an example of the manner in which
Radioactivity of Lead and other Metals.
768
the reductions were made, we have
and
Izp +I2s=90·05
(iv.)
Ils
=
Iip + Ias=49.5.
100
28.6
(v.)
Iat
(vi.)
Iip
= 33·05
From which we have
Lis
= 57·00
Ias = 16.3,
1.735 Ip,
Ize =
or
I as
and
•493 Itp.
Similar calculations were made on the readings obtained
with cylinders Nos. 2 and 3, and the results of the three are
recorded in columns 4 and 5 of Table V. From these it
will be seen that the ionization produced in the air in a lead
cylinder by the gamma rays from radium is only one-half
that produced in it by the secondary rays excited in the lead
walls by these same rays. On the other hand, with gamma
rays of the same intensity entering an aluminium cylinder
of the same size as the lead one, the results show that the
ionization produced by the penetrating gamma rays is ap-
proximately twice that produced by the secondary rays
excited by these gamma rays.
It will also be seen from the numbers given in the above
table, that we have sufficient data to calculate the ionization
produced by the radium in a cylinder of any material of the
same dimensions as those used in this investigation, provided
it was placed in the standard position indicated above.
For example, Column 6 of Table V. gives the reduced
readings corresponding to the gamma rays alone which
entered the respective cylinders. From Table III. the ab-
sorption powers of these cylinders are known in percentages;
and by means of these numbers values can be calculated for
the ionization which would be produced in the same volume
of free air by the gamma rays from the radium. Column 7
of Table V. contains the values of Ip corrected in this way,
and the mean of the results is 36.3. This number, it will be
seen, represents the ionization which would be produced by
the gamma rays from the radium, used in these experiments
in a cylinder of any metal 60 cms. high, and 24 cms. in
diameter, situated in relation to this radium exactly as the
cylinders were in the experiments described above on the
769
Prof. J. C. McLennan on the
supposition that no absorption of the rays took place on
traversing the walls of the vessel.
If absorption did occur, an the absorption constant for
the cylinder was known, the value 363 could be modified
accordingly, and the ionization produced by the gamma rays
alone within the cylinder be deduced.
Suppose, for example, that the cylinder was an aluminium
one 0.73 mm. in thickness, the absorption from Table III. could
be neglected, and 36-3 would represent the ionization pro-
duced by the gamma rays in the air which it enclosed. From
the results given in Column 5, Table V. the corresponding
ionization due to the secondary radiation excited in the
aluminium by the gamma rays would amount to 57 per cent.
of 36-3 or 20-7, so that the total ionization within the
aluminium cylinder due to the gamma rays from the radium
and to the secondary rays which they excited, could be
represented by (36·3+20·7) or 57 would be the estimated
reading.
In an actual experiment with an aluminium cylinder of
the dimensions given above, and situated approximately in
the position indicated, the reading 62 was obtained as the
mean of a number of observations. This difference between
the experimental and the calculated values for the ionization
is not more than 8 per cent.; and it is not surprising when
it is remembered that no special precautions were taken to
place the aluminium cylinder exactly in the position occupied
by the lead cylinders with which the measurements were
made upon which the present calculations are based. It is
possible that the aluminium cylinder may have been as much
as a centimetre out from the position it was supposed to
occupy during the measurements. From the agreement
presented by these measurements, it seems warrantable to
conclude that the relation which has been established between
the relative amounts of ionization produced by primary and
secondary radiations within a mass of air confined in lead or
aluminium cylindrical vessels with the dimensions described
above, is a reliable one.
III. On the Character of the Radiation from different Metals.
From the foregoing discussion it is evident that with the
cylinders examined, a definite proportion existed between
the ionization produced by the gamma rays and that p:o-
duced by the secondary rays which they excited. With lead
cvlinders the amount contributed by the secondary rays was,
as we have seen, twice that arising from the passage of the
gamina rays. But with aluminium cylinders the relation
Radioactivity of Lead and other Metals.
770
was almost exactly the inverse of the preceding, the ioniza-
tion due to the primary rays being nearly twice that due to
the corresponding secondary radiation.
It will be remembered, too, that the radium from which
the gamma rays were obtained was surrounded by a block
of lead 3 cm. in thickness, so that the radiation which issued
from it must have been of a very penetrating nature, and
therefore similar in its characteristics to the penetrating
radiation which has its source in the earth, and contributes
to the natural ionization observed in air or other gases con-
fined in metallic vessels.
It seems fair to conclude then, that in natural or spon-
taneous ionization in air confined in metallic vessels a pro-
portion should hold between the ionization due to the primary
and that due to the secondary rays, similar to the one which
was found to hold experimentally with the gamma rays
from
radium, and the secondary rays emitted by them.
Assuming this relation to hold, it is possible to establish
a connexion between the conductivity of air confined in a
vessel of one metal with that of air enclosed by a second of
the same dimensions but of different material, provided
neither metal contains any radioactive impurities.
With this relation established it is possible then to check
the results obtained experimentally in particular cases, and
by so doing arrive in a measure at a knowledge of the re-
lative importance of the different factors which determine
the ionization.
In Section I. of this paper it has been shown that with the
lead cylinder No. 1 there was generated on the average
23 ions per c.c. per second. Assuming that no part of this
was due to any impurity in the metal, it follows from the
numbers given in Table V. that one-third of this number
was due to the penetrating radiation which entered the
cylinder, that is 767 of the 23 ions were generated by the
penetrating radiation which traversed the air in the vessel.
Allowing for the absorption by the cylinder of 15.36 per cent.
of the penetrating radiation, it follows that 9.06 ions were
generated per c.c. per second in free air by the penetrating
radiation from the earth. Turning now to the aluminium
cylinder No. 10, it is fair to assume, since its absorption of
the gamma rays has been shown to be negligible, that 9′06
may be taken, without sensible error, to be the number of
ions generated per c.c. per second by the penetrating radiation
which entered it. The number produced per c.c. per second
by the induced secondary radiation would then, according to
Table V., be 57 per cent. of this number, that is 5'16, and
1
ا
771
Prof. J. C. McLennan on the
therefore 14.22 would be the total number generated per c.c.
per second by the combined radiations. In the same manner
it can be shown, by taking Eve's value of 53.7 per cent. as
representing the amount of secondary radiation excited in a
zinc radiating surface compared with that obtained with a
lead one, that 17.88 is the number of ions which should be
generated by the penetrating radiation from the earth, and
by the secondary rays excited by these in each cubic centi-
metre of air enclosed in the zinc cylinder No. 8.
TABLE VI.


Column 1. Column 2. Column 3. Column 4.
Column 5.
Column 6.
Cylinder Metal,
No.
Percentage of penetrating
rays absorbed. Per cent.
Total No. of ions generated
per c.e. per second (ub-
served)..
Total No. of ions generated
per c.c. per sec. by pene-
trating rays from earth and
excited
secondary rays.
(Calculated on basis of no
active impurity in No. 1.)
Difference in "q" repre-
senting active impurities
in substance of cylinder.
1
Lead
15.36
23
23
2
16.29
160
22.77
137.23
11
co
3
9.36
37
24.63
12:37
4
11.2
78
24.15
53.85
5
12.92
34
23.67
10.33
""
6
10.12
55
24.42
30.58
$3
7
13.23
61
23.58
37.42
""
8
Zinc
4.62
15
17.88
9
Aluminium
•23
15
14.22
*78
Calculations similar to the above have been made on the
number of ions which, on the basis laid down, should be
generated per c.c. per second in the air enclosed by each of
the lead cylinders, Nos. 2-7, and the deduced values are all
recorded in Column 5 of Table VI. With cylinder No. 9
the calculated value and that found experimentally present a
good agreement; but with cylinder No. 8 the calculated is
slightly greater than the observed value, and may be due to
our making too high an estimate of the ionization produced
by the secondary rays from the zinc walls. Eve states in
his paper that he found the secondary radiation came not
Radioactivity of Lead and other Metals.
772
only from the surface of the different radiators, but from a
considerable depth as well; and since the zinc used in these
experiments was only 1.62 mm. in thickness, it was probably
not so thick as the plates used by him. A smaller value
should then be assigned to the ionization produced by the
secondary radiation from the zinc walls; and if this were
done, the calculated and the observed values for zinc would
come into better agreement.
The argument which has just been used to explain the high
ionization calculated for the zinc cylinder would apply with
still greater force to the secondary rays from aluminium.
With this metal Eve found that the secondary radiation came
from as great a depth as 3 mm.; and if this condition holds
generally for aluminium, it follows that we have assigned for
this metal also too high a value to the ionization produced
by the secondary rays. A reduction should then be made in
the calculated value for "q" of 14.22 ions per c.c. per second,
and as this value is already slightly below the observed value
of the number of ions generated per c.c. per second in the
aluminium cylinder, this reduction would leave a correspond-
ingly greater number of ions to be accounted for, very
probably by the presence of active impurities in the substance
of the receiver.
It is of special importance, however, to note the fair agree-
ment which exists between the calculated and the observed
values, in these experiments, for the ionization produced in
air enclosed in cylinders of lead, zinc, and aluminium, as
illustrated by the numbers given in Table VI. for Cylinders
Nos. 1, 8, and 9, since it emphasizes the view that ordinary
metals do not possess any intrinsic radiation, and that when
any high conductivity is observed in air confined in metallic
vessels, it must be due to the existence of quantities, more or
less considerable, of some foreign radicactive substance in
the metals.
Examples of such contamination are clearly in evidence in
the results given in Table VI. for the lead cylinders Nos. 2 to 7
inclusive; and the numbers given in Column 6 give an
estimate of the relative amounts of the active impurities
present in the different samples of lead used in their con-
struction.
In what has preceded in this Section the discussion has
rested upon the assumption that the lead in Cylinder No. 1
contained no active impurity; and while the experimental
results rather fit in with the deductions which have been
made on this hypothesis, there still remains the possibility
that some part of the ionization observed with this cylinder
773
Prof. J. C. McLennan on the
i
may have been due to traces of some foreign active
substance.
If one could surround the cylinder with some substance
which would act as a screen, and so cut off entirely the
earth's penetrating rays, and consequently also the induced
secondary radiation, any ionization within the cylinder would
then be due to active impurities present in the metals. The
difficulty, however, is in finding a suitable screen.
In some
experiments made in this direction by the writer* in colla-
boration with E. F. Burton some years ago, screens of water
were used, and with them it was found possible to make a
reduction as high as 37 per cent. in the ionization within a
closed cylinder. About the same time H. L. Cooket, in
studying the conductivity of air enclosed in a brass vessel,
was able to reduce the ionization 30 per cent. by sur-
rounding the brass with a screen of lead. Later still Elster
and Geitel‡ observed a fall of 28 per cent. in the con-
ductivity of the air enclosed in an aluminium cylinder on
removing the apparatus from the surface of the earth to a
closed space in a mine surrounded by a wall of rock salt.
But in none of these experiments is there clear evidence that
the penetrating radiation was entirely cut off. On the other
hand, in several of the experiments which have been made.
with this object in view, it has been found that active im-
purities were present in the substances used as screens, and
the screens themselves were observed to contribute a pene-
trating radiation which masked any falling off in the intensity
of the external radiation arising from absorption.
Although many of the surface waters of the earth which
have been examined, among other substances, by different
experimenters, have been shown to contain minute traces of
radium, it is possible that such waters as those of the great
lakes of Canada might be fairly free from such an impurity,
and if so might serve to screen off radiations from an ioniza-
tion chamber immersed in them. Some experiments made a
few years ago by the writer failed to show the existence of
any measurable amount of the emanation from radium in the
water of Lake Ontario; and from this result it would appear
that the water of this lake would seem to afford the substance
requisite to carry out an experiment such as that just indi-
cated. The experimental difficulties, however, are con-
siderable, and it is doubtful if they could be overcome in a
* McLennan and Burton, Phys. Rev. no. 3 (1903); Burton, Phys. Rev.
no. 3 (1904).
+ H. L. Cooke, Phil. Mag. [6] vi. p. 403 (1903).
Elster and Geitel, Phys. Zeit. Nov. 1, 1905, p. 733.
Radioactivity of Lead and other Metals.
774
manner to give satisfactory results. If this water proved to
be an efficient screen, it would be interesting to see whether
all ionization would disappear from the air confined in a
cylinder such as No. 1 of this investigation, if it were
immersed to a considerable depth in it; for on the assumption
that the material used in the construction of the cylinder
contained no active impurity, this is what one should expect.
to find.
IV. On the Rise in Conductivity of Air confined in
Metallic Vessels.
In the course of the experiments described above, it was
repeatedly found when one of the cylinders was filled with
fresh air filtered through cotton- and glass-wool, and after-
wards sealed up, that the conductivity of the enclosed air
steadily rose for a number of days, and finally reached a
steady value.
This phenomenon, which has been described already by the
writer and E. F. Burton in the paper cited previously, has
also been observed by a number of experimenters, including
Elster and Geitel, Evet, Wood and Campbell, and
others, but up to the present has not received a satisfactory
explanation.
During the present investigation special observations were
made on this effect in connexion with air confined in the
lead cylinders Nos. 1 and 2, on account of the great difference
observed in the values of the conductivity impressed upon
the air introduced into them.
When cylinder No. 1 was thoroughly scoured and cleaned
in the manner described in the beginning of this paper, and
freshly filtered air blown through it for twenty minutes, a
reading of 7.7 divisions per minute, or a number within 1 or
2 per cent. of it, was regularly and repeatedly obtained
throughout the period, now nearly six months, during which
the observations have been carried on. If the air after being
introduced into this cylinder was allowed to remain un-
disturbed for some time, and measurements made on its con-
ductivity at stated intervals, it was found that the ionization
steadily increased, and after a period of a week or ten days
reached a value of approximately 11 divisions per minute.
If when this stage was reached filtered air was blown through
the cylinder for twenty minutes, it was always found that a
* Geitel, Phys. Zeit. ii. pp. 560–563 (1901); Elster and Geitel, ibid.
ii. pp. 116-119 (1900).
+ Eve, Phil. Mag. [6] xii. p. 189 (1906).
Wood and Campbell, Phil. Mag. Feb. 1907, p. 265.
775
Prof. J. C. McLennan on the
CONDUCTIVITY,
(Arbitrary Scale)
12
21
10
drop in the conductivity occurred to between 8 and 8.5
divisions per minute.
If the air was again left undisturbed in the cylinder, its
conductivity rose once more and finally reached the maximum
value of approximately 11 divisions per minute. In con-
ducting these operations it was not found necessary to draw
fresh air through the cylinder for more than twenty minutes
in order to lower the conductivity to the minimum value.
TABLE VII.
Cylinder No. 1.
Date.
May 16
17
18
19
21
22
24
Conductivity
(Arbitrary Scale).
7.7
9.0
9.6
10.32
10.65
11.09
11.02
Fresh air blown through cylinder for
twenty minutes:
May 24
25
27
29
June 1
4
8.54
8.91
9.83
10.16
10.79
11.3
Fresh air again blown through cylinder for
twenty minutes:
June 4
8.1
Fig. 1.

7
6
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1
2 3 4 5
MAY
JUNE
Radioactivity of Lead and other Metals.
776
With cylinder No. 2 thoroughly cleaned in the manner
already described, a reading of 545 divisions per minute
with but slight variations was regularly obtained with freshly
filtered air. With this cylinder, too, when the air was un-
disturbed in it the conductivity steadily rose, and after a
time approached a maximum value. The time required for
the steady state to be reached was, however, much greater
than with cylinder No. 1.
A set of readings which exhibit this rise are given in
Table VIII. and a curve representing them is shown in
fig. 2.
TABLE VIII.
Lead Cylinder No. 2.
Time.
June 5
7
9
12
15
Conductivity
(Arbitrary Scale).
54.6
58.3
61
62
63.2
64
17, 5 P.M.
Filtered air was now blown through cylinder
for twenty minutes:
June 17, 5.30 P.M. ...
60.6
18, 1.00 P.M.
64.4
27, 10.30 a.M.
67
Filtered air again blown through for
twenty minutes:
June 27, 12.30 P.M. ... 62.4
Filtered air blown through for one hour:
June 27, 3.30 P.M. ...
62.8
From this it will be seen that when fresh air was intro-
duced into the cylinder on June 17 the reading dropped from
64 to 606, and again on June 27 the introduction of fresh
air was followed by a drop from 67 to 62·4. Air was then
drawn through the cylinder for one hour, but no further
drop in the conductivity ensued. When similar observations
were made on other occasions with this cylinder similar
results were obtained. After a rise occurred, the introduc-
tion of fresh air was always followed by a drop in the
777
Prof. J. C. McLennan on the
conductivity, but the initial value of 54:5 divisions was never
reached without re-cleaning and washing the inner surface
of the lead.
68
67
66
Fig. 2.

CONDUCTIVITY
(Arbitrary Scale)
63
65
64
8 3 རྒྱ 3 ཆ
62
61
GO
59
58
57
56
55
54
5 7 9 44 13 15 17 19 21 23 25 27 29
JUNE
From these observations it would appear that the rise in
the conductivity of the air in both cylinders may be divided
into two parts and ascribed to different causes; the one being
associated with some change in the surface of the metals
used in the construction of the cylinders, and the other with
some substance which becomes diffused throughout the air,
and can be blown out with it.
With cylinder No. 1 the part due to the first cause was
very small, and in no case exceeded 10 or 12 per cent. of the
minimum reading obtained for the conductivity.
The second part, too, was very definite with this cylinder,
and when the maximum conductivity had been reached it
corresponded to a reading of between 25 and 3 divisions
per minute.
With cylinder No. 2 both parts of the rise in conductivity
were well marked. But as the numbers in Table VIII. show,
both parts exhibited a steady increase during the time the
Radioactivity of Lead and other Metals.
778
conductivity was under observation, and owing to the length
of time required for the maximum state to be reached, it is
not possible at present to express them as definite percentages
of the minimum reading obtained with this vessel.
Repeated observations on both cylinders have invariably
given the results just described, and as the greatest care was
taken throughout the investigation to prevent contamination
of the cylinders by foreign active substances except what
might be introduced along with the filtered air, it would
seem clear that a process is going on in the metals, possibly
a diffusion from the interior, whereby the surface becomes
coated with a layer of active matter which makes an important
contribution to the ionizing power of the metal.
From the observations which have been made so far, it has
been impossible to decide whether the second part of the rise
n the conductivity of the confined air was due, in whole or
in art, to an active substance introduced with the air or to
an emanation from the walls of the vessel, but as observations
are still being made with the cylinders, it is possible that some
additional facts may be obtained which will clear up this
difficulty, and also throw light on the nature of the active
impurity which has been shown to be present in varying
amounts in the different samples of lead examined.
V. Summary of Results.
1. The conductivity of air enclosed in lead cylinders has
been shown to vary widely with the samples of lead selected.
The lowest conductivity observed in air enclosed by this
metal corresponded to the production in the air of 23 ions
per c.c. per second, and the highest to the production of
160 ions per c.c. per second.
2. These wide variations show that the high activity of
lead which has been observed generally is due to the presence
of active impurities in varying amounts in the lead, and not
to a high intrinsic radiation from the metal.
3. Calculations made on the observations show that the
differences in the conductivities of air confined in vessels of
different metals, including lead, when free from active im-
purities, arise from and are due to differences in the secondary
radiations from these metals.
4. Experiments made with the gamma rays from radium
showed that of the ionization produced by these rays in air
enciosed in lead receivers, two-thirds was due to the excited
secondary rays and one-third to the gamma radiation itself.
.
779
Radioactivity of Lead and other Metals.
With aluminium cylinders on the other hand, the measure-
ments show that approximately two-thirds of the ionization
was due to the gamma rays, and one-third to the secondary
rays excited by this radiation in the metal.
5. Calculations based on observations on the conductivity
of air confined in different receivers lead to the conclusion
that approximately 9 ions per c.c. per second are generated
in free air by the penetrating radiation from the earth.
Before concluding I wish to acknowledge my very great
indebtedness to Mr. V. E. Pound, for his kindness in repeating
and verifying many of the observations described in the first
part of this paper.
The Physical Laboratory,
University of Toronto,
July 1, 1907.


UNIVERSITY OF TORONTO STUDIES.
The "Papers from the Physical Laboratories," to be issued as a specia
series of University of Toronto Studies, date from the year 1900. No. 1-17 were
published by the Physical Department in a very limited edition and are no longer
Desint For the sake of a complete record the numbering of the Papers, as
ming a series of University of Toronto Studies, is made continuous with the
earlier series and commences with No. 18. The earlier numbers, except those to
which a price is attached, are not now available either for sale or gift.
No. 1: Electric Screening in Vacuum Tubes, by J. C. Mc-
No.
Njo.
No.
No.
No.
No.
LENNAN.
Trans. Roy. Soc. of Canada, second series, Vol.
VI, Sec. III (1900).
2: Electrical Conductivity in Gases traversed by Cathode
Rays, by J. C. MCLENNAN.
(a) Phil. Trans., A. Vol. 195 (1900), pp. 49-77
(6) Zeitschrift f. Physik. Chemie XXXVII, : 1901).
3: On a kind of Radioactivity imparted to certain Salts by
Cathode Rays, by J. C. MCLENNAN.
(a) Phys. Zeit. Vol. 2, No. 49, pp. 704-706 1901).
(b) Phil. Mag., Feb. 1902.
4: On excited Radioactivity, by R. M. STEWART
Trans. Roy. Soc. of Canada, second series, Vol.
VIII, Sec. III (1902).
5: Induced Radioactivity excited in Air at the Foot of
Waterfalls, by J. C. MCLENNAN...
(a) Phys. Zeit., 4, No. 10, pp 295-298 (1903).
(b) Phil. Mag., April 1903.
(c) Physical Review, Vol. XVI, No. 4 (1903).
(d) University of Toronto Studies, Physical Science
Series, No. 1.

0.50
6: Some Experiments on the Electrical Conductivity of
Atmospheric Air, by J. C. MCLENNAN and E. F. BURTON. 0.50
(a) Physical Review, Vol. XVI, No. 3 (1903).
(b) University of Toronto Studies, Physical Science.
Series, No. 2 (1903).
7: On the Radioactivity of Metals Generally, by J. C. Mc-
LENNAN and E. F. BURTON..
(a) Trans. Roy. Soc. of Canada, Second Series Vol.
IX, Sec. III (1903).
(b) Phys. Zeit. 1903, No. 20, pp. 553-556.
(c) Phil. Mag., Sept. 1903.
(d) University of Toronto Studies, Physical Science
Series, No. 3 (1903).

an
8: On the Potential Difference required to produce Elec-
trical Discharges in Gases at Low Pressure;
Extension of Paschen's Law, by W. R. Carr.
Trans. Roy. Soc. of Canada, Second Series, Vol.
VIII, Sec. III (1902).
0.25


No. 9: On the Laws governing Electric Discharges in Gases
at Low Pressures, by W. R. CARR.
Phil. Trans. A. Vol. 201, pp. 403-433 (1903).
No. 10: On the Character of the Radiation from Ordinary
Metals, by E. F. BURTON.
Phys. Rev. Vol. XVIII, No. 3 (1904).
No. 11: The Metric System of Weights and Measures, by
J. C. MCLENNAN.
Proc. of Select Standing Committee on Agriculture
and Colonization of the House of Commons, Canada,
March 29, 1904.
No. 12: On the Radioactivity of Natural Gas, by J. C. MCLENNAN.
Trans. Roy. Soc. of Canada, Second Series, Vol. X,
Section III (1904).
No. 13: On a Radioactive Gas from Crude Petroleum, by
F F. BURTON..
a) University Studies, Phys. Sci. Series, No. 4(1904).
(3) Phil. Mag. Oct.. 1904.
No. 14: On the Radioactivity of Mineral Oils and Natural :
Cases, by J. C. MCLENNAN.
Proceedings of the International Electrical Congress
of St. Louis, 1904.
No. 15: Note on the Use of Sensitive Quadrant Electrometers
by J. C. MCLENNAN.
Phys. Rev. Vol. XX, No. 3 (1905).
No. 16: On the Decay of Excited Radioactivity from Natural !
Gases, by Miss L. B. JOHNSON.
Phys. Rev. Vol. XX, No. 3 (1905).
No. 17: On the Secondary Radiation excited in Different Metals
by the y Rays from Radium, by H. F. Dawes.
Phys. Rev. Vol. XX, No. 3 (1905).
No. 18: On a New Method of Determining the Specific Heat
of a Gas at Constant Pressure, by H. F. DAWES..
Trans. Roy. Soc. of Canada, Second Series, Vol. XII,
Section III (1906).
No. 19: On the Magnetic Susceptibility of Mixtures of Salt?
Solutions, by J. C. MCLENNAN and C. S. Wright
Phys. Rev. Vol. XXIV, No. 3 (1907).
No. 20: On the magnetic properties of Heusler's alloys, b
J. C. MCLENNAN
0.25

0.25
i
0.25
f
0.25
Phys. Rev., Vol. XXIV.
No. 21: On the radioactivity of lead and other metals, by J.
MCLENNAN and V. E. POUND
0.25
Roy. Soc. of Canada, N. S., Vol. XII.
No. 22: The radioactivity of lead, by J. C. MCLENNAN
0.25
Phil. Mag., Dec. 1907.
!