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ity denote the true percentage obtained by correcting in this
manner.

A typical set of measurements is given below :

Emanation passed through P,0^ and glass-wool into
vessel containing air at 1 atmosphere. +975 volts
on case. Exposure about 2 hours.

Equilibrium rate with added capacity: 6*17.

Activity on case measured at 10 and 16 minutes after
removal of emanation ; rates with added capacity : '19
and '17 respectively.

Activity on cathode measured at 20 and 25 minutes
after removal of emanation ; rates witli added capac-
ity : 1*05 and '98 respectively.

Calculated maximum activity on case : -38.

Ditto cathode: 2*53.

Batio of max. cathode to max. case activity : 666 : 1.

Percentage cathode activity : 86-9.

True percentage cathode activity as corrected for dif-
fusion of uncharged carriers to cathode : 86*6.

It is difficult to estimate with precision the experimental
error in the determination of the percentage cathode activity.
A slight error probably arises from the fact that the cathode



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490 Welliach and Bronsofi — Distribution of the Active

and case activities are determined by means of ionization
which is differently distributed. This has been shown experi-
mentally to have a negligible effect on the relative values of
the percentage cathode activities for different potentials, tbe
determination of which was really the object of the experi-
ment. As mentioned above, it was important to make an accu-
rate determination of the maximum activity on the case. A^n
error in »he determination of this activity leads to approxi-
mately the same percentage error in the determination of the
percentage cathode activity; this error in all our experiments
16 probably less than 2 per cent.

The plotting of a curve such as that of fig. 2 is obviously
only possible if the percentage cathode activity is independent
of the amount of emanation used in the experiment. This fact
was demonstrated repeatedly during the couree of our inves-
tigations ; as an instance of the wide range over which this
holds it may be mentioned that the percentage cathode activity-
was the same with 900 volts on the case when the amounts of
emanation in the vessel were such as to afford equilibrium rates
of 5*32 without any added capacity and 16*1 with the added
capacity, i. e. rates which are roughly in the ratio 1 : 65. When
we are working in the earlier part of the curve, i. e. with
relatively small potentials, this independence no longer holds
good ; but for the higher potentials the effect of the amount
of emanation within a large range becomes negligible.

The curve bears a striking resemblance to the curves which
have been experimentally determined for ionization by a-par-
ticles. Many observers have drawn attention to the "lack of
saturation" which is a marked feature of such curves ; in par-
ticular, this phenomenon has been the subject of special inves-
tigation by Bragg,* Kleeman,t Moulin*:): and Wheelock.§
Reference is made to this point later (section 7). The similar-
ity was so striking as to suggest a more detailed study of the
ionization curves for emanation in equilibrium and with differ-
ent applied potentials.

It was found that within the limits of error the ratio of the
two ionization currents obtained for two potentials not too
low II was, like tlie percentage cathode activity, independent of
the amount of emanation in the vessel ; and, moreover, that

• Bragg, Phil. Mag. (6), xi, p. 466, 1906.

t Kleeman, Phil. Mag. (6), xii, p. 273, 1906.

X Moulin, Compt. Rend., cxlviii, p. 1757, 1909; Le Radium, vii, p. 350,
1910.

S Wheelock, this Journal, xxx, p. 238, 1910.

I The potentials should be sufficiently large to prevent volume recom-
bination, as distinguished from columnar recombination, t. e. should be
sufficient to saturate a uniform distribution of Rontgen-ray ions equal in
number to those produced by the a- radiation.



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Deposit of Radium in an Electric Field. 491

this ratio was identical with the ratio of the percentage cathode
activities corresponding to these two potentials.

As an example of the results obtained in this connection it
will suffice to compare the ratio 1*09 of the percentage cathode
activities obtained with potentials 975 and 160 volts respec-
tively with the corresponding ionization currents. From our
values for the ionization currents in air at 1 atmosphere
pressure obtained in three experiments when widely different
quantities of emanation were employed, we deduce the follow-
ing figures :

Ratio of current with V volts to that with 160 volts :
= 1-10 when V=:875,
= 1-10 when V=ft80,
= 1-07 when V= 900.

It would serve no useful purpose to reproduce in full any of
the ionization curves because, in the first instance, the early
slope of the curve depends markedly on the amount of emana-
tion employed, and, secondly, because the comparison could
only be made over the limited range from about 100 to 1000
volts, as the ionization current could not be accurately
measured when the larger potentials were employed.

That the equality of the ionization and activity ratios still
holds approximately at low voltages when a constant amount
of emanation was employed was verified by measuring the per-
centage cathode activitv with 18 volts applied, and the ioniza-
tion currents for 18 volts and higher potentials, it being known
that the percentage cathode activity at the higher potentials
was independent of the amount of emanation in the vessel.
The following results were obtained :

Percentage cathode activity with 18 volts : 42-7

Ditto 160 volts: 79-1
Ratio : '54

Ionization current with 18 volts : 6*6

Ditto 160 volts : 11-4
Ratio : -68

The agreement is only rough, and further experiments will
be necessary at these small potentials.

The percentage cathode activity increases so slowly with the
higher potentials applied that it appeared as if some definite
fraction of the activity was always bound to be deposited on
the case. Subsequent experiments made with large potentials
obtained by using the Wimshurst machine showed, nowever,
that the percentage cathode activity continually increased with
the potential. It is not out of place to mention briefly here
two sets of experiments which were conducted prior to the use
of the Wimshurst machine.



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492 Wellisch. and £7*0118071 - Distribution of the Active

It was suggested that, if a large uniform electric field were
applied, possibly the number of uncharged carriers might
become insignificant. For this purpose the emanation was
introduced into a vessel consisting of two parallel electrodes of
aluminium (each 58""° in diameter) insulated by an ebonite
ring 20°'™ thick ; the vessel contained air at a pressure of 1
atmosphere. Experiments made with applied potentials of
160 and lOOQ volts gave values of 75 and 82*6 respectively for
the percentage of positively charged carriers.

The second set of experiments had as object to determine
whether KaA was deposited on the case. To test this point
the cylindrical vessels were employed as usual and large
potentials applied, but exposures of only 1 minute* duration
were made. It was found that the resulting curve of decay of
the case activity had the characteristic properties of the curves
of decay for the activity due to a short exposure to the radium
emanation.

Mention might also be briefly made here of some experi-
ments which were performed to ascertain whether there was
any alteration in value of the percentage cathode activity
resulting from a long exposure with large positive potentials
applied to the case when throughout the exposure th? ioniza-
tion current was greatly increased by the passage of Rontgen
rays through the vessel. The alteration, if any, was very
small, and for the present, at any rate, we must assume that
the action of the Rontgen rays is without effect on the dis-
tribution of the activity.

5. Experiments with Air at lieduced Pressure.

The experimental results described in the preceding section
strongly suggest that over a wide range of potentials the per-
centage of the total activity which is deposited on the case
represents the percentage lack of saturation of the positive
ionization current. These experiments related to the activity
distribution in air at a pressure of 1 atmosphere. Now
Moulin and Wheelock have shown {loc, cit) that the ioniza-
tion produced by a-particles is more readily saturated when
the pressure of tlie gas is reduced ; it was, therefore, of interest
to determine whether the percentage cathode activity would
follow the positive ionization current when the gas pressure
was reduced. For this purpose the emanation was introduced
into the testing vessel, which contained air at a pressure of
260™™, and the percentage cathode activity was determined for
various applied positive potentials. The results are given
below (Table II), and are exhibited as a curve in fig. 2.



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Deposit of Radium in an Electric Field. 493



Table II.
Emanation in air at preasore of 200""".



Potential in


Percentage cathode


volts.


activity


80


82-5


150


82-5


775


81-8


790


82-2


1130


81-8


2250


83-8


2250


84-2



It is noticeable that although the percentage cathode activity
for 150 volts is greater than the corresponding value for a
pressure of 1 atmosphere, nevertheless when the higher
potentials are reached the values are smaller at the lower
pressure.

When the ionization due to the emanation in equilibrium
with its activity and in air at a pressure of 260™" was measured
for various applied positive potentials, it was observed that
the alteration of ionization with potential was so extremely
slow as to suggest saturation. Over the range for which the
ionization could be measured this alteration was too small to
justify comparison with the figui-es given for the percentage
cathode activity; the striking feature is that both curves
approach more nearly to the horizontal than the corresponding
curves for a pressure of 1 atmosphere. The figures for the
lower pressure show that for large potentials the activity is
farther from saturation than at 1 atmosphere ; it is therefore
only reasonable to conclude that in the case of ionization the
percentage lack of saturation is greater at the lower than at
the higher pressure.

The figures given in Table II, although suflScientlj' con-
sistent to justify the conclusion just given, nevertheless exhibit
slight irregularities which are being made the subject of
further investigation. In the first instance the values for the
percentage cathode activity for potentials in the neighborhood
of 1000 volts show a slight falling oflF as compared with those
corresponding to the smaller potentials. The explanation of
this effect appears to lie in a distortion of the field in the
neighborhood of the ebonite plug, arising from some action of
the a-radiation on the ebonite ; this distortion would result in
some of the cathode activity being deposited on the plug
instead of on the central electrode. Corresponding difliculties
arose for the ionization measurements at the reduced pressures.



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494 Wellisch and Bronson — Distribution of the Active

The increase of the percentage cathode activity in the neigh-
borhood of 2000 volts is probably real, although the number
of experiments in this connection is not yet sufficient to justify
any definite statement. It is, however, of interest to record
that throughout the exposure with these high potentials a
large cuiTent due to ionization by collision was passjng through
the ^as.

Fmally, it might here be mentioned that a complete set of
observations for the distribution of the activity with various
applied potentials, both for air at 1 atmosphere and for air at
a pressure of 260""", had previously been made, using the same
test-vessel, but with the guard-tube projecting a short distance
into the volume of the gas. By reason of this some of the
activity which belonged properly to the cathode was measured
as " case activity," so that the resulting: values for the percent-
age cathode activity were too small. The curves obtained both
for activity and ionization exhibited the same general features
as those previously described. Some of the results obtained
are given below; these figures are not comparable with those
given above, which were all obtained after the guard-tube had
been cut down.

Emanation in Air at 1 atmosphere.



Potential in
volts


Percentage cathode
activity


160

880

2750


76-2 *

84-1

86-2


Emanation in Air


at 250"™ pressure.


Potential in
volts


Percentage cathode
activity


160

920

2600


81-2
81-9
82-1



6\ £jrperiment8 xoith Air at Pressures greater than
1 atmosphere.

Several experiments were performed to ascertain the distri-
bution of activity when the air in the testing vessel was at a
pressure greater than 1 atmosphere. The results are given in



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Deposit of Radium in an Electric Field.



495



Table III : they were obtained in the early stages of the
research when little information was at hand concerning the
distribution of the activity for various potentials. For this
reason the experiments appear very unsystematic ; however,
the results will serve to convey a good idea of the difficulty of
obtaining approximate saturation of the activity, even with
large applied potentials.





Table III.




Pressnre,


Potential in


Percentage cathode


atm.


volte


activity


1-6


+ 700


68-8


2-0


650


64-1


2-5


160


55-2


2-5


750


60-1


3-13


880


57-5


3-33


3750


80-6


3-63


750


53-8



The difficulty of saturating the ionization current through
the gas likewise made itself manifest at these pressures ; as an
example it may be mentioned that with an applied potential
of 1100 volts the ionization current due to a constant source
of radiation increased as the pressure was increased from 1 to
2^ atmospheres, and then continually decreased with any
further increase in the pressure.

These considerations afford an explanation for the maximum
of cathode activity which has l>een found by several experi-
menters to set in at certain pressures when a constant potential
is employed.

No indication of any decided effect upon th^ distribution of
the activity was obtained by subjecting the gas throughout the
exposure to the influence of a strong source of Rdntgen rays.
In this experiment the vessel contained emanation in air at a
pressure of 3^ atmospheres; the applied positive potential was
1100 volts; the equilibrium rate with added capacity was 5*""*
per sec, and with the Rontgen rays acting was 25°"° per sec.
The percentage cathode activity was 56"2, which is of the order
that might have been expected from the previous results.

7. Discussion of Experimental Resxdts,

In the interpretation of the experimental results which hav^
been obtained in connection with the distribution of activity
the question arises immediately : what is the reason for the



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496 WeUisch and Bronson — Distribution of the A^^ti'^^^

diflSculty in saturating the cathode activity % Even nuclei- tLe
most favorable conditions which have been employed in out
work there is still about 10 per cent of the activity de|>osite<i
on the walls of the testing vessel. In an attempt to ans^wer
this question we are- at once led to the corresponding problem
in connection with the ionization current which passes tlirou^li
the gas during the activation of the electrodes. Mention lias
alreaidy been made of the researches of Bragg, Kleeman, M^ovl-
lin, and Wheelock in connection with this aspect of the prob-
lem. Bragg explained the ditBculty of saturating the ioni^za-
tion due to a-particles by introducing the conception of iaitial
recombination ; viz., that the electron which is expelled ia tlie
process of ionization returns in a number. of cases to its pai^ent
atom, and exceptionally strong electric fields are neeaed to
exercise any appreciable preventive effect upon this tendenc v
to recombine. Kleeman extended the experimental wart,
using Bragg's theory as a basis. Moulin ascribed the difficulty
of obtaining saturation to the fact that the a-particles ionized
in columns, so that the density of ionization is not uniform
throughout the gas when sufficiently small volumes of gas are
considered. This localization of the ions would naturally
result in a recombination more intense than that which would
correspond to a uniform distribution in the usual acceptation
of the term. Moulin showed that saturation appeared to be
most difficult when the columns were parallel to the lines of
force of the electric field ; when, however, the a-particles
moved across the lines the ionization tended more readily to
saturation. Wheelock continued the work, adopting the idea
of columnar ionization ; in particular, he showed that when
the pressure was reduced to about one-third of an atmosphere
saturation set in fairly readily. There can be little doubt as
to the reality of this columnar effect and the intense recom-
bination resulting from the local distribution of the ions ; this
is clearly brought out by the slowness with which the ioniza-
tion current increases with the potential in the early stage of
any curve for a-particle ionization. The experimental results
obtained in the present research appear to lead to a radically
different explanation of the shape of the ionization curves at
the higher potentials. It has been shown that for potentials
which are not too low the ratio of the percentage cathode
activities for two different potentials is equal to the ratio of the
corresponding ionization currents due to the a-particles, and
over a wide range is independent of the amount of emanation
employed, that is, of the intensity of the intrinsic ionization.
This experimental result has already led to the suggestion that
the fraction of the total activity which is deposited on the walls
of the testing vessel is a measure of the lack of saturation of



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Deposit of Radium in an Electric jFteld. 497

the ionization current, so that in general the percentage case
activity is equal to the percentage lack of saturation of the
current. The values given in section 5 for the percentage
cathode activities at a pressure of 260"°* are smaller for the
higher potentials than tne corresponding values for atmospheric
pressure ; we are consequently constrained to reeard the per-
centage lack of saturation as being greater for the ionization
currents at the lower pressure than at the higher pressure.
The horizontality of the curves would thus app^r to furnish
no evidence as to the degree of saturation. Tne curves both
for activity and ionization current do not appear to have hori-
zontal asymptotes such as belong to the ordinary saturation
curves for Rontgen-ray ionization. We must rather look upon
the curves as having a continued upward slope, even when we
are considering the ionization curves corresponding to low
pressures. This upward slope suggests that extra ionization
16 produced by the electric iield after the a-particle has ceased
to ionize.

On this view there must be present in the gas certain mole-
cules, or neutrons, which are in a condition allowing of rela-
tively easy ionization. It is highly probable that the molecules
have already been put in this unstable condition by the action
of the a-particle ; we may look upon this condition as the result
of ineffectual attempts by the a-particle at ionization, possibly
as the result of actual ionization immediately succeeded by
initial recombination. However, these molecules are left by
the a-particle in an electrically neutral although unstable con-
dition, and a certain number of them are afterwards resolved
into ion^, probably as a result of collision with the ions already
established in the columns.

These neuti'ons are in all probability formed most numer-
ously during the early part of the range of the a particle, when
it is moving with its largest velocity. The approximately hori-
zontal parts of the ionization curves such as are obtained at
low pressures, or when the a-particle moves perpendicularly to
the lines of electric force, would therefore only represent satu-
ration in the sense that all the free ions have been brought over
to the electrodes because in these cases it is unlikely that an
appreciable number of neutrons would be resolved into ions.

As far as the active deposit particles are concerned we may
regard them as neutral restatoms which have been exposed to
the action of the a-particle at the moment of disintegration of
the emanation atom ; we would thus expect them to be for the
most part radio-active neutrons which have in general to be sub-
jected to further influence before acquiring a positive charge.
Experiments which are now in progress suggest also that the
pe of the Bragg-Kleeraan ionization curve can be accounted



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498 WelUsch and Bronsati — Active Deposit of Radium,

for by the formation of nentroos and ions in different propor-
tions along the entire range of the a-partiele.

8. Summary,

(1) The distribution in an electric field of the activity result-
ing from a long exposure to the emanation of radinm has been
determined for various conditions of pressure, potential, etc.

(2) There appear to be no negatively charged carriers,
all the so-called anode activity being due to the diffusion of
uncharged carriers.

(3) The effect on the distribution obtained by causing Ront-
gen rays to pai^s through the gas during the exposure has been
mvestigated ; this effect was appreciable only when the activity
and ionization were far from saturation.

(4) The difficulty of obtaining saturation both for the cath-
ode activity and for the a-ray ionization currents has been
explained as being due to the formation by the a-particle of
neutrons, some of which are subsequently resolved into ions,
probably by collision with ions already established in the
columns.



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Chemistry and Physics. 499

SCIENTIFIC INTELLIGENCE.

I. Chemistry and Physics.

1. The Melting-point of Spodumene. — Endell and Rieke have
made some new observations upon this subject and have drawn
interesting geological conclusions from their results. They used
spodumene from Branch ville, which appears to be much purer
than the material from Stirling which has been used by Doelter
for melting-point determinations. The authors find that the min-
eral, when heated for an hour or more to about 950° C, changes
its specific gravity from 3-147 to about 2-37, a gain in volume of
24 per cent. At the same time, between 920° and 980° C, the
monoclinic substance loses its double refraction and acquires a
lower average index of refraction ; and the heating-curve shows
a change at nearly the same temperature. It was found that the
material must be finely powdered to show these changes accu-
rately. The authors consider 960° C. as the true "melting-point"
of spodumene, although it is perfectly hard and solid at this tem-
perature and it does not actually fuse until a temperature of about
1380° C. is reached. They do not admit the existence of a solid,
isotropic modification of the mineral, because there is no differ-
ence between the specific gravity and index of refraction of the
un fused solid and the glass.

Since spodumene occurs at Branchville, Conn., in a pegmatite
vein, the authors suggest that it may serve as a measure of geo-
logical temperature, as they believe that it could not be formed
above about 950° C. They consider the effect of pressure as prac-
tically of little importance, since from Clapeyron's formula they
calculate the effect as only 7° C. per kilometer in depth. — Zeitschr.
anorgan, Chem.^ Ixxiv, 33. h. l. w.

2. The Detection of Nitric Acid in the Presefice of Nitrous
Acid, — Sen and Dky have found that hydrazine reacts with
nitrous acid according to the following equations :

N,H, 4- 2HN0,= N, -f. N,0 + 3H,0
N.H, + HNO. = NH, + N,0 + H,0

They have, therefore, used hydrazine sulphate for the purpose of
removing nitrites, in order that nitrates, upon which the reagent
has no action, may be detected. The method appears to be much
more accurate than that of Piccini, which depends upon the remo-
val of nitrous acid by means of urea in the presence of dilute sul-
phuric acid, because this reaction is not rapid enough to prevent



Online LibraryJohn Elihu HallThe American journal of science → online text (page 49 of 61)