AN EPOCH-MAKING DISCOVERY
When radium was discovered by Mme. Curie in 1898, the effect upon
the scientific world was startling, not to say “catastrophic”--as
one author wrote at the time--since its activities ran counter to
every known principle of physical science. “Some of the most solid
foundations of science were destroyed, some of its noblest edifices
wrecked, and scientists had to nerve themselves to face and investigate
a new form of energy.”
So soon as radium compounds (salts) became available, however, the
amount of energy given out in radioactive processes--the emission of
powerful radiations which can be transformed into light and heat--was
measured; and it was found that radium, weight for weight, gives out
as much heat as any known fuel every three days, and in the course of
fifteen years releases a quantity of energy nearly 2,000 times as much
as is obtained from the best fuel, with no signs of exhaustion (Soddy).
In the combustion of coal, the heat evolved is sufficient to raise a
weight of water some 80 to 100 times the weight of the fuel from the
freezing-point to the boiling-point. The spontaneous heat from radium
is sufficient to heat a quantity of water equal to the weight of radium
from the freezing-point to the boiling-point every three-quarters of an
hour. In other words, a pound of radium contains and evolves in its
changes the same amount of energy as 100 tons or more of coal evolve in
their combustion.
In ordinary chemical changes it is the _molecules_ (groups of
atoms) which are altered or rearranged; in radioactive change
the atoms themselves suffer disintegration and rearrangements.
The energy of radioactivity, then, is--according to the accepted
view--intra-atomic--stored-up energy within the atom itself. It was
calculated by Prof. Curie that the energy of one gram of radium would
suffice to lift a weight of 500 tons to a height of one mile. If it
were possible to obtain one cubic centimeter (a thimbleful) of the
“emanation” from radium in the form of a gas, we should find that it
possessed the power, altogether, of emitting more than seven million
calories of heat! A thimbleful of this invisible gas would be more
than sufficient to raise 15,000 pounds of water 1°. But in every mass
of radium, small or large, not more than 13 trillionths of it is
undergoing change per second.
“The processes occurring in the radio-elements,” says Rutherford again,
“are of a character quite distinct from any previously observed in
chemistry. Although it has been shown that the radioactivity is due
to the spontaneous and continuous production of new types of active
matter, the laws which control this production are different from the
laws of ordinary chemical reactions. It has not been found possible in
any way to alter either the rate at which the matter is produced or its
rate of change when produced. Temperature, which is such an important
factor in altering the rate of chemical reactions, is, in these cases,
entirely without influence. In addition, no ordinary chemical change is
known which is accompanied by the expulsion of charged atoms with great
velocity.... Besides their high atomic weights, [they] do not possess
in common any special chemical characteristics which differentiate them
from the other elements.”
It was early observed by Curie and Laborde that the temperature of a
radium salt is always a degree or two above that of the atmosphere,
and they estimated that a gram of pure radium would emit about 100
gram-calories per hour. Giesel later showed that radium was always at a
temperature 5° higher than the surrounding air, regardless of what the
temperature of the air might be. This continues unchanged whether the
temperature of the surroundings be 250° below zero Centigrade, or in
the intense heat of an electric furnace.
“Perhaps,” remarks a writer in _The Scientific American_ (February,
1922), “there will come a time when we shall use the energy in the
atoms to drive our machines, cook our food and heat our rooms. Besides,
already today we are actually using--even if only a very tiny part--the
atomic energy. Thus, for instance, the rays emanating from radium
are used for therapeutic purposes and the electrons emanating from a
glowing filament can be directed so easily that they can be used in a
large number of apparatus for wireless telegraphy and telephony. Most
probably plants also make use of this energy in their growth because
it has been demonstrated that the rays of the sun liberate electrons
from the green leaves, and lastly it may also be mentioned that we
humans use a little of this intra-atomic energy when seeing with
our eyes, which we are enabled to do by the photoelectric action of
light.”[A]
During the course of the process of disintegration, atoms of uranium
and thorium and their products give rise to no fewer than 36 different
substances (A. S. Russell), and of these at least a dozen are “new
elements.”
All of the 36 radioactive elements are disintegration products of one
or the other of the two parent elements, uranium and thorium. They are
arranged by the chemist in three series: namely, Uranium 1, Uranium 2
(the Actinium Series), and Thorium. In the first series there are known
to be 15 transmutations of matter; in the second, 11; and in the third,
10. The periods of “half change”--the period required for one-half of a
given quantity of a radioactive element to decompose--of the different
radioactive elements vary all the way from thousands of millions of
years for the longest lived primary elements--2.6x10^{10} years for
thorium, 8x10^9 for uranium 1--to .002 second for actinium A. In the
case of radium itself, 1,670 years are demanded for the disintegration
of half of any portion, according to the exact measurements of Profs.
B. O. Boltwood and Ellen Gleditsch. The stable end product appears to
be in each case an _isotope_ of lead--leads having similar chemical
properties but of different _atomic weights_ (_i.e._, different atomic
composition).
[A] See Shipley, Maynard, “Electricity and Life,” ch. vi.,
Little Blue Book No. 722.
Isotopes are groups of elements which cannot be distinguished (or
separated from) one another by any known chemical methods, and which
differ only in the atomic weights of the members of the group. In the
radioactive groups, the various elements differ also in degree of
stability of their atoms.
Chemists cannot actually weigh the mass of an atom of an element on a
pair of scales, or by any other method. But if we put down 16 as the
“atomic weight” of oxygen, and ascertain the “combining weight” (ratio)
of hydrogen to oxygen, we can determine the “atomic weight” of hydrogen
(1.008). (See Shipley, “The A B C of the Electron Theory of Matter,”
p. 14, Little Blue Book, No. 603.) The ratio of the masses of _any_
two elements in a chemical compound can be very accurately determined.
Without going into the details here, it may be said that the _relative_
weights of the atoms of any element can be determined to 0.01% in many
cases (by chemical analysis and synthesis); while the _actual_ weight
of any atom has not yet been determined to better than 0.1%.
HOW RADIUM IS CONVERTED TO LEAD
Lead is produced from uranium by a successive series of losses of Alpha
particles--or helium atoms. Omitting the less essential outcomes, or
transition stages, we find that each atom of uranium spontaneously
ejects three atoms of another element, helium, and thereby is
converted into still another element, radium. By losing one atom of
helium, radium, in turn, is converted into the so-called emanation,
or _niton_. The latter quickly loses four more atoms of helium and
is converted into lead, “uranium lead,” having an atomic weight of
206.08. Ordinary (common) lead, constituting the vast bulk of the lead
of the world, has a much higher atomic weight, namely, 207 (Prof.
Theodore Richards). Lead from thorium has an atomic weight of 208; from
actinium, 206. So we have, in fact, four kinds of lead.
Omitting the less stable transition products, we may say, then, that
an atom of uranium is converted into lead by the loss of eight atoms
of helium--losing three to become radium, then one to become the
emanation, and finally four to become lead. No known human agency can
either retard or hasten this breaking down of the uranium atom into
radium, or of the radium into emanation, with the final production of
lead.
This statement has been universally accepted as true. Nevertheless,
Dr. A. Glaschler stated (_Nature_ [London], September 12, 1925) that
he had succeeded in accelerating the change of uranium to uranium X
(the first product of uranium 1) by submitting uranium oxide to “strong
rushes of momentary high-tension currents.” As early as 1923, A. Nodon
(_Comp. rend., 176_, 1705 [1923]) brought forward strong evidence of
an increase of the activity of radioactive substances when outdoors
and enclosed by envelopes of small absorbing power for Gamma rays as
contrasted to the smaller radioactivity of the same substances in
cellars and when heavily enveloped by lead. For a tentative explanation
of this phenomenon, see _Science_, January 8, 1926 (Vol. LXIII, No.
1619), pp. 44–45.
Both uranium and thorium, as we have just stated, break down and become
radium, then change to helium and lead.
Says Rutherford: “Although thorium is nearly always present in old
uranium minerals and uranium in thorium minerals, there does not appear
to be any radioactive connection between these two elements. Uranium
and thorium are to be regarded as two distinct radioactive elements.
“With regard to actinium, there is still no definite information of its
place in the scheme of transformations. Boltwood has shown that the
amount of actinium in uranium minerals is proportional to the amount
of uranium. This indicates that actinium, like radium, is in genetic
connection with uranium....”
The recently discovered product, _protoactinium_,--isolated by Hahn and
Soddy,--is the hitherto missing link between uranium Y and actinium.
“This substance emits Alpha rays and has an estimated period of 10,000
years. The actinium series is believed to have its origin in a dual
transformation of uranium X. The first branch product, representing
about 4% of the total, is believed to be uranium Y, a Beta-ray product
of period one day. This is directly transformed into protoactinium.”
This element has not yet been obtained in a pure state.
Many of the radioactive elements are isotropic with known chemical
elements--_i.e._, alike in their chemical properties, but dissimilar
in radioactive properties. Since they cannot be distinguished--or
separated--from the ordinary elements with which they are isotropic, by
any chemical methods, they must occupy the same place in the periodic
classification of the elements. Radium and mesothorium, for example
(as Soddy was first to show) do not have the same atomic weight, but
they cannot be distinguished from each other by any chemical methods.
Therefore they both have the atomic _number_ 88, though the atomic
_weight_ of radium is 226 and of mesothorium 228. (See Shipley, “Origin
and Development of the Atomic Theory,” p. 64, Little Blue Book, No.
608.) Radium D and lead, and thorium and ionium, are examples of
radioactive isotropes.
The nature of the end-product was first suggested by Boltwood, who
pointed out the invariable presence of lead in old radium minerals,
and in amount to be expected from their uranium content and geologic
age. “Thus,” says Prof. T. W. Richards, of Harvard University, “we
must adopt a kind of limited transmutation of the elements,” although
not of the immediately profitable type [gold] sought by the ancient
alchemists.”
Sir Ernest Rutherford, who succeeded Sir J. J. Thomson as Cavendish
Professor of Physics at Cambridge University, was first to recognize
that the rays from uranium and radium were not all alike, but consisted
of three distinct kinds. In order to distinguish them clearly, without
committing himself in advance as to their exact nature, he christened
them Alpha, Beta, and Gamma rays--the first three letters of the Greek
alphabet. We know now that the Alpha rays are positively charged helium
atoms, with two negative electrons missing; that the Beta rays are
negatively charged electrons (disembodied “particles” of electricity,
exactly like cathode rays); and that the Gamma rays are a type of
X-rays, not material particles but merely extremely short magnetic
waves or oscillations, akin to ordinary light waves or rays.
Dr. R. A. Millikan calls them “the wireless waves of the denizens of
the sub-atomic world. They are ether waves, just like light or just
like wireless waves, except that the vibration frequency ... amounts to
30 billion billions per second. These are the Gamma rays.” This means
that this number of light waves would pass a given point in space each
second. Since these rays do not consist of charged particles they are
not deflected by electromagnetic or electrostatic fields, as are the
Alpha and Beta rays. It has been found that one gram of radium ejects
136,000,000,000 particles a second!
The Gamma rays of radium have such penetrating power that a half-inch
sheet of lead will reduce their original intensity by only one-half,
and they are not absolutely stopped by 20 inches. These invisible
light waves, thousands of times shorter than those of visible light,
are produced whenever a cathode ray (negative electron) hits matter.
Of the atoms forming the substance penetrated, perhaps only one in a
billion is struck. It has been said that the Gamma rays (and X-rays)
are the result of the back-kick of ejected electrons. Prof. Comstock
says that the connection between the Beta rays and the Gamma rays “is
probably similar to that between the bullet and the sound in the case
of a gun.” However this may be, we know that the Gamma rays are, after
all, in essence only excessively minute light waves. While the longest
visible light waves are 0.00008 centimeter, the longest Gamma rays are
0.000000013 centimeter; and whereas the shortest visible light waves
are 0.00004 centimeter, the shortest Gamma rays are but 0.0000000007
centimeter.
The Beta particles are ejected with a velocity of from 90,000 to
160,000 miles a second.
Prof. Gustave Le Bon calculated that it would require 340,000 barrels
of powder to discharge one bullet at this inconceivable speed! These
negatively charged electrons normally revolve around the positively
charged nucleus. Under certain conditions, an electron will make 2200
billion revolutions within an atom in one second.
Radium is not only continually losing matter and energy as electricity,
but it is also losing energy as heat. Professor and Mme. Curie
discovered that any substance placed near radium becomes itself
a _false_ radium. This applies to all substances. The acquired
radioactivity persists for many hours, or even days, after the removal
of the radium. In the case of zinc, these secondary radiations were
found to be four times as intense as ordinary uranium. It vanishes
sooner or later upon the removal from the neighborhood of the potent
radium.
The radioactive something which passes out of radium was not the
already known group of Alpha, Beta and Gamma rays, but an _emanation_
akin to gas. Rutherford, its original discoverer, was not sure that
it was a gas, so he cautiously gave it the name _emanation_. When the
radium was heated, or dissolved in water, the quantity of emanation
was greatly increased, which seemed to show that it was a gas of
some kind occluded (bound up) in the radium. The quantity obtained
was insufficient to bring the emanation within the testing power of
spectroscope or balance.
Nevertheless, the emanation has been detected, and investigated by the
electroscope, which measures the radium rays by the power to discharge
its electrified gold leaves. “The electroscope is about a million
times more sensitive than the most sensitive spectroscope and yet the
spectroscope is capable of detecting easily the millionth part of a
milligram of matter” (Duncan).
Calculations made by Rutherford show that if a thimbleful of this
active gas could be collected, the bombardment of its powerful rays
would heat to a red heat, or even might melt down, the walls of the
glass containing it. The emanation emits only Alpha rays (or particles)
forming helium.
The radium from which the emanation has been abstracted, after the
lapse of an hour or so, loses 75% of its activity. During the course
of a single month, radium will be found to have restored all its lost
emanation. In thirty days it will have regained all its original
activity. It was soon discovered that the emanation abstracted from the
radium loses its radioactivity at the same rate and according to the
same laws as the de-emanated radium regains it. The radium is therefore
said to be “in equilibrium with its products.”
Since these processes are wholly outside the sphere of known
controllable forces, and cannot be created, altered or destroyed--“since
the process is independent of the chemical form of the radium, whether
bromide, chloride, sulphate, etc., we are absolutely shut up to the
conviction that it is a function of the atom. We are in the presence of
an actual decay of the atom. The atom of radium breaks down into atoms
of emanation and the atoms of emanation in their turn break down into
something else. The activity of emanation decays and falls to half value
in about 3.7 days.”
Although the amount of emanation produced from a gram of radium
does not amount to more than a needle-point of the gas (= 1.3 cubic
millimeter), this is sufficient to raise the temperature of 75 grams
of water 1° per hour, which is enough heat to melt _more than its own
weight of ice_ in an hour, and to raise it to the boiling-point in the
next hour, which is equivalent to 60,000 horse-power days! In other
words, the heat evolved by the radium emanation is more than 3,500,000
times greater than that produced in any known chemical reaction: such
as, for example, the union of oxygen and hydrogen to form water.
It was soon discovered that if the spectrum of this mysterious gas--or
radium emanation--be examined again after an interval of about four
weeks, it has changed into a familiar spectrum easily recognized as
that of the gaseous element known as helium. Here the chemist comes
face to face with the astounding fact that the element radium is
decomposed and produces another element, helium--a discovery made by
Ramsay and Soddy in the summer of 1903.
In the successive radioactive changes, one Alpha particle (sometimes
called “ray”) is ejected from each atom disintegrated by the change--in
some cases, at least, accompanied by Beta particles (negative
electrons). The Alpha particle, as already stated, is really an atom of
helium carrying two atomic charges of positive electricity--twice that
of an atom of hydrogen. Strictly speaking, the Alpha particle is only
the _nucleus_ of a helium atom, since it has lost two of its negatively
charged electrons, which are combined in the ordinary helium atom. The
exact velocity of the expelled Alpha particle “varies in the different
radioactive elements” (Joly)--say from 10,000 to 18,000 miles each
second--a velocity sufficient to carry the particle around the earth in
less than two seconds, if unchecked.
But these relatively heavy particles (of atomic size) are actually soon
checked, even by seven centimeters (about a third of a foot) of air.
The Beta particle (1,845 the mass of a hydrogen atom) “shoots a hundred
times as far [as the Alpha particle] and the Gamma rays are a hundred
times more penetrating still” (Millikan). But the Alpha particle is
sometimes ejected with a velocity nearly 40,000 times that of a rifle
bullet,--the velocity of the latter being about half a mile a second.
Even the super-guns which bombarded Paris could not eject a projectile
with a speed of more than about a mile a second. Rutherford observes
that if it were possible to give an equal velocity to an iron cannon
ball, the heat generated on a target would be many thousand times more
than sufficient to melt the cannon ball and dissipate it into vapor.
The flashes of light seen when the Alpha rays bombard a screen of zinc
sulphide, as in Crookes’ spinthariscope, are due to cleavages produced
in the zinc sulphide crystals by the impact of the Alpha rays (positive
ions). Each impact on a crystal produces a splash of light big enough
to be seen by a microscope.
In the phosphorescence caused by the approach of an emanation of radium
to zinc sulphate, the atoms throw off the Alpha (helium) particles to
the number of five billion each second, with velocities of 10,000 miles
or more a second. If the helium projectile should chance to “crash”
into an atom of nitrogen or of oxygen, an atom of hydrogen can be
knocked out of it, as was discovered by Sir Ernest Rutherford, perhaps
the most distinguished of Mme. Curie’s pupils. (Strictly speaking, the
disintegration particles are isotropes of helium, of atomic weight
3, the atomic weight of helium being 4.) Despite its large size as
compared with an electron (or Beta particle), the Alpha particle passes
through a glass wall without leaving a hole behind, and without in any
way interfering with the molecules of the glass. It shoots through
hundreds of thousands of atoms without ever going near enough to them
to be deflected from its course.