F
RANCIS
W. A
STON
Mass spectra and isotopes
Nobel Lecture, December 12, 1922
Dalton’s statement of the Atomic Theory, which has been of such incalcu-
lable value in the development of chemistry, contained the postulate that
"atoms of the same element are similar to one another, and equal in weight".
The second part of this postulate cannot, in general, be tested by chemical
methods, for numerical ratios are only to be obtained in such methods by
the use of quantities of the element containing countless myriads of atoms.
At the same time it is somewhat surprising, when we consider the complete
absence of positive evidence in its support, that no theoretical doubts were
publicly expressed until late in the nineteenth century.
There are two methods by which the postulate can be tested experi-
mentally, either by comparing the weights of the individual atoms, or al-
ternatively by demonstrating that samples of an element can exist which
though chemically identical yet have different atomic weights. The latter
method, by which the existence of isotopes was first proved, has been fully
dealt with in the previous lecture by Professor Soddy. The more direct
method, with which this lecture is concerned, can be applied by means of
the analysis of positive rays.
The condition for the development of these rays is briefly ionization at
low pressure in a strong electric field. Ionization, which may be due to col-
lisions or radiation, means in its simplest case the detachment of one electron
from a neutral atom. The two resulting fragments carry charges of electricity
of equal quantity but of opposite sign. The negatively charged one is the
electron, the atomic unit of negative electricity itself, and is the same what-
ever the atom ionized. It is extremely light and therefore in the strong electric
field rapidly attains a high velocity and becomes a cathode ray. The re-
maining fragment is clearly dependent on the nature of the atom ionized.
It is immensely more massive than the electron, for the mass of the lightest
atom, that of hydrogen, is about 1,850 times that of the electron, and so will
attain a much lower velocity under the action of the electric field. However,
if the field is strong and the pressure so low that it does not collide with other
atoms too frequently, it will ultimately attain a high speed in a direction
8
1 9 2 2 F . W . A S T O N
opposite to that of the detached electron, and become a "positive ray". The
simplest form of positive ray is therefore an atom of matter carrying a
positive charge and endowed, as a result of falling through a high potential,
with sufficient energy to make its presence detectable. Positive rays can be
formed from molecules as well as atoms, so that it will at once be seen that
any measurement of their mass will give us direct information as to the
masses of atoms of elements and molecules of compounds, and that this
information will refer to the atoms or molecules individually, not, as in chem-
istry, to the mean of an immense aggregate. It is on this account that the
accurate analysis of positive rays is of such importance.
In the parabola method of analysis devised by Sir J. J. Thomson, the rays
generated by means of an electric discharge, after reaching the surface of the
cathode, enter a long and very fine metal tube. By this means a narrow
beam of rays is produced which is subjected to deflection by electric and
magnetic fields, and finally falls upon a screen of fluorescent material or a
photographic plate. The fields are arranged so that the two deflections are
at right angles to each other. If we call the displacement on the plate due to
the electric field x and that due to the magnetic field y for any particle, (x, y)
will be the rectangular coordinates of the point where it strikes the plate.
Simple dynamics show that if the angle of deflection is small, for a particle
of mass m, charge e and velocity
V
, the electric deflection x = k(Xe/mv
2
)
and the magnetic deflection y = k’ ( He/mv) where X and H are the magnetic
and electric fields, and k and k’ constants depending solely on the dimensions
and form of the apparatus. Hence if both fields are on together, the locus
of impact of all particles of the same e/m but varying velocity will be a
parabola. Since e must be the electronic charge, or a simple multiple of it,
measurement of the relative positions of the parabolas on the plate enables
us to calculate the relative masses of the particles producing them, that is,
the masses of the individual atoms. The fact that the streaks were definite,
sharp parabolas, and not mere blurs, constituted the first direct proof that
atoms of the same element were, even approximately, of equal mass.
Many gases were examined by this method and some remarkable com-
pounds, such as H
2
, discovered by its means. When in 1912 neon was in-
troduced into the discharge tube, it was observed to exhibit an interesting
peculiarity. This was that whereas all elements previously examined gave
single, or apparently single, parabolas, that given by neon was definitely
double. The brighter curve corresponded roughly to an atomic weight of
20, the fainter companion to one of 22, the atomic weight of neon being
M A S S S P E C T R A A N D I S O T O P E S
9
20.20. In consequence of reasoning adduced from the characteristics of the
line 22, Sir J. J. Thomson was of the opinion that it could not be attributed
to any compound, and that therefore it represented a hitherto unknown ele-
mentary constituent of neon. There was no room for such an element in
the Periodic Table; but the suggestion that atoms of different weight could
exist having identical chemical properties had just been promulgated, and
the facts could be explained very well by the supposition that neon was a
mixture of two such bodies. I therefore undertook to investigate this point
as fully as possible.
The first line of attack was an attempt at separation by fractional distilla-
tion over charcoal cooled with liquid air, but even after many thousands of
operations the result was entirely negative. The density of the fractions was
determined by means of a special quartz microbalance and was found to
agree in every case with the accepted atomic weight of 20.20 to an accuracy
of I in 1000.
The second method I employed was that of fractional diffusion through
pipeclay, which after months of arduous work gave a small but definite
positive indication of separation. A difference of about 0.7 per cent between
the densities of the heaviest and lightest fractions was obtained. If V is the
initial and v, the residual volume of gas, Rayleigh’s theory of diffusion gives
an enrichment of approximately (V/v)
I/21
in respect to the heavier con-
stituent in the residue in the case of neon. This is the result of a single opera-
tion, and the effective ratio V/
V
can be increased indefinitely by repeated
operations-in these experiments it amounted to several thousand- but
from the nature of the work cannot be stated exactly. If, to be perfectly safe,
we take V/
V
as between 500 and 10,000, the theoretical increase in density
of the heaviest fraction over normal should lie between 0.34 and 0.55 per
cent. It was actually 0.4, quite a reasonable figure considering the inefficiency
of the method. The decrease in density in the lightest fraction was rather less
owing to the loss of a portion of the gas during the experiment. When the
war interrupted the research, it might be said that several independent lines
of reasoning pointed to the conclusion that neon was a mixture of isotopes,
but none of these could be said to carry absolute conviction.
By the time the work was resumed in 1919 the existence of isotopes among
the products of radioactivity had been put beyond all reasonable doubt by
the work on the atomic weight of lead and was accepted generally. This fact
automatically increased both the value of the evidence of the complex nature
of neon and the urgency of its definite confirmation. It was realized that
10
1922 F.W.ASTON
separation could only be very partial at the best, and that the most satis-
factory proof would be afforded by measurements of atomic weight by the
method of positive rays. These would have to be so accurate as to prove
beyond dispute that the accepted atomic weight lay between the real atomic
weights of the constituents, but corresponded with neither of them.
The parabola method was not capable of this, for the measurements ob-
tained by means of the best apparatus so far constructed were only reliable
to about one per cent. I therefore started to examine systematically all pos-
sible alternative methods, particularly those in which the fine circular tube
could be replaced by a pair of parallel slits. The reason for this is that the
intensity of the beam of rays will only be reduced in proportion to the
square of the breadth of the slits, whereas for the circular tube the reduction
is proportional to the fourth power of its breadth.
It is clearly only possible to use slits satisfactorily if the electric and magnet-
ic deflections are both in the same plane at right angles to the slits, and during
a mathematical investigation originally started to determine the most fa-
vourable positions of the slits, fields, and plate, I was fortunate enough to
hit on the focussing principle used in the mass spectrograph. A diagram of
this apparatus is given in Fig.
I.
The exact mathematical analysis of the
principle has now been worked out, but it will be enough to give the ap-
proximate theory here.
The rays after arriving at the cathode face pass through two very narrow
parallel slits of special construction S
1
S
2
, and the resulting thin ribbon is
spread out into an electric spectrum by means of the parallel plates P
1
P
2
.
Fig.I. Diagram of mass spectrograph.
M A S S S P E C T R A A N D I S O T O P E S
I I
After emerging from the electric field the rays may be taken, to a first order
of approximation, as radiating from a virtual source Z half-way through
the field on the line S
1
S
2
. A group of these rays is now selected by means
of the diaphragm D, and allowed to pass between the parallel poles of a
magnet. For simplicity the poles are taken as circular, the field between them
uniform and of such sign as to bend the rays in the opposite direction to the
foregoing electric field.
If
θ and ϕ be the angles (taken algebraically) through which the selected
beam of rays is bent by passing through fields of strength X and H, then
and
(1)
(2)
where I, L are the lengths of the paths of the rays in the fields. Eq. (
I
) is
only true for small angles, but exact enough for practice. It follows that over
the small range of
θ selected by the diaphragm, θ v
2
and
V
are constant for
all rays of given e/m, therefore
o, and
o,
so that
when the velocity varies in a group of rays of given e/m. This equation
appears correct within practical limits for large circular pole-pieces.
Referred to axes OX, OY the focus is at r cos
Y
sin
or
r,
so that to a first-order approximation, whatever the fields, so long
as the position of the diaphragm is fixed, the foci will all lie on the straight
line ZF drawn through Z parallel to OX. For purposes of construction, G
the image of Z in OY is a convenient reference point,
ϕ, being here equal to
4
'.
It is clear that a photographic plate, indicated by the thick line, will be
in fair focus for values of e/m over a range large enough for accurate com-
parison of masses.
Since it is a close analogue of the ordinary spectrograph and gives a
<> depending upon mass alone, the instrument is called a <
spectrographs and the spectrum it produces a <>.
Fig. 2 shows a number of typical mass spectra obtained by this means.
The numbers above the lines indicate the masses they correspond to, on the
12
1922 F. W.ASTON
scale O = 16. It will be noticed that the displacement to the right with
increasing mass is roughly linear. The measurements of mass made are not
absolute, but relative to lines which correspond to known masses. Such lines
due to hydrogen, carbon, oxygen, and their compounds are generally pres-
ent as impurities or purposely added, for pure gases are not suitable for the
smooth working of the discharge tube. The two principal groups of these
reference lines are the C
I
group due to C (12), CH (13), CH
2
(14), CH
3
(15), CH
4
or O(16), and the C
2
group (24 to 30) containing the very strong
line C
2
H
4
or CO (28). These groups will be seen in several of the spectra
reproduced, and they give, with the CO, line (44), a very good scale of
reference.
It must be remembered that the ratio of mass to charge is the real quantity
measured by the position of the lines. Many of the particles are capable of
carrying more than one charge. A particle carrying two charges will appear
as having half its real mass, one carrying three charges as if its mass was one-
third, and so on: Lines due to these are called lines of the second and third
order. Lines of high order are particularly valuable in extending our scale
M A S S S P E C T R A A N D I S O T O P E S
13
of reference. When neon was introduced into the apparatus, four new lines
made their appearance at 10, 11, 20, and 22. The first pair are second-order
lines and are fainter than the other two. All four are well placed for direct
comparison with the standard lines, and a series of consistent measurements
showed that to within about one part in a thousand the atomic weights of
the isotopes composing neon are 20 and 22 respectively. Ten per cent of the
latter would bring the mean atomic weight to the accepted value of 20.20,
and the relative intensity of the lines agrees well with this proportion. The
isotopic constitution of neon was therefore settled beyond all doubt.
Spectrum I on the plate shows the first-order lines of neon and some of
the reference lines with which they were compared.
The element chlorine was naturally the next to be analysed, and the ex-
planation of its fractional atomic weight was obvious from the first plate
taken. Its mass spectrum is characterized by four strong first-order lines at
35, 36, 37, 38. There is no sign whatever of any line at 35.46. The simplest
explanation of the group is to suppose that the lines 35 and 37 are due to the
isotopic chlorines, and lines 36 and 38 to their corresponding hydrochloric
acids. The elementary nature of lines 35 and 37 is also indicated by the
second-order lines at 17.5, 18.5, and also, when phosgene was used, by the
appearance of lines at 63, 65, due to CO
35
CI and CO
37
CI.
Later it was found possible to obtain the spectrum of negatively charged
rays. These rays are formed by a normal positively charged ray picking up
two electrons. On the negative spectrum of chlorine only two lines, 35 and
37, can be seen, so that the lines at 36 and 38 cannot be due to isotopes of
the element. These results, taken with many others which cannot be stated
here in detail, show that chlorine is a complex element, and that its isotopes
are of atomic weight 35 and 37. Spectra II, III, and IV show the results with
chlorine taken with different magnetic field strengths.
The mass spectrum of argon shows an exceedingly bright line at 40, with
second-order line at 20, and third-order line at 135. The last is particularly
well placed between known reference lines, and its measurement showed
that the triply charged atom causing it, had a mass 40 very exactly. Now
the accepted atomic weight of argon is less than 40, so the presence of a
lighter isotope was suggested. This was found at 36, and has now been fully
substantiated; its presence to the extent of about 3 per cent is sufficient to
account for the mean atomic weight obtained by density determinations.
The elements hydrogen and helium present peculiar difficulties, since their
lines are so far removed from the ordinary reference scale, but, as the lines
14
1 9 2 2 F . W . A S T O N
were expected to approximate to the terms of the geometrical progression
1, 2, 4, 8, etc., the higher terms of which are known, a special method was
adopted by which a two-to-one relation could be tested with some exactness.
Two sets of accumulators were selected, each giving very nearly the same
potential of about 250 volts. The potentials were then made exactly equal
by means of a subsidiary cell and a current-divider, the equality being tested
to well within 1,000 by means of a null instrument. If exposures are made
with such potentials applied to the electric plates first in parallel and then
in series, the magnetic field being kept constant, all masses having an exact
two-to-one relation will be brought into coincidence on the plate. Such
coincidences cannot be detected on the same spectrum photographically;
but if we first add and then subtract a small potential from one of the large
potentials, two lines will be obtained which closely bracket the third. To
take an actual instance - using a gas containing hydrogen and helium, with
a constant current in the magnet of 0.2 ampere, three exposures were made
with electric fields of 250, 500 + 12, and 500 - 12 volts, respectively. The
hydrogen molecule line was found symmetrically bracketed by a pair of
atomic lines (Spectrum VII, a and c), showing within experimental error
that the mass of the molecule is exactly double the mass of the atom. When
after a suitable increase of the magnetic field the same procedure was applied
to the helium line and that of the hydrogen molecule, the bracket was no
longer symmetrical ( Spectrum VII, b), nor was it when the hydrogen mol-
ecule was bracketed by two helium lines (d). Both results show in an un-
mistakable manner that the mass of He is less than twice that of H
2
. In the
same way He was compared with O++ and H
3
. The values obtained by its
use can be checked in the ordinary way by comparing He with C++ and
H
3
with He, these pairs being close enough together for the purpose. The
following table gives the range of values obtained from the most reliable
plates:
M A S S S P E C T R A A N D I S O T O P E S
15
From these figures it is safe to conclude that hydrogen is a simple element
and that its atomic weight, determined with such consistency and accuracy
by chemical methods, is the true mass of its atom.
The heavy inert gases give interesting and complicated results. Krypton
is characterized by a remarkable group of five strong lines at 80, 82, 83, 84,
86, and a faint sixth at 78. This cluster of isotopes is beautifully reproduced
with the same relative values of intensity in the second, and fainter still in
the third order. These multiply-charged clusters give most reliable values
of mass, as the second order can be compared with A (40) and the third
with CO or N
2
(28) with the highest accuracy. It will be noted that one
member of each group is obliterated by the reference line, but not the same
one. The singly and doubly charged krypton clusters can be seen to the right
and left of SpectrumVIII. It will be noticed that krypton is the first element
examined which shows unmistakable isotopes differing by one unit only.
Xenon is even more complex. It consists of five strong components 129,
131, 132, 134, 136 and two fainter ones 128, 130; in addition recent work
has revealed two extremely faint probable ones 124, 126, making nine in
all.
Mercury was early recognized to be a mixture of isotopes. Its components
are however too close to be perfectly resolved by the present apparatus. Its
first, second, third, and higher order lines appear as a series of characteristic
groups around positions corresponding to masses 200, 100,
etc. Some
of these will be easily distinguished on the spectra reproduced in the plate.
In addition to those mentioned above, the elements nitrogen, boron, fluo-
rine, silicon, bromine, sulphur, phosphorus, iodine, nickel, tin, iron, sele-
nium, aluminium, and antimony have been analysed by means of the
spectrograph, using the discharge-tube method of producing the positive
rays. By the analysis of anode rays the constitution of the alkali metals lith-
ium, sodium, potassium, rubidium, and caesium has been determined. The
following table gives the collected results and also includes magnesium, cal-
cium, and zinc, whose complex constitution has been determined by Demp-
ster, using his own method of analysis, and beryllium, which has been
investigated by G. P. Thomson, using the parabola method.
By far the most important result of these measurements is that, with the
exception of hydrogen, the weights of the atoms of all the elements meas-
ured, and therefore almost certainly of all elements, are whole numbers to
the accuracy of experiment, namely, about one part in a thousand. Of course,
the error expressed in fractions of a unit increases with the weight measured,
16
1 9 2 2 F . W . A S T O N
Table of elements and isotopes.
Element
Atomic
Atomic
number
weight
Minimum
number of
isotopes
Masses of isotopes in order of intensity
H
1
He
2
Li
3
Be
4
B
5
C
6
N
7
0
8
F
9
Ne
10
Na
11
Mg
12
Al
13
Si
14
P
15
S
16
Cl
17
A
18
K
19
Ca
20
Fe
26
Ni
28
Zn
30
As
33
Se
34
Br
35
Kr
36
Rb
37
Sn
50
Sb
51
I
53
X
54
Cs
55
132.81
Hg
80
200.6
1.008
3.99
6.94
9.1
10.9
12.00
14.01
16.00
19.00
20.20
23.00
24.32
26.96
28.3
31.04
32.06
35.46
39.88
39.10
40.07
55.84
58.68
65.37
74.96
79.2
79.92
82.92
85.45
118.7
121.77
126.92
130.2
1
1
2
1
2
1
1
1
1
2
1
3
1
2
1
1
2
2
2
( 1 )
(4)
1
6
2
6
7(8)
2
I
7(9)
1.008
4
7, 6
9
11, 10
12
14
16
19
20,22
23
24,25,26
27
28,29,(30)
31
32
35,37
40,36
39,41
40, 44
56,(54)?
58,60
64,66,68,70
75
80,78,76,82,77,74
79,81
84,86,82,83,80,78
85,87
120,118,116,124,119,117,122,(121)
121, 123
127
129,132,131,134,136,128,130,(126),
(124)
133
(197-200),202,204
Numbers in brackets are provisional only.
M A S S S P E C T R A A N D I S O T O P E S
17
but with the lighter elements the divergence from the whole number rule is
extremely small.
This enables the most sweeping simplifications to be made in our ideas
of mass. The original hypothesis of Prout, put forward in 1815, that all
atoms were themselves built of atoms of protyle, a hypothetical element
which he tried to identify with hydrogen, is now reestablished, with the
modification that the primordial atoms are of two kinds: protons and elec-
trons, the atoms of positive and negative electricity.
The Rutherford-Bohr atom consists essentially of a positively charged
central nucleus around which revolve planetary electrons at distances great
compared with the dimensions of the nucleus itself.
As has been stated, the chemical properties of an element depend solely
on its atomic number, which is the charge on its nucleus expressed in terms
of the unit charge, e. A neutral atom of an element of atomic number N
has a nucleus consisting of K + N protons and K electrons, and around this
nucleus revolve N electrons. The weight of an electron on the scale we are
using is 0.0005, so that it may be neglected. The weight of this atom will
therefore be K + N, so that if no restrictions are placed on the value of K
any number of isotopes are possible.
A statistical study of the results given above shows that the natural
restrictions can be stated in the form of rules as follows:
In the nucleus of an atom there is never less than one electron to every two protons.
There is no known exception to this law. It is the expresssion of the fact
that if an element has an atomic number N the atomic weight of its lightest
isotope cannot be less than
2
N. Worded as above, the ambiguity in the case
of hydrogen is avoided. True atomic weights corresponding exactly to
2
N
are known in the majority of the lighter elements up to
36
A. Among the
heavier elements the difference between the weight of the lightest isotope
and the value
2
N tends to increase with the atomic weight; in the cases of
mercury it amounts to 37 units. The corresponding divergence of the mean
atomic weights from the value
2
N has of course been noticed from the be-
ginning of the idea of atomic number.
The number of isotopes of an element and their range of atomic weight appear
to have definite limits - Since the atomic number only depends on the net
positive charge in the nucleus, there is no arithmetical reason why an element
should not have any number of isotopes.
So far the largest number appears to be 9 in the case of xenon, which also
shows the maximum difference between its lightest and heaviest isotopes,
18
1922 F. W.ASTON
12 units. The greatest proportional difference calculated on the lighter weight
is recorded in the case of lithium, where it amounts to one-sixth. It is about
one-tenth in the cases of boron, neon, argon, selenium, krypton, and xenon.
No element of odd atomic number has more than two isotopes.
The number of electrons in the nucleus tends to be even - This rule expresses
the fact that in the majority of cases even atomic number is associated with
even atomic weight, and odd with odd. If we consider the three groups of
elements, the halogens, the inert gases and the alkali metals, this tendency is
very strongly marked. Of the halogens - odd atomic numbers - all 6 (+
1 ?) atomic weights are odd. Of the inert gases - even atomic numbers - 13
(+2?) are even and 3 odd. Of the alkali metals - odd atomic numbers - 7
are odd and 1 even. In the cases of elements of other groups the prepon-
derance, though not so large, is still very marked. Beryllium and nitrogen
are the only elements yet discovered to consist entirely of atoms whose
nuclei contain an odd number of electrons.
If we take the natural numbers 1 to 40, we find that those not represented
by known atomic weights are 2, 3, 5, 8, 13, (17), (18), 21, (33), 34, (38).
It is rather remarkable that these gaps, with the exception of the four in
parentheses, are represented by a simple mathematical series of which any
term is the sum of the two previous terms. In consequence of the whole-
number rule there is now no logical difficulty in regarding protons and
electrons as the bricks out of which atoms have been constructed. An atom
of atomic weight m is turned into one of atomic weight m + 1 by the addi-
tion of a proton plus an electron. If both enter the nucleus, the new element
will be an isotope of the old one, for the nuclear charge has not been altered.
On the other hand, if the proton alone enters the nucleus and the electron
remains outside, an element of next higher atomic number will be formed.
If both these new configurations are possible, they will represent elements
of the same atomic weight but with different chemical properties. Such
elements are called "isobases".
It will be observed that the principal atomic species of argon and calcium
are isobases, each having a weight 40. In the same manner no less than four
seleniums form isobasic pairs with kryptons. In all such pairs known with
certainty to exist, the elements concerned have even atomic numbers and
even atomic weights, also one member of each pair is an inert gas. The
whole-number rule is not to be supposed as mathematically exact, for al-
though the atoms of all elements are made up of the same electrical units
their masses will be affected slightly by the way in which these unit charges
M A S S S P E C T R A A N D I S O T O P E S
19
are put together. This is called the packing effect, and some recent results
suggest that the difference caused by it may amount to two or three parts in
a thousand in the mass relations of the isotopes of tin and xenon. The
packing effect may be clearly expected to be a maximum in the relation
between the masses of helium and hydrogen.
The case of the element hydrogen is unique; its atom appears to consist
of a single proton as nucleus with one planetary electron. It is the only atom
in which the nucleus is not composed of a number of protons and electrons
packed exceedingly close together. Theory indicates that when such close
packing takes place the effective mass will be reduced, so that when four
protons are packed together with two electrons to form the helium nucleus
this will have a weight rather less than four times that of the hydrogen
nucleus, which is actually the case. It has long been known that the chemical
atomic weight of hydrogen was greater than one-quarter of that of helium,
but so long as fractional weights were general there was no particular need
to explain this fact, nor could any definite conclusions be drawn from it.
The results obtained by means of the mass spectrograph remove all doubt
on this point, and no matter whether the explanation is to be ascribed to
packing or not, we may consider it absolutely certain that if hydrogen is
transformed into helium a certain quantity of mass must be annihilated in
the process. The cosmical importance of this conclusion is profound and the
possibilities it opens for the future very remarkable, greater in fact than any
suggested before by science in the whole history of the human race.
We know from Einstein’s Theory of Relativity that mass and energy are
interchangeable and that in c.g.s. units a mass m at rest may be expressed
as a quantity of energy mc
2
, where c is the velocity of light. Even in the case
of the smallest mass this energy is enormous. The loss of mass when a single
helium nucleus is formed from free protons and electrons amounts in energy
to that acquired by a charge e falling through a potential of nearly thirty
million volts. If instead of considering single atoms we deal with quantities
of matter in ordinary experience, the figures for the energy become pro-
digious.
Take the case of one gram-atom of hydrogen, that is to say the quantity
of hydrogen in 9
C
.
C
. of water. If this is entirely transformed into helium
the energy liberated will be
0.0077 x 9 x 10
20
= 6.93 x 10
18
e r g s
20
1 9 2 2 F . W . A S T O N
Expressed in terms of heat, this is 1.66 x 10
11
calories or in terms of work
200,000 kilowatt hours. We have here at last a source of energy sufficient
to account for the heat of the sun. In this connection Eddington remarks
that if only 10 per cent of the total hydrogen on the sun were transformed
into helium, enough energy would be liberated to maintain its present radia-
tion for a thousand million years.
Should the research worker of the future discover some means of re-
leasing this energy in a form which could be employed, the human race will
have at its command powers beyond the dreams of scientific fiction; but
the remote possibility must always be considered that the energy once lib-
erated will be completely uncontrollable and by its intense violence detonate
all neighbouring substances. In this event the whole of the hydrogen on the
earth might be transformed at once and the success of the experiment pub-
lished at large to the universe as a new star.
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