Theories of matter.
(Gr. [Greek: atomos], indivisible, from [Greek: a-] privative, and
[Greek: temnein], to cut), the term given in physical science to the
ultimate indivisible particle of matter, and so by analogy to something
minutely small in size. If we examine such a substance as sugar we find
that it can be broken up into fine grains, and these again into finer,
the finest particles still appearing to be of the same nature as sugar.
The same is true in the case of a liquid such as water; it can be
divided into drops and these again into smaller drops, or into the
finest spray the particles of which are too small to be detected by our
unaided vision. In fact, so far as the direct evidence of our senses
tells us, matter appears to be indefinitely divisible. Moreover, small
particles do not seem to exist in the water until it is broken up; so
far as we can see, the material of the water is continuous not granular.
This conception of matter, _as infinitely divisible and continuous_, was
taught by Anaxagoras more than four centuries before the Christian era,
and in the philosophy of Aristotle the same ideas are found. But some
phenomena are difficult to reconcile with this view; for example, a
cubic foot of air can be compressed into less than one five-hundredth of
a cubic foot, or, if allowed to expand, the air originally occupying the
cubic foot can be made to fill, apparently uniformly, a space of a
million cubic feet or more. This enormous capacity for expansion and
contraction is astonishing if we believe matter to be continuous, but if
we imagine air to be made up of little particles separated by relatively
large empty spaces the changes in volume are more easily conceivable.
Moreover, if we attribute such a structure to gases, we are led to
attribute it to liquids and to solids also, since gases can be liquefied
without any abrupt change, and many substances usually solid can be
converted into gases by heating them. This conception of the _grained_
structure of matter is very ancient; traces of it are to be found in
Indian philosophy, perhaps twelve centuries before the Christian era,
and the Greek philosophers Democritus and Epicurus, in the 3rd and 4th
centuries B.C., taught it very definitely. Their view was that "matter
is not indefinitely divisible, but that all substances are formed of
indivisible particles or atoms which are eternal and unchangeable, that
the atoms are separated from one another by void, and that these atoms,
by their combinations, form the matter we are conscious of." The Roman
poet Lucretius (_De Rerum Natura_) was an eloquent exponent of this
theory, but throughout the middle ages, indeed until the 17th century,
it was eclipsed by the prestige of Aristotle. In the time, however, of
Boyle[1] and Newton, we again find an atomic theory of matter; Newton[2]
regarded a gas as consisting of small separate particles which repelled
one another, the tendency of a gas to expand being attributed to the
supposed repulsion between the particles.
Let us consider some common phenomena in the light of these rival
theories as to the nature of matter. When a few lumps of sugar are added
to a glass of water and stirred, the sugar soon disappears and we are
left with a uniform liquid resembling water, except that it is sweet.
What has become of the sugar? Does it still exist? The atomist would
say, "Yes, it is broken up into its atoms, and these are distributed
throughout the spaces between the particles of water." The rival
philosopher, who believes water to be continuous and without spaces
between its particles, has a greater difficulty in accounting for the
disappearance of the sugar; he would probably say that the sugar, and
the water also, had ceased to exist, and that a new continuous substance
had been formed from them, but he could offer no picture of how this
change had taken place. Or consider a well-marked case of what we are in
the habit of calling _chemical combination_. If 127 parts of iodine,
which is an almost black solid, and 100 parts of mercury, which is a
white liquid metal, be intimately mixed by rubbing them together in a
mortar, the two substances wholly disappear, and we obtain instead a
brilliant red powder quite unlike the iodine or the mercury; almost the
only property that is unchanged is the weight. The question again
arises, what has become of the original substances? The atomist has an
easy answer; he says that the new body is made up by the juxtaposition
of the atoms of iodine and mercury, which still exist in the red powder.
His opponent would be disposed to say that the iodine and the mercury
ceased to exist when the red powder was formed, that they were
_components_ but not _constituents_ of it. The fact that the two
components can be recovered from the compound by destroying it does not
decide the question. It is remarkable that pure chemistry, even to-day,
has no very conclusive arguments for the settlement of this controversy;
but the sister science of physics is steadily accumulating evidence in
favour of the atomic conception.
[Illustation: From Dalton's _New System of Chemical Philosophy_.]
Hydrogen Gas.
Nitrous Gas.
Carbonic Acid Gas.]
[Illustation:
(.) hydrogen.
( ) oxygen.
(|) nitrogen.
(O) carbon.
(.)( ) water.
(.)(|) ammonia.
(.)(O) ethylene.
(O)( ) carbon monoxide.
( )(O)( ) carbon dioxide.
(|)( ) nitric oxide (nitous gas).
(|)( )(|) nitrous oxide.
( )(|)( ) nitrogen peroxide.]
Dalton.
Until the time of John Dalton, the atomic conception remained purely
qualitative, and until then it does not appear to have advanced
chemistry or to have found further confirmation in the facts of
chemistry. Dalton (1803) gave the atomic theory a quantitative form, and
showed that, by means of it, a vast number of the facts of chemistry
could be predicted or explained. In fact, he did so much to make the
atomic theory of matter probable that he is popularly regarded as its
originator. Dalton lived in a period marked by great advances in
experimental chemistry. Rather before the commencement of the 19th
century the work of Lavoisier had rendered it very probable that
chemical changes are not accompanied by any change in weight, and this
principle of the conservation of matter was becoming universally
accepted; chemists were also acquiring considerable skill in chemical
analysis, that is, in the determination of the nature and relative
amounts of the elements contained in compounds. But Sir H.E. Roscoe and
A. Harden, _New View of the Atomic Theory_ (1896), have shown, from a
study of Dalton's manuscript notes, that we do not owe his atomic theory
to such experiments. If their view is correct, the theory appears to be
a remarkable example of deductive reasoning. Dalton, who was a
mathematical physicist even more than a chemist, had given much thought
to the study of gases. Following Newton, he believed a gas to be made up
of particles or atoms, separated from one another by considerable
spaces. Certain difficulties that he met with in his speculations led
him to the conclusion that the particles of any one kind of gas, though
all of them alike, must differ from those of another gas both in _size_
and _weight_. He thus arrived at the conception of a definite atomic
weight peculiar to the particles of each gas, and he thought that he
could determine these atomic weights, in terms of one of them, by means
of the quantitative analysis of compounds. The conclusion that each
element had a definite atomic weight, peculiar to it, was the new idea
that made his speculations fruitful, because it allowed of quantitative
deduction and verification. He drew simple diagrams, three of which,
taken from Dalton's _New System of Chemical Philosophy_, part ii.
(1810), are reproduced here, in which gases are represented as composed
of atoms. Knowing that the gas which he called "nitrous gas" was
composed of oxygen and nitrogen, and believing it to be the simplest
compound of these two elements, he naturally represented its atom as
formed of an atom of oxygen and an atom of nitrogen in juxtaposition.
When two elements form more than one compound, as is the case with
oxygen and carbon, he assigned to the compound which he thought the more
complex an atom made up of two atoms of the one element and one atom of
the other; the diagram for carbonic acid illustrates this, and an
extension of the same plan enabled him to represent any compound,
however complex its structure. The table here given contains some of
Dalton's diagrams of atoms. They are not all considered to be correct at
the present time; for example, we now think that the ultimate particle
of water is made up of two atoms of hydrogen and one of oxygen, and that
that of ammonia contains three atoms of hydrogen to one of nitrogen. But
these differences between Dalton's views and our present ones do not
impair the accuracy of the arguments which follow. The diagrams show
that Dalton formed a very definite conception of the nature of chemical
combination; it was the union of a small number of atoms of one kind
with a small number of another kind to form a compound atom, or as we
now say a "molecule," this identical process being repeated millions of
times to form a perceptible amount of a compound. The conceptions of
"element," "compound" and "mixture" became more precise than they had
been hitherto; in an element all the atoms are alike, in a compound all
the molecules are alike, in a mixture there are different kinds of
molecules. If we accept the hypothesis that each kind of atom has a
specific and invariable weight, we can, with the aid of the above
theory, make most important inferences concerning the proportions by
weight in which substances combine to form compounds. These inferences
are often summarized as the laws of _constant, multiple and reciprocal
proportions_.
Law of constant proportions.
The law of _constant proportions_ asserts that _when two elements unite
to form a compound the weights that combine are in an invariable ratio,
a ratio that is characteristic of that compound._ Thus if Dalton's
diagram for the molecule, or compound atom, of water be correct, it
follows that in all samples of water the total number of the hydrogen
atoms is equal to that of the oxygen atoms; consequently, the ratio of
the weight of oxygen to that of hydrogen in water is the same as the
ratio of the weights of an oxygen and a hydrogen atom, and _this is
invariable_. Different samples of water cannot therefore differ ever so
little in percentage composition, and the same must be true for every
compound as distinguished from a mixture. Apart from the atomic theory
there is no obvious reason why this should be so. We give the name bread
to a substance containing variable proportions of flour and water.
Similarly the substance we call wine is undeniably variable in
composition. Why should not the substance we call water also vary more
or less? The Aristotelian would find no difficulty in such a
variability; it is only the disciple of Dalton to whom it seems
impossible. It is evident that we have in this law a definite prediction
that can be tested by experiment.
Law of multiple proportions.
The law of _multiple proportions_ asserts that _if two elements form
more than one compound, then the weights of the one element which are
found combined with unit weight of the other in the different compounds,
must be in the ratio of two or more whole numbers._ If we compare
Dalton's diagrams of the two oxides of carbon or of the three oxides of
nitrogen that are given in the preceding table, we at once see the
necessity of this law; for the more complex molecule has to be formed
from the simpler one by the addition of one or more whole atoms. In the
oxides of carbon the same weight of carbon must be combined with weights
of oxygen that are as 1 : 2, and in the oxides of nitrogen a fixed
weight of nitrogen must be in union with weights of oxygen that are as 1
: 2 : 1/2, which are the same ratios as 2 : 4 : 1. This law has been
abundantly verified by experiment; for example, five oxides of nitrogen
are known, and independent analyses show that, if we consider the same
weight of nitrogen in every case, the weights of oxygen combined with it
are to one another as 1 : 2 : 3 : 4 : 5. The discovery of this law is
due to Dalton; it is a direct deduction from his atomic theory. Here
again, apart from this theory, there is no obvious reason why the
composition of different substances should be related in so simple a
way. As Dalton said, "The doctrine of definite proportions appears
mysterious unless we adopt the atomic hypothesis." "It appears like the
mystical ratios of Kepler which Newton so happily elucidated." The
chemists of Dalton's time were not unanimous in accepting these laws;
indeed C.L. Berthollet (_Essai de statique chimique_, 1803) expressly
controverted them. He maintained that, under varying conditions, two
substances could combine in an indefinitely large number of different
ratios, that there could in fact be a continuous variation in the
combining ratio. This view is clearly inconsistent with the atomic
theory, which requires that when the combining ratio of two substances
changes it should do so, _per saltum_, to quite another value.
Law of reciprocal proportions.
The law of _reciprocal proportions_, or, as it might well be named, the
law of _equivalence_, cannot be adequately enunciated in a few words.
The following gives a partial statement of it. _If we know the weights a
and b of two elements that are found in union with unit weight of a
third element, then we can predict the composition of the compounds
which the first two elements can form with each other; either the
weights a and b will combine exactly, or if not, these weights must be
multiplied by integers to obtain the composition of a compound._ To see
how this law follows from Dalton's theory let us consider his diagrams
for the molecules of water, ethylene and the oxides of carbon. In water
and in ethylene experiment shows that 8 parts by weight of oxygen and 6
parts of carbon, respectively, are in union with one part of hydrogen;
also, if the diagrams are correct, these numbers must be in the ratio of
the atomic weights of oxygen and carbon. We can therefore predict that
all oxides of carbon will have compositions represented by the ratio of
8m parts of oxygen to 6n parts of carbon, where m and n are whole
numbers. This prediction is verified by the result of analysis.
Similarly, if we know by experiment the composition of water and of
ammonia, we can predict the probable composition of the oxides of
nitrogen. Experiment shows that, in water and ammonia, we have,
respectively, 8 parts of oxygen and 4.67 parts of nitrogen in union with
one part of hydrogen; we can therefore infer that the oxides of nitrogen
will all have the composition of 8m parts of oxygen to 4.67n parts of
nitrogen. Experiment alone can tell us the values of m and n; all that
the theory tells us is that they are whole numbers. In this particular
case, n turns out to be 3, and m has in succession the values 1, 2, 3,
4, 5.
It is evident that these laws all follow from the idea that a compound
molecule can only alter through the addition or subtraction of one or
more complete atoms, together with the idea that all the molecules in a
pure substance are alike. Fortunately, the compounds at first examined
by the chemists engaged in verifying these laws were comparatively
simple, so that the whole numbers referred to above were small. The
astonishing variety of ratios in which carbon and hydrogen combine was
not at first realized. Otherwise Berthollet's position would have been a
much stronger one, and the atomic theory might have had to wait a long
while for acceptance. Even at the present time, it would be too much to
say that all the complex organic substances have been proved by analysis
to obey these laws; all we can assert is that their composition and
properties can be satisfactorily explained on the assumption that they
do so.
The above statement does not by any means exhaust the possible
predictions that can be made from the atomic theory, but it shows how to
test the theory. If chemical compounds can be proved by experiment to
obey these laws, then the atomic theory acquires a high degree of
probability; if they are contradicted by experiment then the atomic
theory must be abandoned, or very much modified. Dalton himself made
many analyses with the purpose of establishing his views, but his skill
as an analyst was not very great. It is in the work of the great Swedish
chemist J.J. Berzelius, and somewhat later, in the experiments of the
Belgian chemist J.S. Stas, that we find the most brilliant and vigorous
verification of these laws, and therefore of the atomic theory.
We shall now give an outline of the experimental evidence for the truth
of these laws.
Experimental evidence.
The law of the conservation of matter, an important element in the
atomic theory, has been roughly verified by innumerable analyses, in
which, a given weight of a substance having been taken, each ingredient
in it is isolated and its weight separately determined; the total weight
of the ingredients is always found to be very nearly equal to the weight
of the original substance. But on account of experimental errors in
weighing and measuring, and through loss of material in the transfer of
substances from one vessel to another, such analyses are rarely
trustworthy to more than one part in about 500; so that small changes in
weight consequent on the chemical change could not with certainty be
proved or disproved. A few experimenters have carried the verification
much further. Stas, in his syntheses of silver iodide, weighed the
silver and the iodine separately, and after converting them into the
compound he weighed this also. In each of a number of experiments he
found that the weight of the silver iodide did not differ by one
twenty-thousandth of the whole from the sum of the weights of the silver
and the iodine used. His analyses of another compound, silver iodate,
confirm the law to one part in 78,000. In E.W. Morley's experiments on
the synthesis of water the hydrogen, the oxygen and the water that had
been formed were separately determined; taking the mean of his results,
the sum of the weights of the ingredients is not found to differ from
the weight of the product by one part in 10,000. It is evident that if
our experiments are solely directed to the verification of this law,
they should, if possible, be carried out in a hermetically closed
vessel, the vessel and its contents being weighed before and after the
chemical change. The extremely careful experiments of this kind, by H.
Landolt and others, made it at first appear that the change in weight,
if there is any, consequent on a chemical change can rarely exceed
one-millionth of the weight of the reacting substances, and that it must
often be much less. The small discrepancies found are so easily
accounted for by attributing them to experimental errors that, until
recently, every chemist would have regarded the law as sufficiently
verified. Landolt's subsequent experiments showed, what was already
noticed in the earlier ones, that these minute changes in weight are
nearly always losses, the products weigh less than the components, while
if they had been purely experimental errors, due to weighing, they might
have been expected to be as frequently gains as losses. Landolt was
disposed to attribute these losses in weight to the containing vessel,
which was of glass or quartz, not being absolutely impervious, but in
1908 he showed that, by making allowance for the moisture adsorbed on
the vessel, the errors were both positive and negative, and were less
than one in ten million. He concluded that _no change of weight can be
detected._ Modern researches (see RADIOACTIVITY) on the complex nature
of the atom have a little shaken the belief in the absolute permanence
of matter. But it seems pretty clear that if there is any change in
weight consequent on chemical change, it is _too minute to be of
importance to the chemist_, though the methods of modern physics may
settle the question. (See ELEMENT.)
The law of constant proportions is easily verified to a moderate degree
of accuracy by such experiments as the following. We can prepare, in the
laboratory, a white powder that proves to be calcium carbonate, that is,
it appears to be wholly composed of carbon dioxide and lime. We find in
nature two other unlike substances, marble and Iceland spar, each of
which is wholly composed of carbon dioxide and lime. Thus these three
substances, unlike in appearance and origin, are composed of the same
ingredients: if small variations in the combining ratio of the
components were possible, we might expect to find them in such a case as
this. But analysis has failed to find such differences; the ratio of the
weights of lime and carbon dioxide is found to be the same in all three
substances. Such analyses, which do not always admit of great accuracy,
have been confirmed by a few carefully planned experiments in which two
components were brought together under very varied conditions, and the
resulting compound analysed. Stas carried out such experiments on the
composition of silver chloride and of ammonium chloride, but he never
found a variation of one part in 10,000 in the composition of the
substances.
The two laws discussed above were more or less accepted before the
promulgation of the atomic theory, but the law of multiple proportions
is the legitimate offspring of this theory. Berzelius saw at once that
it afforded an admirable test for the correctness of Dalton's views, and
he made numerous experiments expressly designed to test the law. One of
these experiments may be described. Two chlorides of copper are known,
one a highly coloured substance, the other quite white. Berzelius took 8
grams of copper, converted it into the coloured chloride, and sealed up
the whole of this in solution, together with a weighed strip of copper.
After some time the colour entirely disappeared; the strip of copper was
then taken out and reweighed, and it was found to have lost 8.03 grams.
Thus the chlorine, which in the coloured compound was in union with 8
grams of copper, appears, in the colourless chloride, to be combined
with 16.03 grams, or almost exactly double the amount. It is easy to
verify this result. In a series of repetitions of the experiment, by
different observers, the following numbers were obtained for the ratio
of the copper in the two chlorides: 1.98, 1.97, 2.03, 2.003, the mean
value being 1.996. It will be noticed that the ratio found is sometimes
above and sometimes below the number 2, which is required by the atomic
theory, and therefore the deviations may not unreasonably be attributed
to experimental errors. Such experiments--and numerous ones of about
this degree of accuracy have been made on a variety of substances--give
a high degree of probability to the law, but leave it an open question
whether it has the exactitude of the law of the conservation of matter,
or whether it is only approximately true. The question is, however,
vital to the atomic theory. It is, therefore, worth while to quote a
verification of great exactitude from the work of Stas and J.B.A.
Dumas[3] on the composition of the two oxides of carbon. From their work
it follows that the ratio of the weights of oxygen combined with unit
weight of carbon in the two oxides is 1.99995, or with somewhat
different data, 1.9996.
The law of reciprocal proportion, of which some examples have been
already given, is part of a larger law of equivalence that underlies
most of our chemical methods and calculations. One section of the law
expresses the fact that the weights of two substances, not necessarily
elements, that are equivalent in one reaction, are often found to be
equivalent in a number of other reactions. The neutralization of acids
by bases affords many illustrations, known even before the atomic
theory, of the truth of the statement. It is universally found that the
weights of two bases which neutralize the same weight of one acid are
equivalent in their power of neutralizing other acids. Thus 5 parts by
weight of soda, 7 of potash and 3.5 of quicklime will each neutralize
4.56 parts of hydrochloric acid or 7.875 of nitric or 6.125 parts of
sulphuric acid; these weights, in fact, are mutually equivalent to one
another. The Daltonian would say that each of these weights represents a
certain group of atoms, and that these groups can replace, or combine
with, each other, to form new molecules. The change from a binary
compound, that is, one containing two elements, to a ternary compound in
which these two elements are associated with a third, sometimes affords
a very good test for the theory. The atomic theory can picture the
change from the binary to the ternary compound simply as the addition of
one or more atoms of the third element to the previously existing
molecule; in such a case the combining ratio of the first two elements
should be absolutely the same in both compounds. Berzelius tested this
prediction. He showed that lead sulphide, a black substance containing
only lead and sulphur, could be _converted_ by oxidation into lead
sulphate, a white compound containing oxygen as well as lead and
sulphur. The whole of the lead and sulphur of the sulphide was found to
be present in the sulphate; in other words, the combining ratio of the
lead and sulphur was not altered by the addition of the oxygen. This is
found to be a general rule. It was verified very exactly by Stas's
experiments, in which he removed the oxygen from the ternary compound
silver iodate and found that the whole of the silver and the iodine
remained in combination with each other as silver iodide; his results
prove, to one part in ten millions, that the combining ratio of the
silver and the iodine is unaltered by the removal of the oxygen.
The above gives some idea of the evidence that has been accumulated in
favour of the laws of chemical combination, laws which can be deduced
from the atomic theory. Whenever any of these laws, or indeed any
prediction from the theory, can be tested it has so far proved to be in
harmony with experiment. The existence of the periodic law (see
ELEMENT), and the researches of physicists on the constitution of
matter (q.v.), also furnish very strong support to the theory.
Atomic weight.
Dalton was of the opinion that it was possible to determine the weights
of the elementary atoms in terms of any one by the analysis of
compounds. It is evident that this is practicable if the number and kind
of atoms contained in the molecule of a compound can be determined. To
take the simplest possible case, if Dalton had been correct in assuming
that the molecule of water was made up of one atom of oxygen and one of
hydrogen, then the experimental fact that water contains eight parts by
weight of oxygen to one part of hydrogen, would at once show that the
atom of oxygen is eight times as heavy as the atom of hydrogen, or that,
taking the atomic weight of hydrogen as the unit, the atomic weight of
oxygen is 8. Similarly, Dalton's diagram for ammonia, together with the
fact that ammonia contains 4.67 parts of nitrogen to one of hydrogen, at
once leads to the conclusion that the atomic weight of nitrogen is 4.67.
But, unfortunately, the assumption as to the number of atoms in the
molecules of these two compounds was an arbitrary one, based on no valid
evidence. It is now agreed that the molecule of water contains two atoms
of hydrogen and one of oxygen, so that the atomic weight of oxygen
becomes 16, and similarly that the molecule of ammonia contains three
atoms of hydrogen and one of nitrogen, and that consequently the atomic
weight of nitrogen is 14. On account of this difficulty, the atomic
weights published by Dalton, and the more accurate ones of Berzelius,
were not always identical with the values now accepted, but were often
simple multiples or submultiples of these.
Formulae.
The "symbols" for the elements used by Dalton, apparently suggested by
those of the alchemists, have been rejected in favour of those which
were introduced by Berzelius. The latter employed the first letter, or
the first two letters, of the name of an element as its symbol. The
symbol, like that of Dalton, always stands for the atomic weight of the
element, that is, while H stands for one part by weight of hydrogen, O
stands for 16 parts of oxygen, and so on. The symbols of compounds
become very concise, as the number of atoms of one kind in a molecule
can be expressed by a sub-index. Thus the symbol or formula H2O for
water expresses the view that the molecule of water consists of one atom
of oxygen and two of hydrogen; and if we know the atomic weights of
oxygen and hydrogen, it also tells us the composition of water by
weight. Similarly, the modern formula for ammonia is NH3.
The superiority of this notation over that of Dalton is not so obvious
when we consider such simple cases as the above, but chemists are now
acquainted with very complex molecules containing numerous atoms; cane
sugar, for example, has the formula C12H22O11. It would be a serious
business to draw a Daltonian diagram for such a molecule.
Dalton believed that the molecules of the elementary gases consisted
each of one atom; his diagram for hydrogen gas makes the point clear. We
now believe that the molecule of an element is frequently made up of two
or more atoms; thus the formulae for the gases hydrogen, oxygen and
nitrogen are H2, O2, N2, while gaseous phosphorus and sulphur are
probably P4 and S6, and gaseous mercury is Hg1,--that is, the molecule
of this element is monatomic. This view, as to the frequently complex
nature of the elementary molecule, is logically and historically
connected with the striking hypothesis of Amadeo Avogadro and A.M.
Ampere. These natural philosophers suggested that equal volumes of all
gaseous substances must contain, at the same temperature and pressure,
the same number of molecules. Their hypothesis explains so many facts
that it is now considered to be as well established as the parts of the
theory due to Dalton.[4] This principle at once enables the weights of
molecules to be compared even when their composition is unknown; it is
only necessary to determine the specific gravities of the various gases
referred to some one of them, say hydrogen; the numbers so obtained
giving the weights of the molecules referred to that of the hydrogen
molecule.
Present position of the atomic theory.
The atomic theory has been of priceless value to chemists, but it has
more than once happened in the history of science that a hypothesis,
after having been useful in the discovery and the co-ordination of
knowledge, has been abandoned and replaced by one more in harmony with
later discoveries. Some distinguished chemists have thought that this
fate may be awaiting the atomic theory, and that in future chemists may
be able to obtain all the guidance they need from the science of the
transformations of energy. But modern discoveries in radioactivity[5]
are in favour of the existence of the atom, although they lead to the
belief that the atom is not so eternal and unchangeable a thing as
Dalton and his predecessors imagined, and in fact, that the atom itself
may be subject to that eternal law of growth and decay of which
Lucretius speaks. (F. H. Ne.)