and helps to wear them out. e All things perish when they
have been rarefied by the ebb of particles, and succumb to
blows from without,' since ( the atoms never cease to destroy
anything by thumping it from without, and to attack and
overpower it by blows.' 3 In consequence of this loss of
1 i. 1070. 2 vi. 1028. Cf. iv. 932-4.
3 ii. 1143-7. Lucretius speaks at times of these ' blows ' as .helping to
hold the world in existence, as at ii. 528-31,
probavi
versibus ostendens corpuscula material
ex infinite summam rerum usque tenere
undique protelo plagarum continuato.
' I have shown that the minute bodies of matter do continually uphold
the world through an unbroken succession of blows on all sides.' The
word prolelo ' appears to denote a number of draught-oxen, yoked one
30 THE ATOMIC THEORY OF LUCRETIUS.
substance, and of the inroads made by the strain of attacking
forces, the world requires to be fed by a fresh stream of atoms,
constantly flowing in from the infinite void. f As the consti-
tution of living creatures, when deprived of food, loses sub-
stance and wastes away, even so the world must be dissolved
as soon as matter has ceased to be supplied, being in any way
diverted from its course.' l The mere impact of the ever-moving
atoms, beating on the world from without like the shock of
waves on the side of a ship, is not simply and merely destruc-
tive. To a certain extent Lucretius thinks it may even help
to hold things together, for a time, 2 but if this great world is
to maintain its being and action, it must be constantly fed with
matter from the infinite sum without. A precarious condition,
for how easily might the supply of matter ' somehow lose its
way ' in the immensity of the void !
materies aliqua ratione aversa viai.
(As we shall see, the direction of these atoms coming to feed
the world is upwards. 3 ) In consequence of collision the atoms
which naturally fall downwards are forced up, but there can
be no constant succession of upward-streaming atoms -unless
in front of the other, and advancing by even, successive pulls ; hence it
well expresses the effect produced by the continuous succession of blows of
atoms ' (Munro). In such a passage as this ' blows ' stands for its result,
viz., the supply of fresh matter, which, without these atomic collisions,
could not rise upwards to feed the world. The reasoning of i. 1041-51 is
conclusive as to this.
nam veluti privata cibo natura animantum
diffluit amittens corpus, sic omnia debent
dissolvi simul ac defecit suppeditare
materies, aliqua ratione aversa viai. i. 1038-41.
2 * Blows from without cannot hold together all the sum ' [i.e. this
world] , though ' they can frequently strike upon and detain a part, until
others come, and the full sum of matter can be completed.' i. 1042-5.
(Plagae occurs in two different meanings at 11. 1042 and 1045.) On this
subject, see ii. 1105-50.
3 See page 48, and note.
THE ATOM OF LUCRETIUS. 31
matter be infinite. The medium through which this loss and
gain come to things is the air ; ( Whatever ebbs from things is .
all borne into the great sea of air, and it in return gives back
particles to things.' 1
This notion of a loss of Energy constantly going on from
the world has much in common with the scientific doctrine of
the Dissipation of Energy. Thus Professor Balfour Stewart
shows that each form of energy is not capable of being trans-
formed directly, so far as we know at present, into every other
form. Thus, for instance, energy of visible motion cannot be
directly transformed into energy of chemical separation or into
radiant energy. The fact that heat can only be transformed
into mechanical energy subject to a certain condition, shows
us that there must be constantly going on a Dissipation of
Energy. So c if we could view the Universe as a candle not
lit, then it is, perhaps, conceivable to regard it as having been
always in existence ; but if we regard it rather as a candle
that has been lit, we become absolutely certain that it cannot
have been burning from eternity, and that a time will come
when it will cease to burn.' 2 Thus Lucretius's belief that old
age and death must necessarily come to the world when its
waste becomes greater than its supply, is not without scientific
basis.
I What then would be the consequence supposing the influx
of atoms from the infinite were to stop ? Lucretius states it
thus, ( Swift as flame the walls of the world would suddenly
break up and fly asunder along the mighty void, and for the
same reason all other things would follow : all the heaven
from its inmost quarters would tumble down, and in an instant
the earth slide from beneath our feet and wholly pass away
along the boundless void, the ruins of the heaven and. of
earthly things all wildly mixed, and the atoms unloosed from
their bonds of union, so that in a moment not a wrack shall
1 v. 275-8. 2 ' Conservation of Energy,' chap. v.
32 THE ATOMIC THEORY OF LUCRETIUS.
be left behind, nought save lone space and the unseen
first-beginnings. For on whatever side atoms shall first be
wanting, this side will be the gate of death for things in
being.'
It is curious how often Lucretius refers to the destruction of
the^ world. 1 The earth, he says, is ceasing to bear with its
former fertility : it is manifestly grown old. Even his own
generation, he thinks, may see the end of all things. 2 No
doubt the frequency of severe earthquakes and volcanic erup-
tions in Italy and Sicily may have impressed him vividly, and
led to such anticipations. 3 But there were far more cogent
reasons in his own theory of Nature. Lucretiusseems to have
been keenly^conscious of the truth that a cEance-made world
| could not well be permanent, that the chance-move^T atoms
i might fly from their orderly cgmbinatioja^ and undg the world
I even more readily than they had joined to upbuild it. Con-
sidering that he held such a thepry o the world's origin, must
J: *
1 See i. 1102-13; ii. 1148-74; v. 91-109, 338-46, and 364-75. In the
last passage, the lines,
neque autem corpora desunt,
ex infinite quae possint forte coorta,
corruere hanc rerum violento turbine summam
aut aliam quamvis cladem importare pericli
remind us of the remarkable theory of ' Spontaneity-in-things,' attri-
buted to Epicurus by M. Guyau. Such an insurrection of atoms out of
the infinite might be conceived thus to arise and imperil the world in con-
sequence of the power of declination, since atoms in the void are not
subject to the laws which restrict and nullify atomic declination in masses
of matter. In a storm of wandering atoms, * Spontaneity ' would be a
mighty and dangerous force.
sed tamen effabor : dictis dabit ipsa fidem res
forsitan et graviter terrarum motibus ortis
omnia conquassari in parvo tempore cernes. v. 104-6.
3 ' They told us, if I remember, that they had an earthquake on this
part of the coast of Italy about once every five years. Italy is a land of
volcanoes, more or less subdued. It is a great grapery, built over a flue.'
'Lord Byron and his Contemporaries,' by Leigh Hunt, chap. iv.
THE ATOM OF LUCRETIUS. 33
he not indeed have been surprised that the torrent of atoms had
not burst loose many a year before. For, in a world of purely,*/
atomic origin, must not any rent or flaw grow ever wider and
deeper, with accelerating increase ? for where is there any
recuperative power, once the elaborate atomic combination has
met with any jar or crack ? Such a world resembles in its struc-
ture a great many-sided crystal which, so soon as the slightest
flaw has touched it, at once falls into fragments and dissolves.
If a world like this had yet held together for so long, must
not its destruction have seemed to Lucretius now inevitably
nigh at hand ?
Moreover, Lucretius conceives the world as liable to destruc-
tion because it exists only in virtue of a certain balance ] or ^
equipoise of natural forces. In part the atomic motion tends
to preserve things in being, and in part it tends to break them
upT These opposing principles show themselves visibly every
day around us in birth and death, in growth and decay, but
they are at work everywhere. Thus mightiest forces, attacking
it within and without, are ever seeking to destroy the world,
forces as mighty are ever at work combating these and restoring
their inroads. So long as this balance of warring powers holds,
so long at least as the destructive forces do not gain the upper
hand, so long the world will last.
sic aequo geritur certamine principiorum
ex infinite contraction temp ore bellum. .
But this warfare may at any time come to a close.
The First Book concludes with these words, to the disciple
who will earnestly ponder his teaching ' one thing shall grow
clear after another, nor shall the blind night rob thee of the road
that thou see not to the full the most secret ways of nature :
so truly will one thing light the torch for another.'
1 The principle illustrated by this equilibrium of antagonistic forces is
an important Epicurean doctrine, under the name of inovofiia^ or * balance.'
See Book v. 381-96 ; ii. 569-80 ; Cicero, ' De Natura Deorum,' i. 50.
D
T
CHAPTEE III.
THE ATOM OF LUCRETIUS (CONTINUED).
HE Second Book begins with the well-known lines,
Suave, mari magno,
* 'Tis pleasant, when the seas are rough, to stand,
And see another's danger, safe at land.' l
Of course Lucretius hastens to explain that this is f not
because it is delightful or a pleasure at all that any one
should be in distress, but because it is sweet to see dangers
from which you yourself are free. It is sweet, too, to see
great armies arrayed on the plains struggling in combat without
yourself sharing in the danger. But,' Lucretius continues,
' nothing is more pleasant than to occupy the calm high places
of philosophy, that are well defended by the learning of the
wise, from which you may look down and see others, wander-
ing hither and thither, and going far astray in their search for
the way of life, the contest of intellect, the rivalry of rank, the
striving night and day with exceeding toil to struggle to the
height of power, and be masters of the world. O, wretched
rninds of man ! O, blind souls ! not to see in what darkness of
life and in how great dangers is this little term of life spent,
not to see that nature demands nothing else than for the body
to be free from pain, and the mind to enjoy a sense of pleasure
free from care and fear.' Of course the ' way of life ' is that
pointed out by Epicurus.
The pleasure described in the first lines of this passage is a
1 Creech.
THE ATOM OF LUCRETIUS. 35
somewhat selfish one. It does, indeed, stir the imagination to
behold danger from a place of safety far away ; but it is only
a cowardly, sentimental soul that can actually enjoy the sight
of danger that it would not face itself. Lucretius, we are con-
vinced, would rather have plunged into the waters to save a
life at the cost of his own, than stand passive, to enjoy a thrill
of poetic sensation at the cost of drowning men. Lord Bacon
was unfair in naming this the c Lucretian pleasure.' Rather
are these the words of one who cries to others still in the storm,
' I have found the shelter come ! ' Lucretius does but use this
as an illustration from which he may pass to the bold figure of
the mountain- tops on which the Epicurean stands. There is
something very characteristic in the next lines. Sometimes we
hear much the same language in our own day from men who have
found for themselves new opinions as to God and Hereafter,
who boast a new creed different from the common creed of men.
Occasionally they look down on the belief of the many with
just such a calm and confident disdain as this ; but their hearts
are not warm enough for the pity which in Lucretius quite over-
powers the disdain. With such a creed as Lucretius profes'sed
to have found for himself, and with his fervent temper, he
must have felt that the mountain-tops, though lofty places of
view, were very cold sometimes. Yet the rarity of their air
could not chill the feeling for humanity in his heart.
Afterwards, in some splendid pictures, Lucretius proceeds
to show how little wealth or birth or kingly power can deliver
men from care and fear. Reason alone can do this. But all
this time the atoms have been waiting, and, with a Nunc age,
Lucretius recalls his reader to the subject.
The Second Book contains, as Professor Jenkin remarks,
what may be called the kinetics of the Atomic theory. In it
Lucretius promises to treat of the motion of the atoms. He
will set forth e by what motion the birth-giving atoms beget
different things, and after they are begotten break them up
36 THE ATOMIC THEORY OF LUCRETIUS.
again, and by what force they are compelled to do this, and
what swiftness in moving through the void they possess.' l The
book opens with the proposition that matter does not ' cohere
inseparably massed together.' 2 It is always in motion coming
and going. This he infers from the continual change in the
world, by which individuals alter and perish while yet the whole
remains the same. The cause of these changes, what we
should call the energy of the universe, Lucretius holds to be
the atoms in motion. The only ultimate form of energy which
Lucretius recognizes is the motion of the atoms. His next
proposition is to the effect that the atoms can never stop. ' No
rest is given to the bodies of the first-beginnings.' 3 After they
have_come into collision with one another, they cannot either
come to a stop or move more slowly, they rebound in opposite
directions, keeping their original velocity. ' For when, being in
motion, they have met and clashed, as they so often do, it hap-
pens that they suddenly leap asunder in different directions,
and no wonder, since they are very hard and of strength pro-
portioned to their solidity, 4 and nothing behind gets in their
way.' In this it is of course implied that the atoms are elastic.
Professor Jenkin has criticised Lucretius very acutely here.
He shows that if the atoms were not elastic, 'they must
gradually slacken speed after striking and rebounding, stop
Quo motu genitalia material
corpora res varias gignant genitasque resolvant,
et qua vi facere id cogantur, quaeque sit ollis
reddita mobilitas magnum per inane meandi. ii. 62-5.
nam certe non inter se stipata cohaeret
materies. ii. 67-8. Cf. i. 340-5.
3 ii. 95-6. So Diog. L. x. 43, KIVOVVTO.I avvtx&Q.
4 Thus Mr. Munro renders,
nam cum cita saepe
obvia conflixere, fit ut diversa repente
dissiliaut ; neque enim mirum, durissima quae sint
ponderibus solidis neque quicquam a tergo ibus obstet.
ii. 85-8.
THE ATOM OF LUCRETIUS. 37
for an inconceivably short time, and then gradually resume
their pace in an opposite direction.' If they rebound, before
moving on again they must stop. Modern science explains
that, even if they do stop, their energy yet remains unchanged,
for the former energy of motion is now transformed into heat,
vibration, or some other form of energy. It will be remem-
bered that Lucretius's atoms_haye no secondary properties,
but only hardness and, as he assumes, ^elasticity. But in a
perfectly hard body such as he conceives, motion cannot be
transformed into heat or anything else. We now know that
a body which is peiiectlyjhaj-dis not elastic. Lucretius did
not know this. His atoms_mus^lmye_.co^ne _to a__s_to_p, and this
( would be equivalemVto the destruction nf ma.tt.ftr.' The next
proposition has been anticipated at the end of the First Book,
where it is rather implied than actually stated. It is that the
atoms, as combined in various bodies, are in motion ; they
' mutually give and receive motions.' * For some reason or
other Lucretius thinks it hardly necessary to state this as a
dogma by itself, probably because he views it as implied in
the last proposition. It is indeed constantly referred to and
implied throughout the poem. He proceeds at once to defend
it. 2 ( You need not marvel at this, why, seeing that the first-
beginnings of things are all in motion, still the sum appears to
stand in perfect rest.' The atoms of any body may move to\
and fro ceaselessly while we can see nothing but a mass of
matter in repose. Just so, a great flock of sheep and lambs,
all cropping the grass or gambolling on it, when seen from a dis-
tance appears to be only ( a white spot standing on the green
hill,' or a mighty army of foot and horse, all in rapid motion,
1 Atque eadem magni refert primordia saepe
cum quibus et quail positura contineantur,
et quos inter se dent motus accipiantque.
i. 817-19; repeated at 908-10.
2 ii. 308-32.
38 THE ATOMIC THEORY OF LUCRETIUS.
if seen from some place on the mountains very far away, appears
only a small bright patch at rest on the plain. Even so, the
smallness of the atoms puts, as it were, a vast distance between
their motions and our eye. *
Lucretius further develops this statement as accounting for
the different densities of various bodies. In some bodies the
atoms rebound, leaving smaller intervals ; in others they leave
larger. In a mass of iron or stone, the ^toms^are^entangled
wjth_one_anp_ther, and can onlyjthrob^ or oscillate, moving to and
fro within very small distances ; in softer bodies, like air or
sunlight, the jitoms rebound at greater intervals. 1 We gather
also as a deduction from the last proposition that the atoms,
even when they form_such a mass of stone or iron, still nioyjeas
swiftly as they^i^ whpn streaming through .thft void. ^Jf some
reboun^L within very sjcnall limits, they must move to and fro
oftener than those which foiTQ_niore porous bodies. ( The
modern explanation of density, of course, is not merely more
molecules within a given space, but perhaps molecules of greater
weight also.)
It is worth while to pause for a moment to think how remark-
able this statement of Lucretius is. A lump of stone or iron
certainly does not give to our senses any impression that its
particles are in motion: the piece of inert matter certainly
appears to be at rest. It is not easy to see what could have
1 sed magis adsiduo varioque exercita motu
partim intervallis magnis confulta resultant,
pars etiam brevibus spatiis vexantur ab ictu.
et quaecumque magis condense conciliatu
exiguis intervallis convecta resultant,
indupedita suis perplexis ipsa figuris,
haec validas saxi radices et fera ferri
corpora constituunt et cetera de genere horum
paucula quae porro magnum per inane vagantur.
cetera dissiliunt longe longeque recursant
in magnis intervallis : haec aera rarum
sufficiunt nobis et splendida lumina solis. ii. 97-108.
THE ATOM OF LUCRETIUS. 39
suggested to the discoverer a thought so opposite to what the
senses tell us. Yet it is accepted by science now as certainly
true, both for solid bodies, liquids, and gases. In solids,
indeed, these motions of the molecules are confined within very
narrow limits, and cannot be detected ; yet Professor Tyndall
says of the atoms composing the hardest body, when heated,
* They collide, they recoil, they oscillate.' x
According to Maxwell, 'the principal difference between a gas
and a liquid seems to be that, in a gas each molecule spends the
greater part of its time in describing its free path, and is for a
very small portion of its time engaged in encounters with other
molecules ; whereas in a liquid the molecule has hardly any
free path, and is always in a state of close encounter with other
molecules.' In both liquids and gases the molecules move
more freely than in solid bodies, and the argument drawn from
the diffusion of gases and liquids forms one of the strongest
proofs of the motion of molecules. How could two different
gases mix so very rapidly, unless the molecules composing
them were in motion ? The molecules of any gas flying about
beat against whatever opposes them, and the constant suc-
cession of these strokes, according to the Atomic theory,
explains the pressure of gas. Further, as Maxwell says, f All
the three kinds of diffusion, the diffusion of matter, of mo-
mentum, and of energy, are carried on by the motion of the
molecules.' Heat, viewed as a mode of motion, furnishes another
argument. Lucretius states that the molecules of bodies are
moving with more or less speed. Now if heat be a mode of
motion of gross matter, then, as all bodies are more or less hot,
the molecules of all bodies must be moving more or less quickly.
This is just what Lucretius says, and this statement of his is \
perhaps his most marvellous anticipation of modern scientific
discovery.
1 * Fragments of Science for Unscientific People,' by John Tyndall.
1871. Page 12.
40 THE ATOMIC THEORY OF LUCRETIUS.
Lucretius continues with an illustration. If you wish to
realize what the motion of the atoms is, observe when the sun-
light streams into a dark chamber, how numberless motes toss
about and dash against one another, meeting and breaking away
again. These may help us to conceive how the atoms are for
ever tossing about in the great void. Moreover, in this case a
small thing illustrates a great truth. For the movements of
the motes have a cause : they imply that ( hidden and invisible
motions of the atoms are at the bottom.' The atoms move
fi rst of themselves, next thejjstrike against those bodiesjwhich
are formed of a few atoms in union, and^make them__move,
tKese again stir up those which are a little larger. ' Thus
motion mounts up from the first-beginnings and step by step
issues forth to our senses.' l
Lucretius next points out that the velocity of the atoms
passing through the void is immense. Notice, he says, at
sunrise, an Italian sunrise, we must remember, after the
first rays have begun to shoot and the birds to sing in the
woods, how soon and how suddenly the heaven is filled with
light. Yet the rays of light are formed of countless molecules,
and have to pass through a medium, the air, the molecules
being pulled back by each other and hindered by the air.
How much more swiftly must the atoms, which are single
bodies, stream through the unresisting void ? Professor Jenkin
remarks that Lucretius e may also have felt that if all the
power of the universe depended on the motion of exceedingly
small particles, it was necessary to suppose them endowed with
great velocity ; but we do not find this argument used, although
it has led the modern believer in atoms to the conviction that,
if their motion does represent energy, their velocity must be
1 ii. 114-41. The tumblings of the motes which our eyes clearly dis-
cern, so to say, make visible to us the invisible plagae of the atoms to
which they are due (11. 134-6). Lucretius evidently feels this illustration
to have scientific importance.
THE ATOM OF LUCRETIUS. 41
enormous. Lucretius would be glad to know that Herapath,
Joule, Kronig, Clausius, and Clerk-Maxwell have been able
to calculate it.'
Dr. Joule calculated the actual velocity of the molecules of
hydrogen, and found it to be exceedingly great, at the rate of
nearly sixty-nine miles a minute. The velocity of other gases
is less. Maxwell has calculated, from the data of Professor
Loschmidt of Vienna, the actual velocity of the molecules of
four different gases at oC. It is as follows:
Hydrogen. Oxygen. Carbonic oxide. Carbonic acid.
Metres per second . . . 1,859 465 497 396
The molecules of calm air, he says, are flying about in all
directions at the rate of about seventeen miles a minute :
( If all these molecules were flying in the same direction,
they would constitute a wind blowing at the rate of seventeen
miles a minute ; and the only wind which approaches this
velocity is that which proceeds from the mouth of a cannon.
How, then, are you and I able to stand here ? Only because
the molecules happen to be flying in different directions.'
'But it is not only against us, or against the walls of the
room, that the molecules are striking. Consider the immense
number of them, and the fact that they are flying in every pos-
sible direction, and you will see that they cannot avoid striking
each other. Every time that two molecules come into collision,
the paths of both are changed, and they go off in new direc-
tions. Thus each molecule is continually getting its course
altered, so that, in spite of its great velocity, it may be a long
time before it reaches any great distance from the point at
which it set out.'
Again, referring to an experiment with ammonia, he says :
' The molecules of ammonia have a velocity of 600 metres
per second, so that if their course had not been interrupted by