Fleeming Jenkin

“ It is only in this sense that we can hope ever to understand the ultimate constitution of matter; but as the undulatory theory of light has both suggested the discovery of new facts, and has connected all known facts concerning light into one intelligible series of logical deductions, so any true theory of the constitution of matter would suggest new inquiries, and would group the apparently disjointed fragments of knowledge, now called the various branches of science, into one intelligible whole. ”

“ But it is clear, in all his work, that Lucretius conceived two things as quite constant: atoms were neither created nor destroyed, and their motion could neither be created nor destroyed. He believed that each atom kept its velocity unaltered. The modern doctrine is that the total energy of the universe is constant, but may be variously distributed, and is possibly due to motion alone ultimately, though this last point has not been yet proved. Many a fierce battle has been waged over the question, whether what was called the " quantity of motion" in the universe was constant. ”

“ Such a power of moulding circumstances, of turning the torrent to the right, where it shall fertilize, or to the left, where it shall overwhelm, but in nowise of arresting the torrent, adding nothing to it, taking nothing from it, — such is precisely the apparent action of man's will; and though we must allow that possibly the deflecting action does but result from some smaller subtler stream of circumstance, yet if we may trust to our direct perception of free-will, the above theory, involving a power in man beyond that of atoms, would probably be our choice. ”

“ The law of gravitation considered as a result is beautifully simple in a few words it expresses a fact from which most numerous and complex results may be deduced by mere reasoning, results found invariably to agree with the records of observation but this same law of gravitation looked upon as an axiom or first principle is so astoundingly far removed from all ordinary experience as to be almost incredible. ”

“ Lucretius thought atoms acted thus we do not, because we observe no action of the kind in matter, but, on the contrary, strict causation or sequence of phenomena. Whether what we call mind act so or not must also be a matter of observation, but as people have not been able to agree as to the results of observation about free-will made during a great many centuries, we fear the path of observation will lead us no further than we have already come. ”

“ We do not think that the fact of gravitation justifies the assumption that atoms can exert a force upon one another across a void, but Newton spoke of gravitation as an action between two distant bodies, and since then we have got quite accustomed to the idea of finite molecules of matter acting everywhere in the universe, and that, too, without any material medium of communication. ”

“ If these arguments seem insufficient, we may refer to the conception of heat as a mode of motion. If heat be a mode of motion of gross matter, then, as all bodies are more or less hot, the molecules of all bodies will be moving with more or less speed,- — precisely what Lucretius taught. Lucretius was led to his conception by considerations very analogous to those which lead us to consider heat and other forms of energy as modes of motion. ”

“ Leibnitz was not to be satisfied with the idea which Lucretius seems to hold, that a thing may exist just big enough to have parts too small in themselves for independent existence. John Bernoulli, however, did not quite abandon atoms in consequence of this attack; like a sensible man he does not like assumptions of infinite hardness and infinite elasticity, but he replies to Leibnitz that atoms may be so constituted that they may be really indivisible by any process to which they can be subjected by other atoms, although they may have an infinity of parts such as the mind can conceive. ”

“ It is conceivable, but not yet proved, that Leibnitz may be right in all cases, and that what we call the potential energy of gravitation, elasticity, etc., may really be due to the motion either of the atoms of gross matter, or of their constituent parts. If matter in motion be conceived as the sole ultimate form of energy, Leibnitz's proposition is absolutely true, and Lucretius must be allowed great merit in having taught that the motion of matter was as indestructible as its material existence, although he knew neither the laws of momentum nor of vis viva. ”

“ We know with much accuracy the conditions to be fulfilled by any hypothesis, and we possess a mathematical machinery by which we can test how completely any hypothesis does fulfil those conditions. The materials for the work are not wanting, though the architect has not appeared. Inertia and motion seem the most indispensable elements in the conception of the materia prima extended in space. Once in motion, it must continue in motion till stopped when at rest, it must not move without a cause when in motion, it represents energy, or power, and can exert force. ”

“ The action of a body on its neighbour can be explained without the idea of a force acting even across a small void, by the simple assumption that two bodies cannot be in the same place at the same time, an assumption only tacitly made by Lucretius, and generally received as undeniable, though it admits of rational doubt, for experiment is by no means conclusive as to its certainty. ”

“ Moreover, it is found that a certain quantity, sometimes called vis viva, sometimes the kinetic energy of the system, is also constant after and before any collision. This quantity is proportional for each body to the mass of the body, and to the square of its velocity. It must be remembered that we are now speaking of two simple bodies which have only the properties of hardness and elasticity, not being compressible, hot, or susceptible of vibration, so that the transformation of energy due to motion into other forms of energy such as heat is excluded by hypothesis. ”

“ Lucretius chose the second alternative as an exit from the difficulty : Atoms with strict causation did exist, and free-will too. We will then grant free-will to atoms, one and all, not in perpetual exercise, but at quite uncertain times. The idea is startling, but not illogical, and the form in which atoms are supposed to exercise their free-will is quite unexceptionable. We cannot but admire the audacity of the man who, called upon to grant freewill as a tertium quid, either to man or to atoms, chooses the atoms without a qualm. ”

“ The conception of an all-pervading imponderable fluid of this kind has formed part of many theories, and ether came to be very generally adopted as a favourite name for the fluid, but caloric was also much thought of as a medium. We even find half-a-dozen imponderable co-existent fluids regarded with favour, — one called heat, another electricity, another phlogiston, another light, and what not, with little hard atoms swimming about, each endowed with forces of repulsion and attraction of all sorts, as was thought desirable. This idea of the constitution of matter was perhaps the worst of all. ”

“ Bacon was very cautious about atomic theories, but on the whole believed in atoms. He devised the idea of groups or knots of atoms, saying, in reference to the argument of Democritus, that if only one kind of atom existed, all things could be made out of all things " there is no doubt but that the seeds of things though equal, as soon as they have thrown themselves into certain groups or knots, completely assume the nature of dissimilar bodies till those groups or knots are dissolved." ”

“ If the molecules of gross matter are made up of atoms in rapid motion, as Lucretius believed, or of a portion of whirling fluid, as Sir William Thomson suggests, and if elasticity itself be only a secondary property, not possessed by the primordia rerum at all, then the proposition that a molecule never can come to rest is undoubtedly true — such rest would be equivalent to the destruction of matter. ”

“ Motion is essentially relative we only know that a body moves by observing that it changes its position relatively to another. When, therefore, treating of two isolated bodies only, we need only speak of their relative velocity. The motion of the centre of gravity of any system of bodies remains quite unaffected by their collision one with another, and, in considering our two isolated atoms, we may as well, for simplicity's sake, assume the motion of their joint centre of gravity to be nil, though this is not necessary to our argument. ”

“ Lucretius fails to perceive that definite physical laws are consistent with the work of God and the difficulty of reconciling the two ideas, unreal as it seems to us, has been felt by able men even now-a- days, when the conception of divine power is very different from any present to the mind of Lucretius. To most of us the very conception of a law suggests a lawgiver, while he, to prove the existence of laws, thought it necessary to deny the action of beings who could set those laws at nought. ”

“ Light gives us a very perfect analogy to illustrate our assertion that the law of gravitation is not an hypothesis, but a result capable of and requiring further explanation. Gravitation is not perceived directly by the senses, except in the case of the attraction of the earth. We have a special sense for the perception of light, yet many phenomena of radiation are not detected by the eye. Similarly, some of the phenomena of gravitation may escape our observation. Newton detected some of these. ”

“ The doctrines of the indestructibility of matter and of the conservation of energy seem at first sight to help the Necessitarians, for they might argue that if free-will acts it must add something to or take something from the physical universe, and [225] if experiment shows that nothing of the kind occurs, away goes free-will but this argument is worthless, for if mind or will simply deflects matter as it moves, it may produce all the consequences claimed by the Wilful school, and yet it will neither add energy nor matter to the universe. ”

“ The pieces of plenum rubbed against one another till they became quite round the dust rubbed off their angles filled up the interstices, — for of course no void could possibly occur once the universe was quite full. ”

“ From observations on the growth of vegetation, the sensation of heat, chemical decomposition, and other facts perceptible to blind creatures, he with vast genius might have discovered that a body existed at a great distance from the earth, from which a peculiar influence was periodically rained upon the earth that this influence could also be produced by fire and in other ways by men living on the earth, and was in a given medium inversely proportional to the square of the distance from the source of light, as we call it. ”

“ Motion in mechanics has no meaning except as denoting a change of relative position all atoms moving, as Lucretius fancied, at one speed, and in parallel lines, would relatively to one another have been in perfect rest. A bag of marbles in a railway train could not be employed as a source of energy in the train they lie at rest and it is only when brought into collision with something moving at a different pace from the train that they can develop any power, which may then be considerable. ”

“ We are next informed by our author that matter exists, or, in the language of Lucretius, " all nature then, as it exists by itself, has been founded on two things : there are bodies, and there is void in which these bodies are placed, and through which they move about." In his first and second propositions, Lucretius uses the word thing, res, which, as we have already explained, comprehended all kinds of things, such as matter, force, motion, thought, life, etc. ”

“ The idea running in the mind of the writer seems to have been that any matter moving in a medium would be impeded by friction, and therefore necessarily move more slowly than a free atom moving in a void he 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 believers in atoms to the conviction that if their motion does represent energy, their velocity must be enormous. ”

“ Of the idea that the universe is composed of four elements, earth, fire, air, and water, no trace remains except in language, but careful investigation might show that the believers in these elements, or in some one or more of them, as the material of the universe, meant something very different and much more sensible than the vulgar interpretation of their doctrine. ”

“ We moderns jump out of the difficulty at once by saying that the hard bodies are elastic, and elasticity is a form of energy, so that the energy or vis viva which at one time was represented by the body in motion, is at another time represented by the potential energy of elasticity. ”

“ Time, he says, exists not by itself, but simply from the things which happen actions do not exist by themselves, but may be fairly called accidents of matter, and of the space in which they severally go on. Even if all this be granted, we shall not necessarily concede that matter and void have alone a separate existence but we must not complain that Lucretius does not support his proposition more strongly at this point, for indeed his six books form one long argument in support of his proposition. ”

“ He would have been perfectly right, as he was about gravitation, but that need not have prevented subsequent philosophers from devising the undulatory theory of light if they had been clever enough; quite similarly, the fact that gravitation as discovered by Newton does exist need not prevent our trying to devise a scheme which shall explain its action, starting from much simpler postulates than that of an universal influence of each atom on all others at a distance. ”