But our business to-day is neither with magic nor morality. It is with physical science. When we survey man’s strivings to understand the world in which he lives, can we detect any secular leanings towards certain types of belief, any deep-lying inclination to guess by preference in one direction rather than another? We surely can. There are some answers, for example, which we refuse to take from experiment and observation. I have already referred to one such case in connection with causation. No man of science can be provoked, by any seeming irregularities, into supposing that the course of nature is subject to lapses from the rule of perfect uniformity. Consider, again, another case, where the tendency is far less strong, but where few can doubt that it is real. I refer to the deep-seated reluctance felt by most physicists to accept as final any scientific explanation which involves a belief in “action at a distance”—a reluctance which is the more remarkable since action at a distance seems a familiar fact of experience, while action by contact, when you attempt to work it out in detail, seems hard to comprehend.
But there are tendencies feebler and less general than these which give much food for reflection. Consider, for example, the familiar history of atomism. At least as far back as Democritus we find the confident assertion that the world consists of atoms, and that its infinite variety is due to the motions and positions of immutable and imperceptible units, which, if they are not exactly alike, at least differ less among themselves than do the visible objects into which they are compounded. Through successive centuries this theory never died. With the revival of learning and the beginning of modern science it burst into fresh life. It was believed in firmly by Bacon, the prophet of the new era. It was treated as almost self-evident by philosophers like Gassendi and Hobbes. Boyle held it in its most uncompromising form. Newton assumed it without question. After a period of varying fortunes in the eighteenth century, a modification of it in the hands of Dalton started a new era in chemistry. Taken over by the physicists, it now lies at the root of the modern theory of gases and liquids; the modern theory of matter, the modern theory of heat, and the modern theory of electricity.
This is a very strange story; and it is not really made less strange by those who emphasise the differences between the atoms of Democritus, which are the theme of its first chapter, and the electrons of Sir Joseph Thomson, which appear in its last. Different indeed they are; but, though the difference be great, the agreement is fundamental.
There are some who think that the achievement sung by Lucretius is lessened by showing that the ancients who believed in atoms had no experimental warrant for their convictions. And this is perfectly true. They had not. Nor had Bacon, nor Gassendi, nor Hobbes, nor Boyle, nor Newton. But this only brings into clearer relief the point I desire to emphasise. If experience did not establish the belief, whence came it? If it represents nothing better than an individual guess, why did it appeal so persistently to leaders of scientific thought, and by what strange hazard does it turn out to be true? It is certainly curious that Tyndal, in a once famous address to the British Association at Belfast, should have sketched the story from Democritus to Lucretius, and from Lucretius to 1874, without ever putting these questions to his audience, or, so far as I know, to himself.
But the Atomic Theory is by no means the only example of tendencies which have played an important part in the evolution of science. There are other beliefs, or kinds of beliefs, of the most far-reaching importance which have almost exactly similar characteristics. They anticipate evidence, they guide research, and in some shape or other they turn out to be true.
Consider, for example, the group of beliefs which may be described generally as beliefs in persistence, or beliefs in conservation—the kind of belief which has been applied at different periods, and by different schools of scientific thought, to matter, mass, bulk, weight, motion, force, heat, and energy. As every one knows, these ascriptions have not always been correct. But this only emphasises the strength of the tendency. Weight was at one time supposed to be invariable. We know now that the weight of a body varies with its position relatively to other bodies. It is different, for example, at the poles from what it is at the Equator. But how was the error discovered? Not by experiment. There were experiments, no doubt. But those who undertook them already believed in the law of gravitation; and the law of gravitation made it necessary to distinguish the mass of any given fragment of matter both from its weight and from the occult quality of gravity, which is one of the factors on which its weight in any given situation depends. The desire for conservation was not, however, defeated; since physicists, till within the last few years, regarded both mass and gravity as unalterable characteristics of all material bodies.
Again, consider the case of heat. This also has been regarded by powerful schools of scientific thought as a substance that was “conserved.” It is so regarded no longer. But is the inclination to believe in conservation thereby defeated? Not at all. Though heat may vanish, energy remains, and heat is a form of energy.
This doctrine of the conservation of energy is indeed the crowning triumph of the tendency I am discussing, and provides the best illustrations of its strength. For natural philosophers, intent on finding conservation wherever they could, started too boldly on their quest. Descartes regarded the conservation of motion as a self-evident inference from the rationality of God. It is true that he neither had experimental evidence of his doctrine, nor could he, under any circumstances, have obtained it; for the energy of motion, as he incorrectly described it, is not conserved. Leibnitz described it correctly, and had as great a confidence as his predecessor in its conservation, and as little proof to support him. So confident indeed was he, and so independent of experimental evidence was his faith, that he dogmatically asserted that, when motion seemed to disappear, what was lost by the bodies which we see, was exactly taken up by their component elements which we do not see; so that nothing in the nature of what he called vis viva was either lost or created. That this transformation of energy from molar to molecular motion is constantly occurring we now have sufficient proof. But Leibnitz had no proof; and apparently thought none was required other than the Cartesian deduction from the rationality of God. He made a bold anticipation of experience, with nothing to support him but a priori inclination.
His anticipation, however, was not only bold; it was fortunate. Kinetic energy may really be transformed from molar to molecular motion, and suffer no variation. It is conserved. On the other hand, it may not. It may altogether cease, and what becomes of conservation then?
The scientific formula which satisfies both the facts of the case and our desire for conservation is well known.[13] Energy, we are taught, is of two kinds: kinetic and potential energy—energy in act and energy in possibility. Each may turn into the other, and is continually so turning. Each, therefore, may vary in quantity, and does vary in quantity. It is only their sum which is indestructible.