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THE words "physical force" in my title probably convey to you the correct notion of what is the main subject of my address without any further explanation. As a matter of fact, the term "force," in a strictly scientific sense, is slightly different from that in which it is popularly employed. The word in the title is to be taken in its popular meaning, which is not the passive force or pressure exerted, for example, by a column supporting a roof, but force actively at work, moving something against a resistance; or, if passive, like the force of a coiled spring, or of an explosive, waiting, as it were, the opportunity to become active and do work. In scientific language energy is the term now used to signify what once was, and still is, popularly called force. Energy is the power of doing work (kinetic energy), or anything which can be converted into work (potential energy). When a gun is fired, for example, the potential energy of the explosive is converted into the kinetic energy of the bullet, and this bullet possesses then the power of doing work, of moving itself against a resistance--the resistance of the air and the resistance of the target it strikes.

In ordinary language physical force is often referred to as "brute force," but science does not

1 Address to the Independent Labour Party, Aberdeen, 17th November 1915.

now put much, if any, weight on the various origins of force, or, if you will allow me henceforth to use the proper word, the various origins of energy. The energy is the important thing, whether it is brute energy or not-the power of working and battling against resistance, either of a living animal or of a mass of dead matter in motion. This theory of energy, or doctrine of work-work in the strictly physical sense, not, for example, brain work or artistic work-is of vital importance in fields very remote from science. A living being is distinguished from a dead one because it is working every second of its life, and death is the stoppage of that work. But it is not only living things that work continuously. A running river, a waterfall, is doing the same. When we speak of this as a live world in distinction to the moon, which is often spoken of as a dead one, we mean not only that there is no life on the moon, but also no movement of anything, and no change of any kind.

Energy, in general, is due to motion. If the things moving are masses large enough to see, we speak of their mechanical energy. If the things moving are too small to see, even with the microscope-the molecules or smallest particles of matter that exist we speak of their energy as heat energy. If the particles are still smaller, not matter at all, but electrons or particles of electricity, we speak of their energy as electrical energy. But everything that moves, or has in it the potentiality of movement, possesses energy, and if we trace this energy to its source we find that, in almost every case, it comes from the sun. Trains and ships bear their burdens across land and sea, living creatures run or swim or fly by virtue of energy that comes to us from the sun in the form of radiation, that is, light and heat. In the processes of agriculture this radiant energy is



converted into the potential energy of food, and so is utilised by life. But the steam and oil engine also work by virtue of the energy of the sun, which, in bygone times, was stored up by great masses of vegetation, and is now preserved in coal and fuel. Potential energy, or energy on the leash, is not capable of being apprehended until it is converted into kinetic energy. Ignite coal and its potential energy-a form of chemical energy-turns, like that of a released spring, into kinetic, and the molecules of the burning coal are suddenly thrown into a state of intense chaotic motion, which we call heat. But vital energy or vital force, the energy of a man working or living, are not special and peculiar to life. They are one with the great flow of inanimate energy reaching us from the sun, which bears the whole world along.

Energy, someone may say, is a mere abstraction, a mere term, not a real thing. As you will. In this, as in many another respect, it is like another abstraction no one would deny reality to, and that abstraction is. wealth. Wealth is the power of purchasing, as energy is the power of working. I cannot show you energy, only its effects. I cannot show you wealth, only its effects and its purely conventional symbol, money. Money is not wealth to a starving man in a deserted place. It is both a symbol and a measure of wealth earned, as work done is a measure of energy expended. Heat energy, mechanical energy, chemical energy, and so on, are different forms rather than different kinds of energy, just as coins, bank-notes, cheques, and so on, are forms of money. The bank-teller totals up a heterogeneous collection of coins, paper money, securities of various kinds and currencies, all as one sum of money, and then thinks of that sum, not of the sovereigns and shillings, notes and cheques he was

handling the moment before. So the scientific man looks into the processes of nature, sunshine and fire, storm and lightning and tempest, the battling of the elements, the rushing tumult of man and his machinery, the majestic circling of the moon and planets; stranger still, the silent, sleeping powers of coal, explosives, food and fertilisers. He is no more lost among them than the bank-teller is among his miscellaneous collection of monies. He totals them all in terms of energy, the power of working. The enormous variety of activities they display bother him not at all.

Now, just as strict watch as the bank-teller keeps on the credit and debit sides of the accounts of all his customers, nature keeps over the energy accounts of all its manifold processes. There is no work done for which the energy required does not have to be supplied, just as no money can be withdrawn from a bank into which none has been paid. Money cannot be spent twice, more must be forthcoming, and so it is with energy. It can only be spent once, and, whether spent usefully or uselessly, whether doing enduring work or dissipated doing nothing permanent, once spent it cannot be recovered. Very easy indeed it is to waste energy. The performance of any work demands so much energy, but any amount more may be demanded if the worker is inefficient. Abstraction or not, energy is as real as wealth,-I am not sure that they are not two aspects of the same thing. The one drives the commercial and industrial activities of men, and the other the whole physical activities of the entire universe.

Human beings and beasts of burden were at first almost the sole sources of useful energy, the only available labourers to overtake the heavy work of the world. For countless ages the inanimate energy of nature, of wind, waterfall and fire, proved too


difficult to harness and control. It is only a century since the new era of inanimate energy began, since science drilled the tumultuous rush of the swarms of molecules, too small to be seen, and out of their infinite variety of motions in all directions at onceout of heat—made the working motion of the steam engine. Animate energy, "brute force," became dwarfed indeed beside the working giant whose food was fuel. What is more, it had to recognise that it indeed was no divinity, no "vital spark" of origin divine animating a mass of clay. It was just energy, no more no less, to that bank-teller keeping count, and it made a very humble sum compared with the accounts of his inanimate customers.

At once there came about an enormous increase in the world's work, done no longer by living workers but by the inanimate labourers, water-power, coal and oil, which science had enslaved. So that to-day a single machine puts forth a continuous round of labour which an army of men could not keep going for an hour.

Steam engines, locomotives, electric trams, and petrol-driven motor cars have made some of the main aspects of inanimate energy very familiar. We all know that if we want such energy or power in any form we have to pay for it, whether we get it as a finished product, as the electrical energy laid on to consumers' houses and paid for by the unit, or in a partially-manufactured state as coal gas, or in its raw state as coal. In neither case do we care at all for the electricity or gas or coal we buy; we are buying energy, the power of doing so much work, the power of producing so much heat, light, and so on, as the case may be. We are all aware how largely this inanimate power has replaced animal labour. Whether at the docks or on ships or trains or cars, some animal-man, horse, mule or ox-has


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