Few scientific generalisations have been more fruitful; few have been accepted on more slender evidence; none are more certain; none more clearly illustrate our natural appetite for beliefs of conservation. For, indeed, to the over-critical this sort of conservation must needs leave something to be desired. When we assert the indestructibility of matter we mean that a real entity continues through time unchanged in quantity. But the word has a less obvious meaning when it is applied to energy. The propriety of describing motion as energy seems indeed clear enough; and if all energy were energy of motion, and if energy of motion were always conserved, the conservation of energy would be on all fours with the conservation of matter. But this is not the case. In spite of Leibnitz, the amount of vis viva is not indestructible. What, then, happens when some of it is destroyed? In that case, says science, energy changes its form but not its quantity. Energy of motion becomes energy of position. What was kinetic becomes potential; and, as the transformation is effected without loss, the principle of conservation is saved.
When, however, energy thus becomes potential, in what sense does it still exist, and why do we still call it energy? Energy suggests “doings” and “happenings.” In the case of “potential” energy there are no “doings” and no “happenings.” It is “stored”; and stored it may for ever remain, hibernating (as it were) to all eternity, neither changing nor causing change.
I do not quarrel with this; but I ask myself why “energy” should be treated more leniently than “force.” Though force is now known not to be “conserved,” ordinary thought attributes to it a certain continuity of existence even when it does not show itself in motion. Force may be exerted though nothing moves; as, for example, by a book pressing on a table. But this view is profoundly unsatisfactory to many scientific thinkers. For them force is nothing apart from “acceleration”; it does not represent a cause, it only measures an effect. And if in our ordinary moments we think otherwise, this (they think) is simply because we illegitimately attribute to matter something which corresponds to muscular effort in man.
It is not, perhaps, so easy as these critics suppose to extrude from scientific thought (I say nothing of scientific language) this notion of latent force—force which would produce movement if it could; and is actively, though imperceptibly, striving to show itself in motion. But why should they try? They welcome potential energy—why should they anathematise latent force?
I think the answer is to be found in the fact that, whether force has, or has not, any being apart from acceleration, it is certainly not conserved; while, if energy be as real when it is potential as when it is kinetic, it certainly is conserved. A lapse into anthropomorphism, therefore, is without excuse in the first case, while a lapse into metaphysics is justified in the second. Any heresy may be forgiven, and any evidence is worth respectful attention when conservation is the thing to be proved.
I have sometimes amused myself by wondering what would have happened about the year 1842 if the conservation of energy had been a theological dogma instead of a scientific guess. Descartes, as I mentioned just now, inferred the conservation of motion from the attributes of God. Colding and Joule used the same argument in favour of the conservation of energy. Now, if a belief in the conservation of energy had been an integral part of religious orthodoxy in the early forties of the last century surely some positivist philosopher would have used Joule’s first investigation on Work and Heat to upset the very dogma they were intended to establish. “Here” (he would have said) “you have a believer in these metaphysico-theological methods of discovering the laws of nature; and mark what happens. In true medieval fashion he begins with some fanciful deductions from the way in which he thinks God must have made the world. Fortunately, however, though his principles are medieval, his methods are modern. Not only is he a most brilliant experimenter, but he has the courage to put his own speculations to an experimental test. He takes the minutest precautions, he chooses the most favourable conditions, and what happens? Does he prove his case? Do his results square with his theories? Does he find a fixed relation between work and heat? Does he justify his views of God? Not at all. Between his lowest determination of the mechanical equivalent of heat, and his highest, there is an immense and lamentable gap. What does he do? He takes their mean value:—a very proper method if he knew there was a mechanical equivalent of heat; a very improper method if the reality of such an equivalent was the thing to be proved. Clearly, if he had not put his theological opinions into his scientific premises when he began his experiment, he never would have got them out again as scientific conclusions when he had reached its end.”
For my own part, I think this imaginary critic would, at that date, have had something to say for himself—supposing always we are prepared to accept his presuppositions about scientific method. If sound reason and intellectual integrity require us to follow the lead of observation and experiment with no antecedent preference for one class of conclusions rather than another, then no doubt Joule and a long line of distinguished predecessors were the spoilt children of fortune. They made their discoveries in advance of their evidence, and in spite of their methods. If they turned out to be right, or, at least, on the right road, what can we do but criticise their credulity and wonder at their luck? unless, indeed, their luck be a form of inspiration.
Before leaving beliefs of conservation, I must say one more word about the most famous of them all—the belief in the conservation of matter. This was an important article in the scientific creed of the early atomists, who had no better evidence for it than they had for the Atomic Theory itself. The material “substance” of the medieval Aristotelians was, I imagine, also conserved; though as all that could be known about it were its qualities, and as these were not necessarily conserved, the doctrine in practice did not, perhaps, amount to much. Then came the theory which, chiefly in the hands of Boyle[14] at the end of the seventeenth century, initiated modern chemistry. What was conserved, according to this view, was not a metaphysical substance with detachable qualities, but elementary kinds of matter with inseparable qualities; and out of these qualified entities was compounded the whole material universe. I may incidentally observe that a company promoter who should issue a prospectus based on no better evidence than Boyle could advance for this tremendous theory would certainly be in peril of the law. Yet Boyle was right: and, notwithstanding subsequent developments, his conjecture remains the corner-stone of modern chemical research.
Now, what is it that we intend to assert when we say that matter is conserved, or is indestructible? We certainly do not mean that its qualities never suffer change: for most of those which are obvious and striking are always liable to change. If you sufficiently vary temperature or pressure; if you effect chemical composition or decomposition, the old characteristics will vanish and new characteristics will take their place. What, then, is conserved?
In the first place, the lost qualities can (in theory) always be restored, though not always without the expenditure of energy. Water never ceases to be convertible into steam, nor steam into water. The characteristics may vanish, but in appropriate conditions they will always reappear.