THE PLACE OF LIFE IN A UNIVERSE

OF ENERGY.1

THERE is often a striking likeness between principles which CHAP. VII. nevertheless belong to very different departments of knowledge. Each branch of the tree of knowledge bears its own precious fruit, and yet there is a unity in this variety-a community of type that prevails throughout. Nor is this resemblance a merely fanciful one, or one which the mind conjures up for its own amusement; while it has produced a very plentiful crop of analogies, allegories, parables, and proverbs, not always of the best kind, yet parables and proverbs are or ought to be not fictions but truths.

We shall venture to begin this article by instituting an analogy between the social and the physical world, in the hope that those more familiar with the former than with the latter may be led to clearly perceive what is meant by the word ENERGY in a strictly physical sense. Energy in the social world is well understood. When a man pursues his course undaunted by opposition, unappalled by obstacles, he is said to be a very energetic man. By his energy, we mean the power which he possesses of overcoming obstacles; and the amount of his energy is measured by the amount of obstacles which he can overcome, by the

A joint communication to Macmillan's Magazine by the author and Dr. Balfour Stewart, F.R.S., who has permitted me to include it here. J. N. L.

Actual

energy.

CHAP. VII. amount of work which he can do. Such a man may in truth be regarded as a social cannon-ball. By means of his energy of character he will scatter the ranks of his opponents and demolish their ramparts. Nevertheless such a man will sometimes be defeated by an opponent who does not possess a tithe of his personal energy. Now, why is this? The reason is that, although his opponent may be deficient in personal energy, yet he may possess Energy of more than an equivalent in the high position which he position. occupies, and it is simply this position that enables him to combat successfully with a man of much greater personal energy than himself. If two men throw stones at one another, one of whom stands on the top of a house and the other at the bottom, the man at the top of the house has evidently the advantage.

So in like manner, if two men of equal personal energy contend together, the one who has the highest social position has the best chance of succeeding.

But this high position means energy under another form. It means that at some remote period a vast amount of personal energy was expended in raising the family into this high position. The founder of the family had doubtless greater energy than his fellow-men, and spent it in raising himself and his family into a position of advantage. The personal element may have long since vanished from the family, but it has been transmuted into something else, and it enables the present representative to accomplish a great deal, owing solely to the high position which he has acquired through the efforts of another. We thus see that in the social world we have what may be justly called two kinds of energy, namely—

1. Actual or personal energy.

2. Energy derived from position.

Let us now turn to the physical world. In this as in the social world, it is difficult to ascend. The force of gravity may be compared to that force which keeps a man down in the world.

If a stone be shot upwards with great velocity, it may be CHAP. VII. said to have in it a great deal of actual energy, because it has the power of overcoming the obstacle interposed by gravity to its ascent, just as a man of great energy has the power of overcoming obstacles.

This stone as it continues to mount upwards will do so with a gradually decreasing velocity, until at the summit of its flight all the actual energy with which it started has been spent in raising it against the force of gravity to this elevated position. It is now moving with no velocity, and may be supposed to be caught and lodged upon the top of a house.

Here, then, it rests, without the slightest tendency to move, and we naturally inquire, What has become of the energy with which it began its flight? Has this energy disappeared from the universe without leaving behind it any equivalent? Is it lost for ever, and utterly wasted? Far from it; the actual energy with which the stone began its flight has no more disappeared from the universe of energy than the carbon which we burn in our fire disappears from the universe of matter.

It has only changed its form and disappeared as energy of actual motion in gaining for the stone a position of advantage with respect to the force of gravity.

Thus it is seen that during the upward flight of the stone its energy of actual motion has gradually become. changed into energy of position, and the reverse will take place during its downward flight, if we now suppose it dislodged from the top of the house. In this latter case the energy of position with which it begins its downward flight is gradually converted into energy of actual motion, until at last, when it once more reaches the ground, it has the same amount of velocity, and therefore of actual energy, which it had at first.

Thus we have also in the physical world two kinds of energy: in the first place we have that of actual motion, and in the next we have that of position. We see from

Energy is not lost, but

changed in form.

CHAP. VII. this how intimate is the analogy between the social and the physical worlds as regards energy, the only difference being that, while in the former it is impossible to measure energy with exactness, in the latter we can gauge it with the utmost precision, for it means the power of performing work, and work (it is needless to mention in this mechanical age) is capable of very accurate measurement.

Forms of energy.

motion.

There are several varieties of energy in the universe, and, Proteus-like, it is always changing its form. Had it not been for this habit, we should have understood it long since, but it was only when its endeavours to escape from the grasp of the experimentalist were of no avail, that it ceased its struggles and told us the truth.

All of these varieties may, however, be embraced under the two heads already mentioned,—namely, energy of actual motion and energy of position.

A railway train, a meteor, a mountain torrent, represent Energy of energy of motion, but there is also invisible molecular motion which does not the less exist because it is invisible. Such for example is heat, for we have reason to believe that the particles of hot bodies are in very violent motion. A ray of light is another example of energy of motion, and so likewise is a current of electricity; and if we associate the latter with a flash of lightning, it ought to be remembered that the flash is due to particles of air that have been intensely heated by electricity becoming changed into heat. Electricity in motion is pre-eminently a silent energy, and it is only when changed into something else that its character becomes violent.

Energy of

Then, again, as representing energy of position we may position. instance our stone at the top of the house, or a head of water, both of which derive their energy from their advantageous position with respect to gravity.

But there are other forces besides gravity. Thus a watch newly wound up is in a condition of visible advantage with respect to the force of the main-spring, and as it continues to go it gradually loses this energy of position,

converting it into energy of motion. A cross-bow bent is CHAP. VII. likewise in a position of advantage with respect to the spring of the bow; and when its bolt is discharged, this energy of position is converted into energy of motion.

forms.

Besides this, there are invisible forms of energy of Invisible position. When we tear asunder a stone from the earth, and lodge the former on the top of a house, we obtain visible energy of position, the force against which we act being gravity. But we may also tear asunder from each other the component atoms of some chemical compound, our act here being performed against the very powerful force called chemical affinity.

Thus, taking a particle of carbonic acid, we may tear asunder the oxygen from the carbon, and, if our scale of operations be sufficiently great, we shall obtain separate from each other one mass of carbon and another of oxygen, -not, however, without the expenditure of a very large amount of energy in producing this separation.

We have, however, obtained a convenient form of energy of position as the result of our labours, which we may keep in store for any length of time, and finally, by allowing the carbon and oxygen to reunite,—that is to say, by burning the carbon,-we may recover in the shape of heat and light the energy which we originally expended in forcing these bodies asunder.

Some of the most prominent varieties of energy of motion and of position have now been described, and the remarks made have induced the belief that this thing, energy, this capacity which exists in matter for performing work of one kind or another, is by no means a fluctuating element of our universe, but has a reality and a permanence comparable to that which we associate with an atom of matter.

The grand principle of the conservation of energy, a Conservaprinciple lately proved by Dr. Joule,' asserts that energy,

1 We ought not to omit the names of W. R. Grove and Mayer in connection with this generalization.

tion of energy.