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This book describes and explains, in a series of graduated lessons, the LOGY FOR SCHOOLS. By T. ALCOCK, M.D. 18mo. - Is. 6d. BOTANY. PROFESSOR OLIVER'S LESSONS IN ELEMENTARY BOTANY. With nearly Two Hundred Illustrations. By DANIEL OLIVER, F.R.S., F.L.S. Seventh Thousand. 18mo. cloth. 45. 6d. This book is designed to teach the Elements of Botany on Professor Henslow's plan of selected Types and by the use of Schedules. The earlier chapters, embracing the Elements of Structural and Physiological Botany, introduce us to the methodical study of the Original Types. The concluding chapters are entitled, "How to Dry Plants," and "How to describe Plants." A valuable Glossary is appended to the volume. In the preparation of this work free use has been made of the manuscript materials of the late PROFESSOR HENSLOW, Professor CHEMISTRY.. ROSCOE'S LESSONS IN ELEMENTARY CHEMISTRY, INORGANIC AND ORGANIC. By HENRY ROSCOE, F.R.S., Professor of Chemistry in Owens College, Manchester. 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OXFORD: Printed at the CLARENDON PRESS, and Published by MACMILLAN & CO., LONDON, Publishers to the University. THURSDAY, APRIL 28, 1870. WHAT IS ENERGY? T is only of late years that the laws of motion have IT been fully comprehended. No doubt it has been known since the time of Newton that there can be no action without reaction; or, in other words, if we define momentum to be the product of the mass of a moving body into its velocity of motion, then whenever this is generated in one direction an equal amount is simultaneously generated in the opposite direction, and whenever it is destroyed in one direction an equal amount is simultaneously destroyed in the opposite direction. Thus the recoil of a gun is the appropriate reaction to the forward motion of the bullet, and the ascent of a rocket to the downrush of heated gas from its orifice; and in other cases where the action of the principle is not so apparent, its truth has notwithstanding been universally admitted. It has, for instance, been perfectly well understood for the last 200 years that if a rock be detached from the top of a precipice 144 feet high it will reach the earth with the velocity of 96 feet in a second, while the earth will in return move up to meet it, if not with the same velocity yet with the same momentum. But inasmuch as the mass of the earth is very great compared with that of the rock, so the velocity of the former must be very small compared with that of the latter, in order that the momentum or product of mass into velocity may be the same for both. In fact, in this case, the velocity of the earth is quite insensible and may be disregarded. The old conception of the laws of motion was thus sufficient to represent what takes place when the rock is in the act of traversing the air to meet the earth; but, on the other hand, the true physical concomitants of the crash which takes place when the two bodies have come together were entirely ignored. They met, their momentum was cancelled-that was enough for the old hypothesis. So, when a hammer descends upon an anvil, it was considered enough to believe that the blow was stopped by the anvil; or when a break was applied to a carriage-wheel it was enough to imagine that the momentum of the carriage was stopped by friction. We shall presently allude to the names of those distinguished men who have come prominently forward as the champions of a juster conception of things, but in the meantime let us consider some of those influences which served to prepare men's minds for the reception of a truer hypothesis. We live in a world of work, of work from which we cannot possibly escape; and those of us who do not require to work in order to eat, must yet in some sense perform work in order to live. Gradually, and by very slow steps, the true nature of work came to be understood. It was seen, for instance, that it involved a much less expenditure of energy for a man to carry a pound weight along a level road than to carry it an equal distance up to the top of a mountain. It is not improbable that considerations of this kind may have led the way to a numerical estimate of work. Thus, if we raise a pound weight one foot high against the force of gravity we may call it one unit of work, in which case two pounds raised one foot high or one pound raised two feet high would represent two units, and so on. We have therefore only to multiply the number of pounds by the vertical height in feet to which they are raised, and the product will represent the work done against gravity. The force of gravity being very nearly constant at the earth's surface, and always in action, is a very convenient force for this purpose; but any other force, such as that of a spring, would do equally well to measure work by. Generalising, we may say, the space moved over against a force multiplied into the intensity of that force will represent the quantity of work done. So much for the definition of work, and it is necessary to know what work is before proceeding to define Energy. Now what does the word Energy really mean? In the first place it does not mean force. Two substances may have an intense mutual attraction, in virtue of which they form a very intimate union with one another; but when once this union has been consummated, although the force still continues to exist, the combination is singularly deficient in Energy. Nor does Energy mean motion, for although we cannot have motion without Energy, yet we may have Energy without motion. By the word Energy is meant the power of doing work; and the energy which a labouring man possesses means, in the strictly physical sense, the number of units of work which he is capable of accomplishing. This is a subject which at this stage we may attempt to illustrate by reference to a very different department of knowledge.* The analogy which we shall venture to institute is between the social and the physical world, in the hope that those who are more familiar with the former than with the latter may be led to perceive clearly what is meant by the word Energy in a strictly physical sense. Energy in the social world is well understood. When a man pur sues his course, undaunted by opposition and unappalled by obstacles, he is said to be a very energetic man. By his energy is meant the power which he possesses of overcoming obstacles; and the amount of this energy is measured (in the loose way in which we measure such things) by the amount of obstacles which he can overcome-the 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, a man of this kind will sometimes be defeated by an opponent who does not possess a tithe of his personal energy. Now, why is this? A reply to this question will, if we do not mistake, exhibit in a striking manner the likeness that exists between the social and the physical world. The reason is that, although his opponent may be deficient in personal energy, yet he may possess more than an equivalent in the high position which he 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 at 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 The subject has previously been discussed from this point of view by Messrs. Stewart and Lockyer in an article in Macmillan's Magazine, August 1868. |