lodged on the roof of a house, has spent its actual energy in separating itself from the earth, and has thus acquired an equivalent in energy of position. We may perhaps compare the heating of a particle to a man whirling round his head a heavy weight attached to his hand by a thick india-rubber string. Part of his energy will be spent in imparting velocity to the weight, and part also in stretching the string. The first is a kinetic, the latter a potential form of energy. Thus we have in the molecular world, as well as in the mechanical world, energy of motion and energy of position. We may carry the analogy yet a step further. In the visible mechanical world, whenever a body is in rapid motion, part of its energy is carried off by the air in the shape of sound and other motions of the air; thus when a string vibrates or a bell is struck the sound which reaches us represents so much of the energy of the moving particles which has been carried off by the air. 106. Radiant Light and Heat (5).—Now there is a medium pervading all space, which we call the ethereal medium, and which carries off part of the energy of motion of molecules, just as the air does in the case of large moving bodies. Thus the molecular energy which we have now described as heat is given out by a hot body to this medium which surrounds it, and by it is transmitted in a series of waves, moving at the enormous rate of 186,000 miles in one second of time. This undulatory energy is known as radiant light and heat. - 107. Electrical and Chemical Group: Electrical Separation (6). Besides the kinds of invisible energy now mentioned, we have those very important forms of it connected with electricity and chemical affinity. Thus when two bodies charged with opposite electricities are apart from one another, we have a species of energy due to the position of advantage occupied by these bodies as respects electrical force, and the bodies will have a tendency to rush together, just as a stone at the top of a cliff has a tendency to rush to the earth. Now, if they be allowed to meet one another, their energy of position will be converted into that of visible motion, just as when the stone is allowed to drop from the cliff its energy of position is converted into that of visible motion. 108. Electricity in Motion (7).—We come next to the energy represented by electricity in motion. Whenever an electric circuit has been completed, there is a power or energy pervading it which we term the electric current; and if part of the circuit be formed of a metallic wire, we can by its means convey this power into any place we choose, and as it were lay on so much energy which, properly applied, may be instrumental in doing useful work. Thus, while in ordinary cases the work is done by the side of the engine, in the case of an electric current we may have the battery or source of energy by our side, and by means of conducting wires perform our work fifty miles away. 109. Chemical Separation (8).-Finally, that description of energy represented by chemical separation has been already referred to and illustrated in the case of carbon, which we supposed separated from oxygen, for which it has an intense attraction. Let us now briefly recapitulate what has been said regarding the various forms of energy. We have, in the first place, visible mechanical energy, both actual and potential; in the next place, we have the heat group of energies-sensible heat probably representing an energy of motion, and latent heat denoting rather an energy of position while belonging to this group we have also radiant light and heat; thirdly we have the electrical and chemical group of energies, embracing that form of energy of position which is represented by the separation of differently electrified bodies, embracing also electricity in motion; and lastly, belonging to this group, we have that form of energy of position represented by the separation of bodies having a strong chemical affinity for each other. The remainder of this work will be chiefly devoted to a description of these various forms of energy, and of the laws according to which they are transmuted into one another. In the meantime let us describe the great principle which governs all such transmutations. LESSON XV.-CONSERVATION OF ENERGY. 110. The most important principle connected with this subject is that known as the conservation of energy. The production of a "Perpetual Motion" has long been one of the dreams of enthusiasts. 66 Their great ideal of mechanical triumphs was a machine that, without requiring to have any labour bestowed upon it, or to be fed with fuel of any kind, should continue to perform work for ever; a clock which could wind itself up, or an engine that could go on without coals, would be a machine of this description. In their endeavours to attain their object the advocates of a perpetual motion" must often have started questions which the natural philosopher is not always able to reply to. We do not know all the properties of matter, and we are not always able to predict what will happen under every conceivable combination of natural forces. At last, in an inspired moment, the philosopher conceived the idea of replying to all the questions of the enthusiast by denying the possibility of perpetual motion, and by asserting that it is just as impossible either to create or destroy energy as it is to create or destroy matter. Now, it is clear that the only way of establishing the truth of a principle of this kind is by trying it in a number of cases; and if it succeeds in explaining the peculiarities of each case, we have strong grounds for believing in its truth it is a tree that must be tested by its fruit. The principle of the conservation of energy has stood the test, and not only so, but it has also greatly assisted us in finding out new facts and laws of matter, so that we have much reason for believing in its truth. 111. Motion of a Stone.-Let us first of all apply it to the case of a stone projected vertically upwards, and to simplify matters, let us suppose that the stone weighs exactly one kilogramme, and that its velocity of projection is that of 196 metres in one second, which, as we have seen, represents 19'6 units of work. Let us consider the state of things at It thus appears that we may have invisible molecular energy as well as visible mechanical energy, and before proceeding further it may be desirable to give a short account of the various forms of energy both visible and invisible. For this purpose it will be convenient to divide the various energies as we know them into three groups. The first of these will be the group of Visible Energies; and this will embrace two varieties, namely (1) the energy of visible motion, and (2) the potential energy of visible arrangements. The second of these will be the group of Heat Energies, consisting (3) of the kinetic energy of absorbed heat, (4) of molecular separation, and (5) of radiant light and heat. The third of these will be the group of Electrical and Chemical Energies, consisting (6) of electrical separation, (7) of electricity in motion, and (8) of chemical separation. 104. Visible Energy of both kinds (I and 2).—In the first place we have visible energy on the large scale, embracing that which is due to position, and that which is due to actual motion. Of that which is due to position we have a head of water, a stone at the top of a cliff, a cross-bow bent (which is in a position of advantage with regard to the elastic force of the bow), a clock wound up, and so on. Then with regard to actual energy, we have that of a cannon-ball, or of a meteor, or of a gale of wind, or of a flowing river. 105. Heat Group: Absorbed Heat and Molecular Separation (3 and 4).—To come now to invisible energy, we have that well-known form of it which we call heat. When a body is greatly heated we have reason to believe that its particles are in a state of intense motion among themselves, although the body as a whole is at rest. But what in this case is latent heat, for (as we shall afterwards see) it requires a large amount of heat to convert boiling water into steam, notwithstanding which the steam is no hotter than the boiling water? In such a case we have reason to believe that much of the species of energy which we call heat has passed from the form of energy of motion into energy of position, and spent itself in forcing the particles of water to a great distance from each other, just as a stone, thrown upwards and lodged on the roof of a house, has spent its actual energy in separating itself from the earth, and has thus acquired an equivalent in energy of position. We may perhaps compare the heating of a particle to a man whirling round his head a heavy weight attached to his hand by a thick india-rubber string. Part of his energy will be spent in imparting velocity to the weight, and part also in stretching the string. The first is a kinetic, the latter a potential form of energy. Thus we have in the molecular world, as well as in the mechanical world, energy of motion and energy of position. We may carry the analogy yet a step further. In the visible mechanical world, whenever a body is in rapid motion, part of its energy is carried off by the air in the shape of sound and other motions of the air; thus when a string vibrates or a bell is struck the sound which reaches us represents so much of the energy of the moving particles which has been carried off by the air. 106. Radiant Light and Heat (5).-Now there is a medium pervading all space, which we call the ethereal medium, and which carries off part of the energy of motion of molecules, just as the air does in the case of large moving bodies. Thus the molecular energy which we have now described as heat is given out by a hot body to this medium which surrounds it, and by it is transmitted in a series of waves, moving at the enormous rate of 186,000 miles in one second of time. This undulatory energy is known as radiant light and heat. 107. Electrical and Chemical Group: Electrical Separation (6).-Besides the kinds of invisible energy now mentioned, we have those very important forms of it connected with electricity and chemical affinity. Thus when two bodies charged with opposite electricities are apart from one another, we have a species of energy due to the position of advantage occupied by these bodies as respects electrical force, and the bodies will have a tendency to rush together, just as a stone at the top of a cliff has a tendency to rush to the earth. Now, if they be allowed to meet one another, their energy of position will be converted into that of visible motion, just |