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Conservation, in Physics, a term employed at present in three definite connections to signify the impossibility of altering the quantity of certain existences in the universe. Each of these existences may be distributed in different ways, but the total quantity thereof cannot be changed. Taking them in the order in which the enunciation of the doctrine of their conservation has been formulated, we have : 1. Conservation of Matter. This doctrine has been reached by the investigations chiefly of chemists. Matter exists in the universe in various forms and with unequal distribution. Whatever process obtains to change the distribution or the forms of matter, whether this process be natural or artificial, it cannot change the total quantity of matter. Thus the burning of a candle involves the combination of particles of candle with particles of oxygen in the surrounding air. There is thus a redistribution of the matter in the system, and portions of that candle pass away in various directions. But every particle still exists; none are annihilated and none created afresh. 2. Conservation of Energy. Energy is generally defined as the power of doing work, and this definition may be here accepted, though from the principle to be enunciated it is obvious that to do work is merely to transfer energy. But whatever energy may be defined to be, the total amount in the universe is held to be constant. If, for instance, a machine has a certain amount of energy given to it, by no means can we make it give out more than that quantity. This we learn by experience, and we argue its truth by reason of the applicability of the doctrine of conservation of energy to every case where results obtained theoretically may be tested experimentally. The doctrine being true, it is impossible to construct a machine that shall be in continual motion and that shall also do useful external work. For if its energy is partially given away less must remain. If the withdrawal of energy is continuous, it must ultimately lose its energy and be therefore motionless. So perpetual motion, as understood by the old philosophers, is an impossibility. 3. Conservation of Electricity. This has only of recent years been advanced by physicists. Whatever electricity may be, whether according to the one-"fluid" theory its effects are observable wherever it exists in quantity above or below the average, or according to the two-"fluid" theory the effects are observable where the two opposing principles do not neutralise each other, this doctrine of conservation of electricity states that like matter its total quantity in the universe is invariable and that its distribution alone varies. The question of conservation of force was investigated experimentally by Faraday, but the results did not lead to any new development, Force is not an existence, but only a rate of change of energy.