The mechanical energy of machinery or working power is exhausted by the very act of working, and cannot be restored except by the action of other forces. In clockwork, the weight must sink to move the wheel, and when the weight is down, the store of energy is gone, and can only be restored by raising the weight through the expenditure of energy in the human arm, and the expenditure of human energy must be restored by food and rest. The heat given off from the bodies of men and animals is restored by the combustion of the oxygen inhaled during respiration and the carbon of the food, and the light and heat given out by the combustion of fuel, whether in the form of coal or wood, is compensated by the light and heat of the sun stored up in living vegetables. It is this equivalent for force or energy which prevails in every department of nature that constitutes the universal and invariable law of the Conservation of Energy, ‘a principle in physics as large and sure as that of the indestructibility of matter or the invariability of gravity. No hypothesis should be admitted nor any assertion of a fact credited, that denies this principle. No view should be inconsistent or incompatible with it. Many of our hypotheses in the present state of science may not comprehend it, and may be unable to suggest its consequences, but none should oppose or contradict it.’[1] Thus, ‘there is a definite store of energy in the universe, and every natural change or technical work is produced by a part only of this store, the store itself being eternal and unchangeable.’[2] Cohesion is a force which acting at inappreciably small distances unites atoms and molecules of the same kind into solids, liquids, and aëriform fluids, exactly according to the law of the conservation of energy; for it requires the very same amount of force to dissolve their union as to form it. Cohesion varies with temperature both in simple and compound bodies, for metals can be fused and vaporized by artificial heat, and ice becomes water and aqueous vapour as the seasons change from winter to summer. In solids the force of cohesion is so strong, that their atoms and molecules always retain their respective places; that power is so weak in liquids, that their atoms and molecules are capable of motion among themselves, and in gases and the ethereal medium the atoms are free and have no cohesion whatever. The resistance offered by substances to compression is an equal and contrary force.