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Nature previously considered separate such as material movement, sound, heat, light, electricity, and magnetism were all measurable in the same units, those of energy, the quantity of which in the universe neither increased nor decreased. The conservation of energy was a mag­nificent extension of Newton's principle of conservation of motion, like it, contained in itself no conception of pro­gressive change. However, the change did indeed follow from the second law, which limited the amount of work that could be got from each ton of coal by an engine of given design. The efficiency of engines at that time sel­dom rose to as much as five per cent.

The principle of the conservation of energy, of which mechanical work, electricity, and heat were only different forms, was the greatest physical discovery of the middle of the nineteenth century. It brought many sciences to­gether. Energy became the universal gold standard of changes in the universe. A fixed rate of exchange be­tween different forms of energy was established - between the calories of heat, the foot-pounds of work, and the kilowatt-hours of electricity. The whole of human ac­tivity - industry, transport, lighting, ultimately food and life itself - was seen to depend on this one common term: energy.

THE LAW OF ENERGY CONSERVATION

Heat, this most active, powerful and mysterious phe­nomenon of Nature, was once a really challenging prob­lem to physicists-professionals as well as non-profession­als. Among the first investigators of the problem were people of all walks of life: a peer of France Laplace and an English manufacturer of beer Joule [dsu:!], the French philosopher and writer Voltair and an English acrobat, a musician and physicist Young, the War Minister Rumford and a French doctor Poll Marat, the leader of the French Revolution.

The first to estimate the mechanical equivalent of heat was Robert Mayer (1842). Soon afterwards it was also proposed by Joule and later by von Helmholtz, a physiologist and a physicist. The same idea, though not so clearly expressed, seems to have occurred to at least five other physicists or engineers. The approaches of the three principal discoverers were different. Mayer was led to the conception by general philosophical considerations of a cosmical kind. He was struck by the analogy between the energy gained by bodies falling under gravity and the heat given off by compressed gases. Joule was led to the idea first by experiments aimed at finding out how far the new electric motor could become a practical source of power. Helmholtz in 1847, by an attempt to generalize the Newtonian conception of motion to that of a large number of bodies acting under mutual attraction, showed that the sum of force and tension, what we would now call kinetic and potential energy, remained the same. This is the principle of the Conservation of Energy in its most formal sense, but it

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