In a steam-engine, heat, which is the best known kind of energy, is produced in the furnace ; when the engine is at work all of this energy does not leave it as heat; some is turned into mechanical work, and the more work the engine does the greater is the difference between the heat generated in the furnace and that leaving- the machine. If, however, we used the work to rub two rough surfaces together we could get the heat back, and if (which of course is impossible in practice) we could avoid all friction between the moving parts of the machine, and have all parts of the engine at the end of the experiment exactly at the same temperature as at its beginning, the quantity of heat thus obtained would be exactly equal to the difference between that amount of heat originally generated in the furnace of the engine, and the quantity which had been carried off from it to the air since its fire was lighted. Having turned some of the heat into mechanical work we could thus turn the work back into heat again, and find it yield exactly the amount which seemed lost.

Can matter, be transmuted ? What is always found when energy is transformed ?

Can man create energy ? Illustrate the fact that energy can be changed in kind but not created. What is meant by the law of the conservation of energy ?

Give an illustration of the conservation of energy.

Or we might use the engine to drive an electro-magnetic machine and so turn part of the heat liberated in its furnace, first into mechanical work, and this afterwards into electricity; and if we chose to use the latter with the proper apparatus, as now used for electric lighting, we could turn more or less of it into light; and so have a great part of the energy which first became conspicuous as heat in the engine furnace, now manifested in the form of light at some distant point. In fact, starting with a given quantity of one kind of energy, we may by proper contrivances turn all or some of it into one or more other forms; but if we collected all the final forms and retransformed them into the first, we should have exactly the amount of it which had disappeared when the other kinds appeared.