THE THEORY OF ENERGY AND THE LIVING "WORLD. 521 



by the action of natural forces. Thus it was evaporated by the heat of 

 the sun, formed into clouds, transferred by the winds, etc. Here, then, 

 is but an example of a complex energy transformation, first from actual 

 to potential energy, and then back again, with neither loss nor gain. 



There are as many forms of energy as of distinct varieties of phe- 

 nomena. Physicists distinguish two species of mechanical energy — the 

 energy of position and energy of motion. There are several varieties 

 of the former species, including distance energy or force, of which we 

 have already spoken ; surface energy, corresponding to the phenomena 

 of surface tension, and volume energy, which corresponds to the phe- 

 nomena of pressure. It would be useless for the purposes we have in 

 view to discuss mechanical energy at great length. It is more impor- 

 tant to show briefly that the various known forms of energy may be 

 transformed, the one into the other. These forms are heat, electrical, 

 magnetic, chemical, and radient energies. 



It is taught nowadays in all elementary treatises on physics that 

 mechanical work may be transformed into heat and reciprocally heat into 

 mechanical work. Friction, collision and percussion, compression and 

 expansion, destroy or annihilate the mechanical energy communicated 

 to a body or to the parts of a machine. At the same time that the 

 motion disappears heat appears. Examples are abundant. There is 

 the box of the wheel heated by the friction of the spindle; the ignition 

 of particles of steel broken off in breaking the stone; the melting of 

 two pieces of ice by Davy by rubbing them together, although sur- 

 rounded by objects below the freezing point; the boiling of water by 

 drilling, as observed by Rum ford in the boring of bronze cannon in 1790; 

 the ignition of particles of metal in beating upon the anvil; the rise of 

 temperature even to the point of fusion in lead balls fired against a 

 resisting obstacle, and, finally, the origin of fire in the fable of Prome- 

 theus by means of rubbing pieces of wood together in the way still 

 called by the Hindoos pra mantha. There is a constant correlation 

 between the phenomena of heat and motion, a correlation which has 

 become so well known that observers have ceased to verify it by indi- 

 vidual cases. There is no destruction in the true sense of the word. 

 That which is lost in one form reappears under another, giving the 

 impression of something indestructible, which manifests itself in suc- 

 cessive disguises. This impression is translated into words in saying 

 that mechanical energy is metamorphosed into thermal energy. 



This interpretation takes on a character of startling precision when 

 these mutations are examined with the almost absolute accuracy of 

 physical measurement. It is then shown that the rate of the exchange 

 is invariable. The transformations of heat into motion and vice versa 

 are accomplished in accordance with a vigorous numerical law, which 

 fixes the quantity of energy of the one kind transformed into the other. 

 The mechanical effect is evaluated, as we have said, in work; that is, in 

 kilogram-meters. Heat is measured in calories, the calorie being the 



