THE PRINCIPLES OF ENERGY 43 



tained in a heat-insulated vessel. He took all possible precau- 

 tions, measured the amount of mechanical work done (by causing 

 a falling weight of known mass to actuate the paddle), and 

 measured the mass of water and the rise of temperature produced 

 by the friction of the paddle. He found what is now called the 

 " mechanical equivalent of heat " — that is, he found that when 

 a weight of 1 kilogram falls through 427 kilometres, 1 kilogram 

 of water is raised in temperature 1° C. This amount of heat 

 communicated to the water is called 1 (large) Calorie. Now 

 imagine the water contained in the mechanism to be heated up 

 1° C. ; if the latter were reversible the paddle would revolve. But 

 it does not, and the apparatus is therefore irreversible. 



Finally, make the end of a poker red-hot in the fire, and then 

 take it out and let it stand ; it will cool down, and in half an hour 

 or so it will have attained the temperature of the air in the room. 

 Its heat has been lost by radiation and convection, and has gone 

 to raise the temperature of the air, furniture, and walls of the room 

 ever so little. Imagine the experiment to be reversed, so that the 

 heat of the room would flow into the end of the poker and raise 

 its temperature to redness ; such an effect has never been observed 

 to occur (though if it did occur physicists would not be incredu- 

 lous !). Therefore the flow of heat, of itself , is irreversible. 



These examples will enable us to formulate two more statements : 



Some energy transformations are approximately reversible and 

 ^ others are irreversible . . . . . . . . • • (3) 



The flow of heat {of itself) is irreversible, only going from a hot 

 to a cold body . . . . . . . . . . • • (4) 



Thus we can, quite easily, cause some energy transformations 

 to occur, but not so easily, or not at all, some others. 



We can easily cause all kinds of energy to transform into heat, 

 and, in fact, they do transform into heat of themselves. 

 Mechanical friction always generates heat, chemical action 

 generally produces heat and sometimes chemical energy com- 

 pletely transforms into heat ; the flow of electricity through a con- 

 ductor, the straining and bending of materials (internal friction), 

 the reception of light by substances — in short, all physical reac- 

 tions — transform, or tend to transform, into heat. Therefore — 



All forms of available energy tend to be transformed into heat, 



' but heat is not at all, or it is only with difficulty and loss, 



transformable into other forms of available energy . . (5) 



