in Energy, the Power of Nature 45 



those accompanying the application of heat. When oxygen 

 and hydrogen unite, the potential energy of separation is 

 changed into kinetic heat -energy, as already explained 

 ( 44). When i Ib. of hot water- vapour radiates out its 

 heat -energy its temperature falls gradually to 212 at 

 ordinary pressure ; but then, in assuming the liquid state, 

 967 heat-units are given out as the particles rush together 

 under the influence of cohesion. One pound of steam at 

 212 if passed into 4 Ibs. of water at 32 gives out heat 

 enough in liquefying to warm up the whole 5 Ibs. of water 

 to 212; hence the great value of condensing steam as 

 a heating agent. One pound of water cooling from 212 to 

 32 gives out 1 80 heat-units, and as the particles come 

 fully under the influence of cohesion and group themselves 

 into solid crystals of ice, the energy that held them apart 

 is changed into 144 units of heat. 



74. Mechanical Equivalent of Heat. The great task 

 of measuring the quantity of heat-energy which is equal to a 

 certain amount of work ( 25, 49), and so of comparing 

 the invisible motion of molecules with the visible motions 

 of masses, was attempted and triumphantly accomplished 

 by Joule in 1843, when the modern theory of energy was 

 founded. He showed that I heat -unit was equal to 772 

 foot-pounds. In other words, if a mass of i Ib. were to be 

 pulled down by gravity through 772 feet, and the whole of 

 its kinetic energy changed into heat in i Ib. of water at 32, 

 the temperature of the water would be thereby raised to 

 33. Thus we can measure the work done by heat in 

 melting i Ib. of ice at 32 ( 69) and find it to be equal to 

 i n,ooo foot-pounds, while that done in evaporating i Ib. of 

 water at 212 ( 70) is 747,000 foot-pourids. It appears 

 that the heating of ij Ibs. of ice at 32 until it becomes 

 steam at 212 requires as much energy as the feat of 

 mountain-climbing described in 49, 52. 



75. Degradation of Energy. It is always possible and 

 easy to change work or electricity or light into heat, and 

 772 foot-pounds of work will always yield the full heat- 

 unit. The inverse operation is different, and from i unit of 

 heat the best machine it is possible to imagine could only 



