354 NINETEENTH CENTURY. PT. ill. 



of ice he found that the temperature of the water would 

 not increase above o Cent, so long as a morsel of ice 

 remained unmelted ; and again, that boiling water never 

 grew hotter than 100 Cent, while it was being turned into 

 steam. Now if we look upon heat as a vibration, we can 

 understand that the motion which is sent into ice from the 

 fire below will all be employed in overcoming the force of 

 attraction and separating the particles of ice so as to turn 

 the solid into a fluid, and it will only be when the last 

 particles are free that there will be any movement to spare 

 so as to produce the quivering motion of heat. Then if 

 you go on heating the water still more, the struggling move- 

 ment will continue between the force of attraction and the 

 force of motion, and so the water will grow hotter and hotter, 

 till at last at 100 Cent, the force of motion wins the battle, 

 and the little particles fly asunder and float away as steam \ 

 and from that moment all the extra movement is employed 

 in forcing asunder particle from particle, till all the water 

 has passed away in vapour. 



It was for this reason that Watt had to use so much 

 more cold water to cool down sttam of 100 Cent than 

 to cool down water of 100 Cent; for in cooling down 

 steam he had not only to get rid of the quivering motion 

 of heat, but of all the extra force which was holding the 

 particles asunder. 



Dr. Joule's Experiments on the Mechanical Equiva- 

 lent of Heat, 1849 It had now been shown that energy 

 could be converted into heat In Rumford's experiment the 

 energy had been derived from the horse, and in Davy's case 

 fiom himself, as he rubbed the pieces of ice or worked the 

 machine. But neither of these experiments' measured 

 accurately the amount of energy which produced a given 

 quantity of heat. Rumford had, indeed, made a rough 



