334 NINETEENTH CENTURY. pt. iii. 



passes on through the glass of the tube into the mercury, and 

 the particles of mercury are also set in motion, and so the 

 mercury swells and rises in the tube. 



The cause of Latent Heat explained. — And now, if 

 you will look back for a moment to chapter xxviii., and 

 read again about the ' latent heat ' which puzzled Dr. Black 

 so much, you will see how beautifully it can be explained 

 by this theory that heat is a kind of motion. You will 

 remember that, however much heat he put under a piece 

 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 ioo° 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 loo" 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 steam 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 



