44 The Realm of Nature CHAP. 



7 1 . Action of Heat on Water- vapour. The work done 

 by 967 heat-units on I Ib. of water at 212 was done once 

 more against cohesion. The vibrating particles have been 

 enabled to increase the amplitude of their oscillations to a 

 great extent, the volume of the gaseous steam being 1700 

 times as great as that of the water from which it was derived, 

 and every particle of the water-vapour is darting with the 

 speed of nearly i mile per second. When heat is supplied 

 to steam every unit raises the temperature by 2 (its specific 

 heat being only half that of water) ; the rise of temperature 

 means increase in the velocity of the darting particles and 

 brings about an increase of volume by \- part for each 

 degree if the pressure upon the vapour remains the same, or 

 a corresponding increase of pressure on the sides of the 

 containing vessel if expansion is prevented. When water- 

 vapour is raised to a very high temperature the heat begins 

 to do the work of breaking up the molecules of water into 

 its components oxygen and hydrogen, thus doing work 

 against chemical attraction and storing up potential energy 

 in the separated gases. 



72. Pressure and Change of State. Under pressure 

 ice melts at a lower temperature than 32, and the few other 

 bodies which contract when they liquefy also have their 

 melting-points lowered by pressure. Bodies which expand 

 when they liquefy like mercury, rocks, and most other 

 substances have their melting temperatures raised by 

 pressure so that more heat is required to liquefy them. The 

 effect of pressure on the temperature at which the change 

 from liquid to gas takes place is much more marked. In 

 every case an increase of pressure delays complete vaporisa- 

 tion or boiling until a higher temperature is reached. Water, 

 for example, cannot be heated in the liquid state to a greater 

 temperature than 68 if the atmospheric pressure is one- 

 fortieth of its average amount, but to 176 at half the usual 

 pressure, and to 250 if the usual pressure is doubled. The 

 boiling-point of a liquid may thus be used to measure 

 atmospheric pressure. 



73. Heat-energy. The changes which take place when 

 heat is withdrawn from matter are the exact opposite of 



