82 HEAT. 



also precisely the same temperature. Neither water 

 or steam are hotter, although both have been con- 

 stantly taking in heat. But the heat has not been 

 without effect, any more than in the conversion of a 

 solid into a liquid. It has combined with the liquid 

 to form the steam. In this case, also, the heat which 

 disappears is called latent heat. 



195. RELATION OF PRESSURE TO BOIL- 

 Howdoespres- ING. In order that a bubble of steam may 



sure oppose f . . 



boiling ? form, it is necessary that a small portion 

 of water, shall expand into a comparatively 

 large portion of steam to form it. But the atmosphere 

 is constantly pressing on the surface of the water, and 

 acting through the water, in all parts of the vessel, to 

 prevent any separation of particles or expansion. The 

 case is similar to that of a piece of india-rubber com- 

 pressed beneath a mass of iron : it cannot expand ow- 

 ing to the weight of the iron. 



196. HEAT OVERCOMES PRESSURE. But 



Explain how 



heat overcomes if we could by some means increase the 

 pressure. elasticity of the india-rubber, it would ex- 

 pand and lift the iron. So, if we can in any way in- 

 crease the tendency of the particles of water to sepa- 

 rate, it will finally be strong enough to overcome the 

 pressure of the atmosphere above arid affect separation. 

 Heat has this effect. As the water becomes hotter, 

 the tendency of its particles to fly apart becomes 

 greater and greater, till, at last, it is sufficient to over- 

 come the pressure which has before crowded them to- 

 gether, and a bubble of steam is formed. Others im- 

 mediately follow, and boiling thus commences. This 



