HEAT. 151 



of 32 . Here, 140 of heat has left the water and com- 

 bined with the snow, and thus caused it to melt; yet the 

 water resulting from this melted snow has the same tem- 

 perature as the snow. Therefore a pound of snow 

 requires 140 of heat to melt it. 



Water after being cooled down to 32 cannot freeze 

 until it has parted with 140 of heat, and ice after being 

 heated to 32 will not melt until it has absorbed 140 o 

 heat ; hence the slowness with which these operations are 

 performed. 



To the quantity of heat which thus combines with a 

 body and causes fluidity, Dr Black gave the name of 

 latent heat. 



The fluidity of all bodies is owing to the same cause, 

 and it may be considered as a general law, that whenever 

 a solid is converted into a liquid, it combines with heat. 

 Metals owe their ductility and malleability to the latent 

 heat which they contain. Hence, we have the reason 

 why they become hot and brittle by being hammered, 



OF STEAM AND EVAPORATION. 



Nearly every liquid body when raised to a certain 

 temperature is gradually converted into an elastic fluid, 

 which like air is invisible. Water when heated suffi- 

 ciently is converted into an elastic fluid, called steam, 

 which is invisible, and has the same mechanical pro- 

 perties as air. One cubic inch of water will make 1800 

 cubic inches of steam. 



When a vessel of water is placed over a fire, the water 

 gradually heats until it reaches 2 12, after which its 

 temperature will not increase ; yet, heat must be gradually 

 and constantly combining with it, and as it does not 

 become hotter, it must combine with the steam which is 

 constantly flying off; but the steam isonly at 21(2, of 

 course the heat must become latent, and we must con- 

 clude that the change of water into steam is owing to this 

 latent heat. 



Steam is water combined with about 1000 of heat, 

 therefore, as has been before shown, a given quantity of 

 steam at 212 , will heat a great deal more water than an 

 equal weight of water at 212, because it contains a 

 great deal more heat, which is given out by changing the 

 steam into water. 



