OBJECTS.] INTRODUCTORY. 61 



much water, derived from the breath and the evapora- 

 tion of moist surfaces, as can maintain itself in the 

 gaseous state at the temperature. The window- 

 panes, being thin, are cooled by the outside air, 

 and of course the gaseous water inside the room, 

 when it comes in contact with the cold window- 

 panes, becomes condensed on them into fine drops 

 of cold water. The panes becoming colder and 

 colder, these minute drops at last freeze, and the 

 water not only becomes solid, but it crystallises ; 

 that is to say, the little solid masses take on more or 

 less regular geometrical forms with flat faces, inclined 

 to one another at constant angles, so that they re- 

 somble bits of glass cut according to particular fixed 

 patterns. All ice is in fact crystalline, but in ice 

 which has been formed from thick sheets of water, the 

 crystals are so packed together that they cannot be 

 distinguished separately. 



43. When Ice is warmed it begins to change 

 back into Water as soon as the Temperature 

 reaches 32. 



A lump of ice brought out of the open air in very 

 cold weather may have a temperature of 30, or 20, or 

 lower. If such a lump is brought into a warm room 

 it gradually becomes warmer, but remains unchanged 

 otherwise, until it has risen to 32. Then it begins to 

 melt, and remains at 32 as long as it is melting ; and 

 the water which proceeds from it is at first also at 32. 



If you were to throw a lump of ice into the middle 

 of a hot fire, so long as a particle of ice remained as 

 such, it would have a temperature of 32 and no more. 

 This is a fact exactly parallel to that which is observed 



