THE ACTION OF FROST IX WATER-PIPES. 31 



Fortunately for us, the freezing of water is always a slow 

 process, for if this conversion of every 15 gallons into 16 took 

 place suddenly, all our pipes would rip open with something 

 like explosive violence. But such sudden freezing of any 

 considerable quantity of water is practically impossible, on 

 account of the " latent heat" of liquid water, which amounts 

 to 142. All this is given out in the act of freezing. It is 

 this giving out of so much heat that keeps the temperature of 

 freezing water always at 32% even though the air around may 

 be much colder. No part of the water can fall below 32 

 without becoming solid, and that portion which solidifies 

 gives out enough heat to raise 142^ times its own quantity 

 from 31 to 32. 



The slowness of thawing is due to the same general fact. 

 An instructive experiment may be made by simply filling a 

 saucepan with snow or broken ice, and placing it over a com- 

 mon fire. The slowness of the thawing will surprise most 

 people who have not previously tried the experiment. It takes 

 about as long to melt this snow as it would to raise an 

 equal weight of water from 32 to 174. Or, if a pound of 

 water at 174 be mixed with a pound of snow at 32, the result 

 will be two pounds of water at 32 ; 142 will have disap- 

 peared without making the snow any warmer, it will all have 

 been used up in doing the work of melting. 



The force with which the great expansion due to freezing 

 takes place is practically irresistible. Strong pieces of 

 ordnance have been filled with water, and plugged at muzzle 

 and touch-hole. They have burst in spite of their great thick- 

 ness and tenacity. Such being the case, it is at first sight a mat- 

 ter of surprise that frozen water-pipes, whether of lead or iron, 

 ever stand at all. They would not stand but for another property 

 of ice, which is but very little understood viz. its viscosity. 



This requires some explanation. Though ice is what we call 

 a solid, it is not truly solid. Like other apparent solids it is 

 not perfectly rigid, but still retains some degree of the possi- 

 bility of flowing which is the characteristic of liquids. This 

 has been shown by filling a bombshell with water, leaving the 

 fuse-hole open and freezing it. A shell of ice is first formed 

 on the outside, which of course plugs up the fuse-hole. Then 

 the interior gradually freezes, but the expansion due to this 

 forces the ice out of the fuse-hole as a cylindrical stick, just 

 as putty might be squeezed out, only that the force required to 

 mould and eject the ice is much greater. 



