302 SCIENCE IN SHORT CHAPTERS. 



thickness and tenacity. Such being the case, it is at first 

 sight a matter 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 un- 

 derstood, viz., its viscosity. 



This requires some explanation. Though ice is what we 

 call a solid, it is not truly solid. Like other apparent sol- 

 ids it is not perfect rigid, but still retains some degree of 

 the possibility 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. 



I have constructed an apparatus which illustrates this 

 very strikingly. It is an iron syringe with cylindrical in- 

 torior of about half an inch in diameter, and a terminal 

 orifice of less than fa of an inch in diameter. Its piston 

 of metal is driven down by a screw. Into this syringe I 

 place small fragments of ice, or a cylinder of ice fitted to 

 the syringe, and then screw down the piston. Presently a 

 thin wire of ice is squirted forth like vermicelli when the 

 dough from which it is made is similarly treated, showing 

 that the ice is plastic like the dough, provided it is squeezed 

 with sufficient force. 



This viscosity of ice is displayed on a grand scale in 

 glaciers, the ice of which actually flows like a river down 

 the glacier valley, contracting as the valley narrows and 

 spreading out as it widens, just as a river would; but moving 

 only a few inches daily according to the steepness of the 

 slope and the season, slower in winter than in summer. 



Upon this, and the slowness of the act of freezing, de- 

 pends the possibility of water in freezing in iron pipes with- 

 out bursting them. Even iron yields a little before burst- 

 ing, but ordinary qualities not sufficiently to bear the ex- 

 pansion of -jV of their contents. What happens then ? The 

 cylinder of ice contained in the tube elongates as it freezes, 



