232 WELLS'S NATURAL rHILQSOPHY. 



All these frost-work figures are limited by the laws of crystallization, and the 

 lines which bound them, form among themselves no angles but those of 

 30°, 60° and 120°.. If we fracture thin ice, by allowing a pole or weight to 

 fall upon it, the fracture will have more or less of regularity, being generally 

 in the form of a star, with six equi-distant radii, or angles of 60° 



529. The force exerted by the expansion of water in the 

 force does wa- ^'^^ ^^ freezing is very great. As an illustration, the following 

 ter expand in experiment may be quoted : — Cast-iron bomb-shells, thirteen 



inches in diameter and two inches thick, were filled with wa- 

 ter, and their apertures or fuse-holes firmly plugged with iron bolts. Thus 

 prepared, they were exposed to the severe cold of a Canadian winter, 

 at a temperature of about 19° below zero. At the moment the water 

 froze, the iron plugs were violently thrust out, and the ice protruded, and 

 in some instances the shells burst asunder, thus demonstrating the enor- 

 mous interior pressure to which they were subjected by water assuming a 

 solid state. 



The rounded and weather-worn appearance of rocks is mainly due to tho 

 expansion of freezing water, which penetrates into their fissures, and is ab- 

 sorbed into their pores by capillary attraction. In freezing, it expands and 

 detaches successive fragments, so that the original sharp and abrupt outline ia 

 gradually rounded and softened down. 



The bursting of eartlien water vessels, and of water pipes, by the freezing 

 of water contained in them, are familiar illustrations of the same principle. 



By allowing the water to run in a service-pipe, we prevent its freezing, be- 

 cause the motion of the current prevents the crystals from forming and 

 attaching themselves to the sides of the pipe. 



530. The ordinary temperature at which water freezes is 

 perature does 32°, Fahrenheit's thermometer. This rule applies only to 

 water freeze? fresh water; salt water never freezes until the surface is cooled 

 dowTito 27°, or five degrees lower than the freezing point of winter. 



Under some circumstances pure water may be cooled down to a tempera- 

 ture much below 32° without freezing. Thus, if pure, reccntly-boUed water, 

 be cooled very slowly and kept very tranquil, its temperature may be low- 

 ered to 21° without the formation of ice; but the least motion causes it to 

 congeal suddenly, and its temperature rises to 32°. 



. 531. The ice produced by the freezing of sea or salt water 



ice produced is generally ft-esh and free from salt, since water in freezing, 



by the freez- y sufficient freedom of motion be allowed to its particles, ex- 



ing of salt wa- ^ 



ter free from pels all impurities and coloring matters. The ice formed m 



■^'' ^ the congelation of a solution of indigo is colorless, since tho 



water in which the indigo was dissolved expels the blue coloring matter in 



freezing. 



.„^ , . , Blocks of ice are generally filled with minute air-bubbles ; 



What IS the „ , , . ,. 1 XL 



origin of the this is owing to the fact that the water in freezing expels the 



minute bubbles j^j^ contained in it, and many of the liberated bubbles become 

 Been in ice r ' "^ „ . , 



lodged and imbedded in the thickening fluid. 



