Page:Elementary Text-book of Physics (Anthony, 1897).djvu/227

§ 196] '''195. Change of Volume with Change of State.'''—Substances are generally more dense in the solid than in the liquid state, but there are some notable exceptions. Water, on solidifying, expands; so that the density of ice at zero is only 0.9167, while that of water at 4° is 1. This expansion exerts considerable force, as is evidenced by the bursting of vessels and pipes containing water.

196. Change of Melting- and Freezing-points.—If water be enclosed in a vessel sufficiently strong to prevent its expansion, it cannot freeze except at a lower temperature. The freezing-point of water is, therefore, lowered by pressure. On the other hand, substances which contract on solidifying have their solidification hastened by pressure.

The lowering of the melting-point of ice by pressure explains some remarkable phenomena. If pieces of ice be pressed together, even in warm water, they will be firmly united. Fragments of ice may be moulded under heavy pressure, into a solid, transparent mass. This soldering together of masses of ice is called regelation. If a loop of wire be placed over a block of ice and weighted, it will cut its way slowly through the ice, and regelation will occur behind it. After the wire has passed through, the block will be found one solid mass, as before. The explanation of these phenomena is, that the ice is partially melted by the pressure. The liquid thus formed is colder than the ice; it finds its way to points of less pressure, and there, because of its low temperature, it congeals, firmly uniting the two masses.

Water, when freed from air and kept perfectly quiet, will not form ice at the ordinary freezing-point. Its temperature may be lowered to -10° or -13° without solidification. In this condition a slight jar, or the introduction of a small fragment of ice, will cause a sudden congelation of part of the liquid, accompanied by a rise in temperature in the whole mass to zero.

A similar phenomenon is observed in the case of several solutions, notably sodium sulphate and sodium acetate. If a saturated hot solution of one of these salts be made, and allowed to cool in a closed bottle in perfect quiet, it will not crystallize. Upon opening