56 PHYSIOGRAPHY. [chap. 



By careful observation it has been found that the shrinkage 

 proceeds with great regularity as the air gets cooler, but 

 there is no need to trouble ourselves, at present, with the 

 law of contraction. 



Now watery vapour, such as that present in the atmo- 

 sphere, is a body which may be said roughly to have a 

 constitution similar to that of the air with which it . is 

 associated. But when this vapour is cooled, a limit is soon 

 reached, beyond which any further cooling brings about 

 the condensation of the vapour as liquid water. In fact, 

 watery vapour, or steam, differs from fluids like air chiefly 

 in the readiness with which it can thus be condensed or 

 liquefied. 



Having in this way reduced the vapour to the condition 

 of a liquid, it is important to observe the effect of still further 

 lowering its temperature. As the water becomes cooled, the 

 bulk of the liquid is diminished. With most liquids, this re- 

 duction of bulk continues until their parts lose that freedom 

 of motion upon one another which is characteristic of a 

 liquid, and the mobile liquid passes into a cornpact -rigid 

 solid. The solid thus obtained by the congelation of water 

 is termed /ce. It is important however to note that water 

 and a few other liquids, instead of continuing steadily to 

 contract when cooled, reach a limit at which contraction 

 stops and is succeeded by expansion, so that the solid 

 water actually occupies a good deal more space than did 

 the hquid from which it was derived. When a water- 

 pipe bursts during a frost, or a jug of water cracks as 

 the liquid freezes, we are practically taught that water, 

 during the process of solidification, undergoes a large 

 increase of bulk. 



By reason of this expansion, a piece of ice necessarily 

 weighs much less than an equal bulk of water. If, for 



