But while most substances have a definite melting point, some substances do not.  If a glass rod is held in a Bunsen burner, it will gradually grow softer and softer, and finally a drop of molten glass will fall from the end of the rod into the fire.  The glass did not suddenly become a liquid at a definite temperature; instead it softened gradually, and then melted.  While glass is in the soft, yielding, pliable state, it is molded into dishes, bottles, and other useful objects, such as lamp shades, globes, etc. (Fig. 20).  If glass melted at a definite temperature, it could not be molded in this way.  Iron acts in a similar manner, and because of this property the blacksmith can shape his horseshoes, and the workman can make his engines and other articles of daily service to man.

[Illustration:  FIG. 20.—­Molten glass being rolled into a form suitable for window panes.]

33.  Strange Behavior of Water.  One has but to remember that bottles of water burst when they freeze, and that ice floats on water like wood, to know that water expands on freezing or on solidifying.  A quantity of water which occupies 100 cubic feet of space will, on becoming ice, need 109 cubic feet of space.  On a cold winter night the water sometimes freezes in the water pipes, and the pipes burst.  Water is very peculiar in expanding on solidification, because most substances contract on solidifying; gelatin and jelly, for example, contract so much that they shrink from the sides of the dish which contains them.

If water contracted in freezing, ice would be heavier than water and would sink in ponds and lakes as fast as it formed, and our streams and ponds would become masses of solid ice, killing all animal and plant life.  But the ice is lighter than water and floats on top, and animals in the water beneath are as free to live and swim as they were in the warm sunny days of summer.  The most severe winter cannot freeze a deep lake solid, and in the coldest weather a hole made in the ice will show water beneath the surface.  Our ice boats cut and break the ice of the river, and through the water beneath our boats daily ply their way to and fro, independent of winter and its blighting blasts.

While most of us are familiar with the bursting of water pipes on a cold night, few of us realize the influence which freezing water exerts on the character of the land around us.

Water sinks into the ground and, on the approach of winter, freezes, expanding about one tenth of its volume; the expanding ice pushes the earth aside, the force in some cases being sufficient to dislodge even huge rocks.  In the early days in New England it was said by the farmers that “rocks grew,” because fields cleared of stones in the fall became rock covered with the approach of spring; the rocks and stones hidden underground and unseen in the fall were forced to the surface by the winter’s expansion.  We have all seen fence posts and bricks pushed out of place because of the heaving of the soil beneath them.  Often householders must relay their pavements and walks because of the damage done by freezing water.