t, even though its
temperature remains at 0 deg. C. Because water loses heat when it freezes,
the presence of large streams of water greatly influences the climate
of a region. In winter the heat from the freezing water keeps the
temperature of the surrounding higher than it would naturally be, and
consequently the cold weather is less severe. In summer water
evaporates, heat is taken from the air, and consequently the warm
weather is less intense.

32. Molding of Glass and Forging of Iron. The fire which is hot
enough to melt a lump of ice may not be hot enough to melt an iron
poker; on the other hand, it may be sufficiently hot to melt a tin
spoon. Different substances melt, or liquefy, at different
temperatures; for example, ice melts at 0 deg. C., and tin at 233 deg. C.,
while iron requires the relatively high temperature of 1200 deg. C. Most
substances have a definite melting or freezing point which never
changes so long as the surrounding conditions remain the same.

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 o