exactly equal to the amount of heat absorbed by melting ice.

The number of units of heat required to melt a unit mass of ice is
called the _heat of fusion_ of water.

31. Climate. Water, in freezing, loses heat, even though its
temperature remains at 0° 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° C., and tin at 233° C.,
while iron requires the relatively high temperature of 1200° 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