r
words, it does not undergo decomposition readily. To decompose it into
its elements by heat alone requires a very high temperature; at 2500 deg.,
for example, only about 5% of the entire amount is decomposed. Though
very stable towards heat, water can be decomposed in other ways, as by
the action of the electrical current or by certain metals.

~Heat of formation and heat of decomposition are equal.~ The fact
that a very high temperature is necessary to decompose water
into hydrogen and oxygen is in accord with the fact that a
great deal of heat is evolved by the union of hydrogen and
oxygen; for it has been proved that the heat necessary to
decompose a compound into its elements (heat of decomposition)
is equal to the heat evolved in the formation of a compound
from its elements (heat of formation).

~Water of crystallization.~ When a solid is dissolved in water and the
resulting solution is allowed to evaporate, the solid separates out,
often in the form of crystals. It has been found that the crystals of
many compounds, although perfectly dry, give up a definite amount of
water when heated, the substance at the same time losing its crystalline
form. Such water is called _water of crystallization_. This varies in
amount with different compounds, but is perfectly definite for the same
compound. Thus, if a perfectly dry crystal of copper sulphate is
strongly heated in a tube, water is evolved and condenses on the sides
of the tube, the crystal crumbling to a light powder. The weight of the
water evolved is always equal to exactly 36.07% of the weight of copper
sulphate crystals heated. The water must therefore be in chemical
combination with the substance composing the crystal; for if simply
mixed with it or adhering to it, not only would the substance appear
moist but the amount present would undoubtedly vary. The combination,
however, must be a very weak one, since the water is often expelled by
even a gentle heat. Indeed, in some cases the water is given up on
simple exposure to air. Such compounds are said to be _efflorescent_.
Thus a crystal of sodium sulphate (Glauber's salt) on exposure to air
crumbles to a fine powder, owing to the escape of its water of
crystallization. Other substances have just the opposite property: they
absorb moisture when exposed to the air. For example, if a bit of dry
calcium chloride is placed in moist air, in the course of a few hours it
wil