Quicklime is a substance obtained by heating chalk or limestone to
redness. When pure, it is a white hard solid which can be made to pass
into the liquid and gaseous states only at enormously high temperatures.
If a lump of fresh quicklime be placed in a saucer and about one-third
of its weight of water poured upon it, there will be a great turmoil,
heat will be evolved, the water will disappear, and the lime will
crumble down into a soft white powder. This operation is what
bricklayers call =slaking= lime. And if no more water has been added
than the proportion mentioned, the pure white powder which results will
be solid and dry, the water having, to all appearance, vanished.
In the solution of salt we saw a solid become fluid under the influence
of water; in the slaking of lime the fluid water enters into the
structure of a solid. If more water is added, this solid dissolves or
becomes liquid, as the salt did, and the solution is called =limewater=.
By carefully managed evaporation of the water the lime may be recovered
in the form of crystals, just as the salt was recovered. But there is
this difference, that the salt crystals contain no water, while the lime
crystals not only contain water, but contain exactly the same proportion
as exists in slaked lime, that is to say, 18 parts water to 56 parts
lime.
The water thus bound up with the lime into a new solid holds on so
firmly to the lime that it requires a red heat to separate the two. The
lime and the water are said to be =chemically combined=; and as the
proportion of lime and water in slaked lime, or lime crystals, is always
the same, they are said to be combined in =definite proportions=; and
the slaked lime receives the special name of =hydrate of lime=.
=Gypsum= or =Plaster of Paris= is a dry white powder. If mixed with a
little water it does not slake after the fashion of quick lime, but the
mixture soon =sets= or becomes hard; and, at the same time, the greater
part of the water disappears. In fact, it has combined with the plaster
of Paris and forms part of another hydrate, in which, when the
superfluous moisture dries, not a trace of water is to be seen. It is
this property which is taken advantage of when plaster of Paris is used
in making casts and moulds. The fluid plaster is poured over and round
the body to be cast; as a fluid, it applies itself conveniently to all
the inequalities of its surface; and, when it sets, it retains the shape
which it has thus acquired. Set plaster of Paris may be perfectly dry;
but it nevertheless contains between ⅐ and ⅛ of its weight of water,
fixed and forming an integral part of the solid hydrate. And if the set
plaster is strongly heated, the combined water is driven off and it
returns to its original state.
Gypsum is found abundantly in nature, in the shape of beautiful
transparent crystals which are called =selenite=. These crystals have
the same composition as set plaster, that is to say, they are hydrates.
A thin flake of such a crystal viewed with the highest powers of the
microscope appears perfectly homogeneous. Nevertheless, there is good
reason for the conclusion that it consists of molecules of water and
molecules of gypsum which hold together so strongly that they form a
hard brittle glassy solid. Moreover, the molecules of the hydrate itself
hold together more strongly in some directions than in others. It is
very easy to split the crystals lengthwise; while much more force is
needed to cut them crosswise and then they do not split, but break.
Glauber’s salts and Epsom salts are other examples of solids which
dissolve in water and separate in the crystalline form as the water
evaporates; and which, like lime and gypsum, combine with a definite
proportion of water to form crystalline compounds. In fact, each of
these glassy brittle solids contains more than half its weight of water.
Thus we see that two bodies, of which water is one, may combine together
to give rise to something different from either. And we are thus led to
the science of =chemistry=, which tells us exactly how bodies combine,
what comes of their combination, and how compounds may be separated into
their constituents.
55. =Mineral bodies may take on definite shapes and grow, or increase in
size, by the addition of like parts.=
Water and all the other natural bodies which have hitherto been
mentioned, are what are called =mineral bodies=, although, in common
use, the term mineral is usually restricted to ores and metals. Now we
have repeatedly had occasion to remark that, under certain
circumstances, not only water, but many other mineral bodies, assume
regular shapes. The most familiar example is that of the beautiful
imitation of leaves and foliage which is presented by the ice which
forms on a window in winter. But we have also seen that common salt,
lime, gypsum, Glauber’s salts and Epsom salts, also assume the
crystalline form as they or their compounds with water are deposited
from their solutions. And if a drop of solution of Glauber’s salts or of
Saltpetre, is allowed to evaporate under the microscope, a wonderful
spectacle will be presented. As the salt assumes the solid state, the
crystals suddenly appear in the field of view as needles and plates
disposed in beautiful patterns, which rival those of hoar frost, though
they are quite different from them. In fact, as you will learn if you
study =crystallography=, every crystallizable substance has its proper
crystalline forms and never departs from certain strictly related
geometrical figures.
A crystal of any of these substances will =grow= if placed under proper
conditions. Thus, if a crystal of common salt is hung by a thread in a
saturated solution of salt, which is exposed to the air, so as to allow
the water to evaporate slowly, the molecules of the salt which is left
behind and can no longer be held in solution, deposit themselves on the
crystal in regular order and increase its size without changing its
form. And, in this way, the small crystal may =grow= to a great size.
The large crystals of sugar candy, which consist of sugar and water
deposited from a strong syrup or saturated solution of sugar, grow in
the same fashion, upon threads suspended in the evaporating syrup. In
this mode of growth you will observe that the enlargement is effected by
addition to the outside of the growing body; and moreover the matter
which is added, namely, the salt or the sugar, already exists as salt in
the brine or as sugar in the syrup.