showing that chalk is essentially
nothing but carbonic acid and quicklime. Chemists enunciate the result
of all the experiments which prove this, by stating that chalk is
almost wholly composed of "carbonate of lime."

It is desirable for us to start from the knowledge of this fact,
though it may not seem to help us very far toward what we seek. For
carbonate of lime is a widely-spread substance, and is met with under
very various conditions. All sorts of limestones are composed of more
or less pure carbonate of lime. The crust which is often deposited by
waters which have drained through limestone rocks, in the form of what
are called stalagmites and stalactites, is carbonate of lime. Or, to
take a more familiar example, the fur on the inside of a tea-kettle is
carbonate of lime; and, for anything chemistry tells us to the
contrary, the chalk might be a kind of gigantic fur upon the bottom of
the earth-kettle, which is kept pretty hot below.

Let us try another method of making the chalk tell us its own history.
To the unassisted eye chalk looks simply like a very loose and open
kind of stone. But it is possible to grind a slice of chalk down so
thin that you can see through it--until it is thin enough, in fact, to
be examined with any magnifying power that may be thought desirable. A
thin slice of the fur of a kettle might be made in the same way. If it
were examined microscopically, it would show itself to be a more or
less distinctly laminated mineral substance, and nothing more.

But the slice of chalk presents a totally different appearance when
placed under the microscope. The general mass of it is made up of very
minute granules; but, imbedded in this matrix, are innumerable bodies,
some smaller and some larger, but, on a rough average, not more than a
hundredth of an inch in diameter, having a well-defined shape and
structure. A cubic inch of some specimens of chalk may contain
hundreds of thousands of these bodies, compacted together with
incalculable millions of the granules.

[Illustration: CHALK.

(Magnified nearly 100 diameters.)]

The examination of a transparent slice gives a good notion of the
manner in which the components of the chalk are arranged, and of
their relative proportions. But, by rubbing up some chalk with a brush
in water and then pouring off the milky fluid, so as to obtain
sediments of different degrees of fineness, the granules and the
minute rounded bodies may be pretty well sep