ar towards 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.

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 separated from one another, and submitted to microscopic
examination, either as opaque or as transparent objects. By combining the
views obtained in these various methods, each of the rounded bodies may
be proved to be a beautifully-constructed calcareous fabric, made up of a
number of chambers, communicating freely with one another. The chambered
bodies are of various forms. One of the