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.
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