Discourses: Biological and Geological Essays | Page 4

in the East, chalk has its share in the
formation of some of the most venerable of mountain ranges, such as
the Lebanon.
What is this wide-spread component of the surface of the earth? and
whence did it come?
You may think this no very hopeful inquiry. You may not unnaturally
suppose that the attempt to solve such problems as these can lead to no
result, save that of entangling the inquirer in vague speculations,
incapable of refutation and of verification. If such were really the case,
I should have selected some other subject than a "piece of chalk" for
my discourse. But, in truth, after much deliberation, I have been unable
to think of any topic which would so well enable me to lead you to see
how solid is the foundation upon which some of the most startling
conclusions of physical science rest.
A great chapter of the history of the world is written in the chalk. Few
passages in the history of man can be supported by such an
overwhelming mass of direct and indirect evidence as that which
testifies to the truth of the fragment of the history of the globe, which I
hope to enable you to read, with your own eyes, to-night. Let me add,
that few chapters of human history have a more profound significance
for ourselves. I weigh my words well when I assert, that the man who

should know the true history of the bit of chalk which every carpenter
carries about in his breeches- pocket, though ignorant of all other
history, is likely, if he will think his knowledge out to its ultimate
results, to have a truer, and therefore a better, conception of this
wonderful universe, and of man's relation to it, than the most learned
student who is deep-read in the records of humanity and ignorant of
those of Nature.
The language of the chalk is not hard to learn, not nearly so hard as
Latin, if you only want to get at the broad features of the story it has to
tell; and I propose that we now set to work to spell that story out
together.
We all know that if we "burn" chalk the result is quicklime. Chalk, in
fact, is a compound of carbonic acid gas, and lime, and when you make
it very hot the carbonic acid flies away and the lime is left. By this
method of procedure we see the lime, but we do not see the carbonic
acid. If, on the other hand, you were to powder a little chalk and drop it
into a good deal of strong vinegar, there would be a great bubbling and
fizzing, and, finally, a clear liquid, in which no sign of chalk would
appear. Here you see the carbonic acid in the bubbles; the lime,
dissolved in the vinegar, vanishes from sight. There are a great many
other ways of 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 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