ho 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 slic