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 f