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, tho 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. [Footnote 60: From a lecture delivered to the workingmen of Norwich, England, during the meeting of the British Association in 1868, now included in "Lay Sermons, Addresses and Reviews." By permission of D. Appleton & Co.] 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, tho it may not seem to help us very far toward 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...