ON A PIECE OF CHALK 65 



We all know that if we "burn" chalk the result is quick- 

 lime. 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, van- 

 ishes from sight. There are a great many other ways of 

 showing that chalk is essentially nothing but carbonic acid 

 and quick-lime. Chemists enunciate the result of all the 

 experiments which prove this, by stating that chalk is al- 

 most 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 car- 

 bonate 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 i^rind a slice of chalk down so thin that you can see 

 through it — until it is thin enough, in fact, to be examined 



