154 LAY SERMONS, ESSAYS, AND REVIEWS. [ix. 



We all know that if we &quot; burn &quot; 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 &quot; carbonate of lime.&quot; 



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 lime 

 stones 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 dis 

 tinctly laminated mineral substance, and nothing more. 



