I ON A PIECE OF CHALK 5 



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 Kme 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 quick- 

 lime. 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 



