On a Piece of Chalk 77 



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 pro- 5 

 pose 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 10 

 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 15 

 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 show- 

 ing that chalk is essentially nothing but carbonic acid and 20 

 quicklime. Chemists enunciate the result of all the ex- 

 periments 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 25 

 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 30 

 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 the teakettle 

 is carbonate of lime; and, for anything chemistry tells us, 



