ixj ON A PIECE OF CHALK. 177 



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. 



The lano'uao-e 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- 

 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 

 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 " car- 

 bonate 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 



