254 NATURE STUDY 



STONES 



It is rather difficult to say what stones will be brought 

 in by the children. But the teacher will certainly need 

 to know something about limestones, sandstones, and 

 slates. 



The safest plan with stones, is to take the specimen 

 to a specialist, and then, with the name as a clew and the 

 specimen in your hand, learn all that you can about it 

 both from books and people. 



Limestones : 



The best-known limestone is marble, which is found in 

 nearly every color, — white, yellow, red, blue, black, — but 

 is most familiar to us as the white marble so extensively 

 used for entire buildings, doorsteps, mantels, table tops, 

 and tombstones. 



A piece of marble tested Avith the knife and with 

 dilute acid, strong vinegar, or weak hydrochloric acid 

 will be proved to be a soft rock made up of carbonate of 

 lime. The acid combines with the lime, setting free the 

 carbon dioxide, which causes the effervescence. 



The carbon dioxide, thus produced from marble and 

 acid, is identical both in nature and method of manufact- 

 ure with the carbon dioxide which gives the agreeable 

 pungency to soda water. 



It is identical, also, with the carbon dioxide exhaled 

 by plants and animals, which, while it is not an active 

 poison, produces death by suffocation, since its presence 

 means the absence of oxygen. 



In this connection read the story of the Black Hole of 

 Calcutta so graphically told by Macaulay. 



