THE ELEMENTS OF PLANT FOOD 29 



with oxygen and calcium or magnesium, it appears as 

 limestone, marble, and dolomite. 



Notes. Diamonds are usually found in the form of rounded peb- 

 bles. These are cut into desirable forms by pressing the stone against 

 a revolving wheel covered with a mixture of diamond dust and oil. 

 Carbon in this form is the hardest known substance except carborun- 

 dum. This quality, combined with its brilliance and its rarity, makes 

 it the most precious of gems. 



Graphite is used in making lead pencils, stove polish, electric light 

 carbons, axle grease, and crucibles. It is often called plumbago. 



Charcoal. Charcoal is made by heating wood with 

 a small supply of O. It is made in large quanti- 

 ties by piling up sticks of wood in a heap around 

 a central flue, covering the wood with earth, leaving 

 a hole at the top for a flue and small holes at the bottom 

 to admit a quantity of O to commence the combustion. 

 After the wood is kindled, the holes at the bottom may 

 be closed so as to regulate the supply of O entering the 

 kiln. When the process is complete, the charcoal is 

 in the form of the sticks that were put in, but its char- 

 acter and its color are considerably different. It is 

 very porous and has the power of absorbing gases to a re- 

 markable degree. Beechwood charcoal has been known 

 to absorb 170 times its own volume of dry ammonia. 



It is this property of absorption that gives charcoal 

 its value as a purifier and deodorizer. Oxygen being 

 held in the pores of the charcoal in a condensed form, 

 any offensive gas absorbed by the charcoal is brought 

 into contact with its condensed oxygen, whereupon 

 the impurity is oxidized. This process may be con- 

 tinued for a considerable time. When the charcoal 

 gets full of the impurities, it may be restored to its 

 former condition by reburning. 



