PLANT FOOD AND PLANT GROWTH 27 



bon. It differs in some manner from the carbon in coal or char- 

 coal, probably because of a different number, or a different arrange- 

 ment, of the atoms in the molecule. 



The diamond is very pure carbon in crystallized form. Both the 

 diamond and ordinary carbon may be converted into graphite, 

 and small diamonds have been made artificially from ordinary 

 carbon. 



Free carbon in any form burns with oxygen to form carbon dioxid 

 (CO 2 ), but with graphite and the diamond the reaction occurs 

 only at higher temperatures than are necessary with ordinary 

 carbon. 



Carbon in the free state has never been liquefied, but it has 

 been volatilized at the temperature of 3500 Centigrade (6332 

 Fahrenheit). On the other hand, many of the compounds with 

 carbon are liquids or gases at ordinary temperatures. Examples 

 of liquid compounds are carbon disulfid (CS 2 ), benzene (C 6 H 6 ), and 

 many other hydrocarbons contained in petroleum; while carbon 

 dioxid (CO 2 ) , methane (CH^) , and acetylene (C^Hg) are well-known 

 gases containing carbon. 



In the solid form, carbon occurs not only in the free state, 

 but also in compounds, of which two very important groups are 

 the carbonates and the carbohydrates. Marble, limestone, chalk, 

 and marl are different forms of calcium carbonate (CaCO 3 ), one 

 of the few compounds with a molecular weight of 100, and thus 

 containing 12 per cent of carbon, 40 per cent of calcium, and 48 

 per cent of oxygen. Of these four materials marble is nearly pure 

 calcium carbonate, while the others may be nearly pure or very 

 impure. When calcium carbonate is heated, two bonds are broken 

 and then joined in another way, so that the one compound (CaCO 3 ) 

 is broken into two (CaO) and (CO 2 ), thus: 



=O becomes Ca: = O, or 



CaCO 8 = CaO + CO 2 . 



The calcium oxid (CaO) is burned lime, or quicklime, which 

 remains in the kiln; while carbon dioxid (CO 2 ) is a gas which passes 

 off into the air. It is easy to see that only 56 pounds of quicklime 

 could be made from 100 pounds of pure limestone. 



