164 JUNIOR GRADE SCIENCE 



and black, and so soft that it will mark paper. It is really a crystalline 

 form of carbon, although good crystals are not very common. It 

 occurs naturally in mines, chiefly in California, and was formerly largely 

 obtained from Cumberland. Besides its use for lead pencils, it is also 

 used as a lubricant. 



Amorphous varieties. Other forms of more or less pure carbon in 

 an uncrystallised or amorphous state are coke, and gas carbon, which 

 result from the heating of coal ; lampblack, which is the carbon de- 

 posited by oils, etc., burning in an insufficient supply of oxygen, and 

 wood charcoal, obtained by heating wood in closed retorts or in stacks 

 under earth. 



Charcoal has the power of absorbing colouring matter, and on the 

 latter account it is used for decolorising solutions coloured by organic 

 matter. Animal charcoal is really a misleading term, as the quantity 

 of carbon present is usually only about 10 or 12 per cent., the remaining 

 being chiefly bone ash. 



Both animal and wood charcoal are very porous substances, and 

 they have the power of absorbing gases to a large extent. Wood 

 charcoal is used considerably on the Continent for heating purposes. 

 Both kinds are useful in destroying noxious vapours. 



Coal contains large quantities of carbon, especially the harder or 

 anthracite coals, where the quantity may reach 94 per cent., being, 

 however, only about 65 in brown coal or lignite. 



Whenever any of the kinds of carbon burn freely in a good supply 

 of air or oxygen, carbon dioxide is the compound formed, thus affording 

 evidence that the three varieties are only allotropic forms (see p. 106) 

 of the simple substance carbon. 



80. CARBON DIOXIDE PRODUCED BY BURNING 

 AND BREATHING. 



i. When carbon is burnt carbon dioxide is formed. (a) Heat strongly 

 a piece of charcoal in a closed hard glass test-tube and show that without 

 air it does not burn. 



(6) Suspend a piece of glowing charcoal in a jar containing lime-water, 

 Shake up and show that the lime-water is turned milky. Carbon dioxide 

 can alwuys be distinguished by this action upon lime-water, for it is the 

 only common, colourless, inodorous gas which turns lime-water milky. 



ii. Carbon dioxide is produced by a burning candle. (a) Burn a 

 candle or taper in a clean dry glass jar (Fig. 110). After the flame has 

 been extinguished, withdraw the taper. Pour a little freshly made lime- 

 water into the jar and shake it up. Notice the milkiness of the lime-water. 



(6) Cut a long thin chip of wood, hold it in the flame of a laboratory 

 burner until it burns brightly, then thrust it into a cylinder, the bottom 



