xxi CARBON AND SOME OF ITS COMPOUNDS _>).-, 



quently found in coal mines, and is known as fire-damp. It is 

 an inflammable gas, and hence its presence is a source of danger 

 in collieries, and many disastrous explosions have been caused 

 by its ignition. 1 Water and carbon dioxide (the after-damp) are 

 produced by its combustion, and the effects of the products in 

 causing suffocation are often more fatal than those of the explo- 

 sion itself, while the presence of small quantities of carbon 

 monoxide adds greatly to the loss of life. 



In the laboratory, methane is usually prepared by heating a 

 mixture of sodium acetate and caustic soda. It is colourless, 

 odourless, insoluble in water, and burns with feebly luminous 

 flame. 



Ordinary coal gas consists of a mixture of various hydrocarbons, 

 chiefly marsh gas, together with hydrogen. By its combustion 

 carbon dioxide and water are formed. If sufficient air be intro- 

 duced into the flame it has been seen (Chap. XV.) that this 

 combustion proceeds without the separation of any carbon. If 

 air be not introduced into the gas some of the hydrocarbons 

 become decomposed the hydrogen burning, to form water, and 

 the carbon being separated. This carbon is raised to incan- 

 descence and afterwards burned, when there is sufficient air. 



CHIEF POINTS OF CHAPTER XXI. 



Carbon is present in all living matter. When organic substances 

 are moderately heated, a black residue largely composed of carbon is 

 left. If the temperature is raised, the carbon burns away and an 

 almost colourless ash is left. 



Carbon exists in several allotropic forms. Two of these, diamond 

 and graphite, are crystalline. Non-crystalline or amorphous carbon 

 is known in varying degrees of purity as coke, gas carbon, wood 

 charcoal and animal charcoal. 



Lime, which is used in making lime-water, is a white solid which 

 is unchanged on heating. But when heated intensely it glows and 

 emits a brilliant white light. It dissolves in hydrochloric acid to 

 form calcium chloride. Wet lime turns a red litmus paper blue. 

 Lime is obtained from chalk and limestone by strongly heating them. 

 When freshly made it is called quick lime which combines with water, 

 evolving much heat and becoming converted into xlaktd lime. 



When Chalk or Limestone is heated it loses about 44 per cent, of 

 its weight. This is due to the loss of carbon dioxide. The same 

 gas is evolved when either of the substances is acted upon by hydro- 



i Finely divided coal dust has also been shown to be capable of giving rise to 

 explosions, and it is possible that many regarded as due to fire-damp were in reality 

 due to coal dust, which appears to produce more violent explosions than the 



