140 CHEMISTRY. 



light it. The same effect will be produced if you use bitu- 

 minous coal, or oil, or tallow. The explanation is this : The 

 heat sets free the hydrogen and the carbon in the form of 

 carburetted hydrogen, just as it does in the case of the can- 

 dle. The gas that we bum in our houses is made from coal 

 substantially in the way indicated in the above experiment. 

 It is made in large iron retorts. It has many impurities 

 mingled with it, which are removed by certain chemical 

 processes before the gas is distributed in the pipes. Gas 

 which is made from oil is purer, and gives a stronger light, 

 than that which is made from coal. 



179. Results of Combustion. The results of combustion 

 are of two kinds those which pass off in the form of gas or 

 vapor, and those which are deposited in a solid form. When 

 a gas burns, the results are all aeriform. The vapor, how- 

 ever, that is formed by the burning of hydrogen may be 

 condensed into a liquid form, or even be made solid in the 

 form of snow, hail, or ice. The results of the combustion 

 of some solids are wholly aeriform, as, for example, in the 

 case of the candle, whether it be tallow, wax, or stearine. 

 The results in the case of some solids, on the other hand, 

 are wholly solid. When a metal, as iron, burns, not a par- 

 ticle of it passes off as gas, but it all falls as a solid oxide. 

 When wood or coal burns, the results are both aeriform and 

 solid, the latter being in the form of ashes. The ashes of 

 different substances vary much both in character and in 

 quantity. When wood is burned, out of every 100 pounds 

 about 2 are ashes, while 98 pounds fly off into the air by 

 uniting with oxygen to form carbonic anhydride and water. 

 Of what ashes are composed we shall speak particularly in 

 another place. 



180. Expedients for Increasing Combustion. When any 

 thing is burning, the greater the supply of oxygen the 

 more brisk and perfect will be the combustion. If we blow 



