THE CHEMISTRY OF COMBUSTION 85 



materials. Wood and many other fuel materials decay at 

 ordinary temperatures. They are really undergoing what is 

 called SLOW OXIDATION. At higher temperatures the rate 

 is much more rapid. For each substance that burns, there is 

 a temperature at which it burns rapidly in the air. This 

 temperature is called the KINDLING TEMPERATURE. When a 

 fire is kindled, the aim is to heat the fuel to be burned by burn- 

 ing the kindling, the kindling having a lower kindling tem- 

 perature than the fuel. Thus sulphur is used to kindle the iron ; 

 kerosene, paper, or shavings are used to kindle the coal or 

 wood because these kindling agents have lower kindling tem- 

 peratures than the fuel which is to be burned. When the 

 fuel is once ignited, it then liberates heat fast enough to keep 

 itself at the kindling temperature and so the fire continues 

 as long as the concentration of the oxygen is sufficient. Wet 

 fuels often require so much heat to dry them that the burning 

 portion can not supply heat enough to dry the unburned por- 

 tion and to raise it to the kindling temperature. Hence the 

 fire goes out. Water is thrown on a fire to cool the burning 

 material below its kindling temperature. Also the steam 

 arising from the water serves to dilute the air and thus to lower 

 the concentration of the oxygen so that the fire goes out. 



96. Study of Hydrogen. Hydrogen is the second of the 

 fuel elements. It is to be prepared and studied. 



Exercise 26. The Preparation and Properties of Hydrogen 



Set up the apparatus as shown in Fig. 68. Cover the bottom of the 

 flask with granular zinc, replace the stopper and see that all joints of 

 the apparatus are tight. Prepare to collect the gas, hydrogen, just 

 as oxygen was collected. Do not have any flames closer than 4 ft. 

 from the hydrogen generator. Pour water down the thistle tube until 

 the bottom of the flask is well covered and the lower end of the 

 thistle tube is submerged; then pour in concentrated hydrochloric 

 acid slowly until the action between the zinc and the acid is rapid. 

 Do not spill the acid on the hands, clothing, or desk. Allow the 

 hydrogen to escape from the delivery tube for about one minute; 

 then collect the gas in bottles as in the last exercise. Fill one bottle 

 half full of the gas. Finally remove the delivery tube from the water, 



