THE CHEMISTRY OF COMBUSTION 81 



The following table gives the constituents of the air out of 

 doors. 



Oxygen, 21 volumes in 100 volumes of dry air. 

 Nitrogen, 78 volumes in 100 volumes of dry air. 

 Water vapor, variable within wide limits. 

 Carbon dioxide, 3 volumes in 10,000 volumes of dry air. 



Since the burning of fuels requires oxygen and produces car- 

 bon dioxide and water vapor, it is evident that combustion will 

 tend to change the proportions of the constituents of the air. 

 Why the oxygen is not all used up and why the volume of the 

 carbon dioxide does not largely increase in amount will be ex- 

 plained in Art. 381. 



92. Study of Oxygen. Since oxygen plays so important a 

 part in all burning and the liberation of energy from fuels, it 

 is well for us to study it. We want it unmixed with other 

 gases. There are many compounds of oxygen which may be 

 made to give up a part or all of their oxygen by heating or 

 by other means. One such compound is potassium chlorate, a 

 white crystalline compound composed of potassium, chlorine, 

 and oxygen. When it is heated, it liberates its oxygen. If it is 

 mixed with manganese dioxide, it liberates oxygen at a much 

 lower temperature. 



Exercise 26. The Preparation and Properties of Oxygen 



Set up the apparatus as shown in Fig. 67. Remove the test tube, 

 fill it about one- third full of potassium chlorate; then place about an 

 equal amount of manganese dioxide in the tube. Clqse the tube with 

 the hand and shake it until the chlorate and the dioxide are well 

 mixed. Replace the stopper in the apparatus, making sure that the 

 stopper fits tightly. Apply gentle heat to the mixture in the tube, 

 beginning at the end which has the stopper, but do not heat the 

 stopper, for it may catch fire and make serious trouble. As the oxy- 

 gen is given off, gradually and carefully extend the heat toward the 

 closed end of the tube. At no time should so much heat be applied 

 that the escaping gas carries the oxygen-producing mixture along 

 with it bodily, thus tending to clog the delivery tube and to prevent 

 the escape of the gas. Fill as many bottles as possible fro^n the gas 

 produced. To do this, submerge the bottle in the water in the pan, 



