12 ELEMENTARY BIOLOGY 



the fire as it was before, although it now exists in different 

 forms and in different chemical combinations. 



18. Air and fire. We know from experience that rnost 

 substances will not burn unless there is a supply of air. The 

 dependence of the fire upon the air may be due to the fact 

 that burning uses up air (or something in the air) just as it 

 uses up the combustible. Or it may be that in the course of 

 the burning the fuel is changed into a new substance (probably 

 invisible) that interferes with further burning unless it is car- 

 ried off in the air. We can find out by means of a simple 

 experiment that the fire takes from the air certain materials, 

 and that it also discharges into the air certain other substances. 

 We should also feel confident that the " new substances " are 

 merely rearrangements of the particles of the original fuel and 

 the stuff taken from the air. 



19. Burning a synthesis. We have already learned that 

 compounds are substances consisting of two or more elementr-, 

 and that the formation of compounds is called synthesis, or 

 "putting together." Experiments can be made to show that 

 although the burning process may result in breaking tip com- 

 pounds — the compounds of the tallow, for example — it ah o 

 results in the formation of new compounds. Accordingly the 

 prodjicts of combustion represent more matter than is present 

 in the fuel. The ashes and smoke and the invisible substances 

 resulting from a fire together weigh as much as the original 

 combustible plus the amount of material taken from the air. 



20. The gases in the air. The ordinary air is a mixture 

 of gases. Three of these are well known. In addition to the 

 dust and the water vapor, and some gases which are not 

 easily reached through our ordinary experiences and laboratory 

 methods, the atmosphere may be said to have approximately 

 this composition : 



Nitrogen about 79% 



Oxygen about 20% 



Carbon dioxid less than -^-5^0 



