THE CHARACTERISTICS OF LIVING THINGS 13 



burn; that is, combine with oxygen to form carbon dioxide 

 (C02)} and only mineral matter or ashes remain. Another 

 example is the familiar fact that when wood (or any plant 

 matter) is burned there is formed a mass of red-hot coals 

 or embers, which if allowed to cool quickly become charcoal ; 

 but if the coals remain heated, they soon change to ashes, 

 because the carbon is burned. Obviously, the charcoal 

 obtained by heating ordinary animal or plant substances 

 is a mixture of combustible carbon and incombustible 

 mineral matter or ashes. Charcoal made by heating sugar 

 leaves no ashes, because it .is pure carbon, which in burning 

 combines with oxygen and forms carbon dioxide. 



A chemist could prove by careful analysis* that the ashes 

 from either plant or animal substances are a mixture of 

 several compounds containing the elements calcium, sulphur, 

 iron, potassium, sodium, and several other elements. These 

 are all found in the soils, and in the water of the lakes, rivers, 

 and the sea. 



16. Summarizing our inquiry into the composition of 

 living things, we have found that water, carbon, gases, and 

 mineral matters may be obtained by analysis of living 

 things. All these substances are also found in lifeless or 

 inorganic matter. It is evident then that chemical composi- 

 tion alone does not enable us to distinguish between living and 

 lifeless matter. And yet in most cases we have no difficulty 

 in deciding whether a certain thing is dead or alive. How do 

 we know? The answer will be found in the life-activities 

 discussed in the next lesson. 



II. LIFE-ACTIVITIES 



17. Distinguishing between Living and Lifeless. The 



conclusion reached in the preceding lesson was that all the 

 materials entering into the composition of living things 

 (animals and plants) are also found in inorganic things, 



