108 PRACTICAL LESSONS IN SCIENCE. 



The most marked characteristic of oxygen is its prompt and 

 vigorous support of combustion. But combustion, whether slow 

 or rapid, is a chemical process, and when we remember the wide 

 dispersion of oxygen, being found in chemical combination every- 

 where, we judge that it must be an extremely active chemical 

 agent, and yet at ordinary temperatures its action is by no 

 means vigorous. But if substances are heated to a greater or 

 less degree before being brought into contact with oxygen, it 

 unites promptly with most of the elements, making an endless 

 variety of compounds. 



Sulphur burns in the air; it burns with greater vigor in oxy- 

 gen than in air. Sulphur has but a slight odor, oxygen has 

 none; and yet, after burning sulphur, whether in air or oxygen, 

 a pungent, suffocating odor is present, the odorless sulphur and 

 oxygen have disappeared and something with a pungent odor 

 remains. If a piece of charcoal, nearly pure carbon, be ignited 

 and thrust into a jar of oxygen it burns fiercely for a few moments, 

 then gradually ceases. Some of the charcoal has disappeared, 

 the oxygen is gone and in their place is a gas in which the char- 

 coal will not burn and which quickly extinguishes a lighted candle 

 thrust into it. 



In the first case, the oxygen combined with the sulphur form- 

 ing a gas whose molecule contains one atom of sulphur and two 

 atoms of oxygen, its chemical symbol is S0 2 and it is called sul- 

 phur dioxide. The chemical equation is S + 2 = S0 2 . In the 

 other case, oxygen united with carbon forming a compound 

 whose molecule contains one atom of carbon and two atoms of 

 oxygen called carbon dioxide. The equation is C + 2 = C0 2 . If 

 we had used phosphorus, only a slight elevation of temperature 

 would have been necessary to start the chemical action. More 

 was needed in the case of sulphur, more still in the case of carbon ; 

 but when started in each case it continued vigorously till the 

 oxygen was all gone. 



As soon as one atom of sulphur or carbon combined with 

 two atoms of oxygen their activity ceased, their attractive 

 powers had been satisfied. As these atoms rushed together 



