Fire and Heat 75 



he named it nitrogen. This is the name by which it is now 

 generally known. 



Properties and uses. Nitrogen constitutes about four 

 fifths of the total volume of the atmosphere. It does not sup- 

 port combustion, nor does it support animal life if confined in 

 it. But it is indispensable to animal and plant life, and forms 

 an essential part of their substance and food. Although they 

 live in an atmosphere containing nitrogen in such quantity, 

 animals do not directly absorb and assimilate it. They depend 

 for their supply upon plants, which in turn depend upon com- 

 pounds of nitrogen in the soil, as nitrates (saltpeter) and am- 

 monia. Some plants, as beans, peas, and alfalfa, known as 

 legumes, are important for the quantities of nitrogen they 

 derive from the air through the activities of bacteria, which are 

 plants of microscopic size, living in the nodules of their roots 

 (page 573). These minute plants absorb nitrogen from the air 

 in the soil, use it as food, and throw off in waste products such 

 compounds of nitrogen as the legumes need for food. 



A summary of the points already considered will be helpful 

 in the study of other problems in the process of combustion. 



First. It appears that lead, and some other metals, as iron 

 and copper, are changed when heated in the air into substances 

 that weigh more than the metals from which they are produced. 



Second. It appears that the air or some part of it is necessary 

 to this change in the metals ; if the air supply is shut off, the 

 change is not effected, no matter how much heat is added. 



Third. The experiments with a candle show that air is neces- 

 sary to combustion, for when shut off entirely, the candle ceases 

 to burn. 



Fourth. The proportion of the air concerned in combustion 

 is found to be about one fifth. The approximate four fifths 

 left, called nitrogen, does not support combustion, but extin- 

 guishes flame and fire. 



From these facts it may be inferred that about one fifth of 

 the air, the active part, may combine in combustion with other 



