116 PRACTICAL LESSONS IN SCIENCE. 



air over red-hot copper, the oxygen combines with the copper 

 forming copper oxide, leaving the nitrogen free. Oxygen enters 

 into many combinations, and compounds of oxygen are gener- 

 ally stable, while nitrogen enters into comparatively few combi- 

 nations and they are generally unstable, breaking up under the 

 least provocation. 



Besides occurring in the air nitrogen is an essential ingredient 

 of animal and vegetable substances, and is an important ingredi- 

 ent in many other interesting compounds. Besides the oxygen 

 and nitrogen the air always contains more or less water vapor, 

 carbon dioxide or carbonic acid, ammonia and small quantities 

 of other gases, and the lower portions are often loaded with 

 various kinds of dust. The air seems to be simply a mechanical 

 mixture of these various gases and not a chemical compound. 

 If the oxygen and nitrogen were in chemical combination we 

 would expect the proportions by weight to be as 14 of nitrogen 

 to 16 of oxygen, or in some multiple of those numbers, but the 

 proportion is as 23.10 of oxygen to 76.90 of nitrogen, and when 

 these gases are mixed in this proportion, under no circumstances 

 is there any generation of heat, light or electricity, as occurs in 

 chemical combinations. As far as known the gases of the air 

 exist everywhere in uniform proportions. The carbon dioxide 

 and ammonia are small but important ingredients of the air, as 

 it is from the air that plants get the greater part of the carbon 

 dioxide and ammonia necessary to their existence. 



Ammonia is a colorless gas, formula NH 3 , composed of one 

 atom of nitrogen and three atoms of hydrogen. Formerly it 

 was made from the horns of harts, hence the name spirits of 

 hartshorn. The chief source of ammonia and ammonium com- 

 pounds is now the ammoniacal liquors of gas works derived as a 

 by-product in the distillation of coal. It is greedily absorbed by 

 water, and a solution of the gas in water is the ammonia of the 

 shops. This solution has a strong alkaline reaction, turning red 

 litmus to blue, neutralizing strong acids, forming salts of 



ammonium. 







In certain compounds a group of nitrogen and hydrogen 



