104 The Realm of Nature CHAP. 



When an electric discharge passes through air, a small 

 quantity of nitrogen is always caused to combine with 

 hydrogen and oxygen to form salts of ammonia. 



153. Oxygen was originally known as Vital Air, for it 

 is the ingredient of the atmosphere which sustains life, and 

 by its ready combination with other elements supports com- 

 bustion. The oxygen of the atmosphere is a great store of 

 potential energy when taken into account with the uncom- 

 bined substances in the Earth ( 56, 44). Oxygen in the 

 pure state combines very energetically with carbon, hydrogen, 

 and almost all the other elements ; but when it is diluted 

 with four times its volume of inert nitrogen, combustion is 

 slower and quieter, although the same amount of energy is 

 ultimately set free as would be the case if no nitrogen 

 were present. Under the influence of electric dis- 

 charge, and of the growth of some trees, oxygen is partly 

 changed into a condensed form called ozone, and partly 

 combined with water to form peroxide of hydrogen. These 

 substances exist in the air in very minute proportions, but 

 when either of them is present it is believed to increase 

 the healthfulness of a neighbourhood. Oxygen in small 

 quantities is a colourless and transparent gas, but in the 

 atmosphere it absorbs a good deal of sunlight, giving 

 broad black bands in the red part of the spectrum. The 

 blue tint of the sky may be due in part to the true colour 

 of oxygen. The proportion of oxygen in the free air of the 

 country is a very little greater than in crowded towns. 



154. Carbonic Acid, though present in small amount, 

 has an important part to play in the economy of the atmo- 

 sphere. Green plants in sunlight absorb it, decompose it, 

 retain the carbon to build up in their own substance, and 

 breathe back the oxygen into the air. Animals and also 

 plants ( 399, 400) breathe in air, absorb the oxygen, which 

 is ultimately combined with carbon and breathed out as car- 

 bonic acid. There is a large proportion of carbon in coal, oil, 

 wood, fat, and almost all combustible substances, which thus 

 produce carbonic acid as the principal result of their union 

 with oxygen. The amount present in the atmosphere 

 varies considerably ; 3 parts in 10,000 is the proportion in 



