8 



Oxygen (0) at ordinary temperatures is a colorless, odor- 

 less gas. It is the most abundant of the elements, forming as 

 it does one fifth of the air, eight ninths of the water, and 

 about one half of the rocks and soil. It combines with a great 

 number of other elements, forming oxides, and is necessary 

 for all ordinary combustion and for the respiration of animals 

 and plants. With hydrogen and carbon it forms the carbo- 

 hydrates, of which the greater part of the plant body consists. 

 Iron and other metals burn in pure oxygen. 



Hydrogen (H) is a colorless, tasteless, and odorless gas, and 

 is the lightest substance known. It does not occur free, but 

 is most abundant combined with oxygen in the form of water. 

 It burns with a blue flame and is a constituent of all acids. 

 Hydrogen and oxygen, if mixed at ordinary temperatures, 

 will remain a mere mixture, but if heated or ignited they 

 combine with a violent explosion and form water. 



Nitrogen (N) is a heavy, inert, colorless, tasteless, odorless 

 gas that occurs free in the air, of which it forms about four 

 fifths. It is extremely inert, does not support combustion, nor 

 readily enter into combination with other elements. It is four- 

 teen times as heavy as hydrogen, and without it the air would 

 have little weight, birds could not fly, and the sails of ships 

 and windmills would be practically useless. Nitrogen also 

 serves to dilute the oxygen in the air; otherwise oxidation 

 in our bodies would proceed so rapidly as to be harmful. 

 Ammonia gas is largely nitrogen, and gunpoj^ler, guncot- 

 ton, and nitroglycerin owe their effectivenes^JRhe fact that 

 these are unstable compounds containing this element. Nitro- 

 gen does not exist in the mineral matter of the earth, though 

 the soil is the source of most of this element used by plants. 

 Here it- exists largely in the form of nitrates derived from the 

 decaying organic matter. Nitrogen is necessary to the formation 

 of proteins and is an essential constituent of the protoplasm of 

 all animals and plants. 



