NITROGEN. 165 



the evolution of gas may otherwise become too rapid. Collect the gas, and 

 notice its properties mentioned below. 



Properties. Nitrogen is a colorless, inodorous, tasteless gas; 

 which, at a temperature of 130 C. (202 F.) and a pressure of 

 280 atmospheres, may be condensed to a colorless liquid. It is neither, 

 like oxygen, a supporter of combustion, nor, like hydrogen, a com- 

 bustible substance ; in fact, nitrogen is distinguished by having very 

 little affinity for any other element, and it scarcely enters directly into 

 combination with any substance. Nitrogen is not poisonous, yet not 

 being a supporter of combustion it cannot sustain animal life. 

 Nitrogen is trivalent in some compounds, quinquivalent in others. 



Atmospheric air is a mixture of about four-fifths of nitrogen and 

 one-fifth of oxygen, with small quantities of aqueous vapor, argon, 

 carbon dioxide, and ammonia, containing frequently also traces of 

 nitrous or nitric acid, and occasionally hydrogen sulphide, sulphur 

 dioxide, and hydrocarbons. Besides these gases there are always 

 suspended in the air solid particles of dust and very minute cells of 

 either animal or vegetable origin. 



100 volumes of atmospheric air contain of 



Oxygen 20.60 volumes. 



Nitrogen 77.16 " 



Argon 0.80 volume. 



Carbon dioxide . . . . 0.03-0.04 " 

 Aqueous vapor .... 0.5 -1.40 " 



Ammonia } . traces. 



Nitric acid -* 



Omitting all minor constituents, the composition of air by volume 

 is about 79 per cent, of nitrogen and 21 per cent, of oxygen, corre- 

 sponding in weight to 77 per cent, of nitrogen and 23 per cent, of 

 oxygen. 



That atmospheric air is a mixture and not a compound of oxygen and 

 nitrogen is shown by the facts that the composition is not absolutely constant, 

 that the two elements may be mixed in the proper quantities without showing 

 the least evidence that chemical change has taken place, and that pure water 

 absorbs from air the two elements in quantities different from those in which 

 they occur in air. 



Humidity, specifically called relative humidity, designates the amount 

 of aqueous vapor in the atmosphere, compared with that which is 

 required to saturate it at the respective temperature. When the air 

 is completely saturated the humidity is expressed at 100; if perfectly 



