74 NON-METALS AND THEIR COMBINATIONS. 



and it scarcely enters directly into combination with any sub- 

 stance. Nitrogen is not poisonous, yet not being a supporter of 

 combustion it cannot sustain animal life. Nitrogen is a trivalent 

 element, combining with three atoms of hydrogen. 



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

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

 carbon dioxide, and ammonia, containing frequently also traces 

 of nitric acid and carburetted hydrogen, occasionally sulphu- 

 retted hydrogen and sulphur dioxide. Besides these gases there 

 are frequently 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, 20.61 volumes. 



Nitrogen, 77.95 " 



Carbon dioxide, 0.04 " 



Aqueous vapor, 1.40 " 



Ammonia, ) 



. ' > traces. 



Nitric acid, J 



An analysis of air may be made by the following method : 

 A graduated glass tube, containing a measured volume of air, 

 is placed with the open end downward into a dish containing 

 mercury. A small piece of phosphorus is then introduced and 

 allowed to remain in contact with the air for several hours, 

 when it gradually combines with the oxygen. The remaining 

 volume of air is chiefly nitrogen, the loss in volume represents 

 oxygen. 



For the determination of carbon dioxide and water, a measured 

 volume of air is passed through two U-shaped glass tubes. One 

 of these tubes has previously been filled with pieces of calcium 

 chloride, the other tube with pieces of potassium hydrate, and 

 both tubes have been weighed separately. In passing the 

 measured air through these tubes the first one will retain all the 

 moisture, the second one all the carbon dioxide ; the increase in 

 weight of the tubes at the end of the operation will give the 

 amounts of the two constituents. 



That oxygen is found in the atmosphere in a free state is 

 explained by the fact that all elements having affinity for oxygen 

 have entered into combination with it, whilst the excess is left 

 uncombined. Nitrogen is found uncombined, because it has so 

 little affinity for other elements. 



