NITROGEN* AND AIR 223 



crust ; so that nitrogen must be considered, in contradistinction to 

 oxygen, as an element which is purely superficial, and does not extend 

 to the depths of the earth. 1 



Nitrogen is liberated in a free state in the decomposition of the 

 nitrogenous organic substances entering into the composition of 

 organisms for instance, on their combustion. All organic substances 

 burn when heated to redness with oxygen (or substances readily yielding 

 it, such as oxide of copper) ; the oxygen combines with the carbon, 

 sulphur, and hydrogen, and the nitrogen is evolved in a free state, 

 because at a high temperature it does not form any stable compound, 

 but remains free. Carbonic anhydride and water are formed from the 

 carbon and hydrogen respectively, and therefore to obtain pure 

 nitrogen it is necessary to remove the carbonic anhydride from the 

 gaseous products obtained. This may be done very easily by the action 

 of alkalis 4r instance, caustic soda. The amount of nitrogen in 

 organic substances is determined by a method founded on this. 



It is also very easy to obtain nitrogen from air, because oxygen 

 combines with many substances. Either phosphorus or metallic copper 

 are usually employed for removing the oxygen from air, but, naturally, 

 a number of other substances may also be used. If a small saucer 011 

 which a piece of phosphorus is laid be placed on a cork floating on water, 

 and the phosphorus be lighted, and the whole be covered with a glass 

 bell jar, then the air under the jar will be deprived of its oxygen, and 

 nitrogen only will remain, owing to which, on cooling the water will 

 rise to a certain extent in the bell jar. The same object (procuring 

 nitrogen from air) is attained much more conveniently and perfectly 

 when air is passed through a red-hot tube containing copper filings. 

 At a red heat, metallic copper combines with oxygen and gives a black- 

 powder of copper oxide. If the layer of copper be sufficiently long and 

 the current of air slow, all the oxygen of the air will be absorbed, and 

 nitrogen alone will pass from the tube. 2 



1 The reason why there are no other nitrogenous substances within the earth's mass 

 beyond those which have come there with the remains of organisms, and from the air 

 with rain-water, must be looked for in two circumstances. In the first place, in the in- 

 stability of many nitrogenous compounds, which are liable to break up with the forma- 

 tion of gaseous nitrogen ; and in the second place in the fact that the salts of nitric acid, 

 forming the product of the action of air on many nitrogenous and especially organic 

 compounds, are very soluble in water, and on penetrating into the depths of the earth 

 (with water) give up their oxygen. The result of the changes of the nitrogenous organic 

 substances which fall into the earth is without doubt frequently, if not always, the forma- 

 tion of gaseous nitrogen. Thus the gas evolved from coal always contains much nitrogen 

 (together with marsh gas, carbonic anhydride, and other gases). 



2 Copper (best as shavings, which present a large surface) absorbs oxygen, forming 

 CuO, at the ordinary temperature in the presence of solutions of acids, or, better still, in 



