NITRO-BENZOL 



NITROGEN 



509 



compounds lieing converted, in the presence of lime 

 or potash, into the corresponding nitrates of lime 

 or potash ; and from these nitric compounds plants 

 denve the most or all of their nitrogen, although 

 some experimentalists maintain that some plants 

 obtain part at least of their nitrogen from am- 

 monia. See Lawes and Gilbert, Proc. Roy. Soc., 

 vol. xlvii. ; Marshall Ward, Phil. Trans., B. 1887, 

 vol. clxxviii. 



Xltro-benzql, CnHjNO.,, is a yellow, oily fluid, 

 of sp. gr. 1"2, which may be distilled without decom- 

 position, and boils at 415 (213 ('.). It has a 

 sweet taste, is insoluble in water, but dissolves 

 freely in alcohol and ether. Its odour is very 

 similar to that of oil of bitter almonds, which has 

 led to its use in perfumery under the name of 

 Essence of Mirbune. It is obtained by treating 

 benzol, C,H 8 , with warm fuming nitric acid, when 

 one atom of hydrogen is replaced by the group of 

 atoms NO, so that the benzol, C B HjH, oecomes 

 converted into nitro-benzol, C,H s NOj. The vapour 

 of nitro-benzol when inhaled produces, after from 

 6 to 12 hours, coma and sometimes death. The 

 fact that it- action is so long delayed seems to 

 indicate that its poisonous action is due to some 

 product of its decomposition rather than to itself. 

 In cases of poisoning artificial respiration and 

 powerful stimulants should be resorted to. 



Nitrogen (Fr. azote; sym. N; atom, wt 14) 

 is an elementary gas, which in the free state forms 

 nearly four-fifths by volume of our atmosphere. 

 In roinl. ination with other elements nitrogen is 

 a necessary constituent of every organised body, 

 and it forms a very large number of most important 

 cni|M>unds. For its relation to that gas, see 

 ARUOX. Its name was given to it shortly after 

 it had been proved to be an essential constituent 

 of nitre or potassium nitrate. Its presence in 

 the atmosphere was discovered in 1772 by Kuther- 

 ford, at tliat time a professor of botany in the 

 university of Edinburgh. It was more particu- 

 larly investigated soon after by Priestley, Scheele, 

 and Lavoisier. It is a colourless, tasteless, inodor- 

 ous gas, and was formerly regarded as permanent 

 or incondensable ; but it can lie reduced to the 

 liquid state by the application of sufficient pressure 

 after it has lieen cooled to a very low temperature. 

 The Critical Temperature (q.v. ) of nitrogen, or the 

 point of temperature alx>ve which it cannot be con- 

 densed into a liquid by the application of any pres- 

 sure however great, is - 146' C. When cooled to 

 this temperature a pressure of thirty-five atmo- 

 spheres must lie applied in order to liquefy it. 



Nitrogen is fourteen times as heavy as hydrogen, 

 and is slightly lighter, bulk for bulk, than atmo- 

 spheric air, its sp. gr. being 01)713, compared with 

 air = 1. It is but slightly soluble in water, one 

 hundred volumes of water at ordinary tempera- 

 tures dissolving only about one and a half volumes 

 of it. 



Whilst nitrogen is a constituent of all plant and 

 animal organisms and of many important com- 

 pounds, it is, in the free state, rather inert towards 

 other elements, and does not readily enter into 

 direct combination with them. It is not combust- 

 ible, nor does it act in the atmosphere as asupporter 

 of combustion, a lighted taper plunged into a jar 

 containing nitrogen being at once extinguished. 

 It is almost unnecessary to say that nitrogen is 

 not poisonous, since it is breathed freely along with 

 oxygen by all animals; but it cannot support animal 

 life, and an animal placed in it will die from suffo- 

 cation for want of tne oxygen necessary for respira- 

 tion. Its function in the atmosphere seems to be 

 mainly that of diluting the oxygen with which it 

 is there associated. Although nitrogen forms about 

 79'1 per cent, of the total volume and 77 per cent. 



of the total weight of the atmosphere, the free 

 gas does not appear to play any important part 

 in supplying nitrogen for the construction of the 

 tissues of plants. Vegetable physiologists are at 

 present actively inquiring into this question ; and 

 it appears to be definitely established that plants 

 cannot directly absorb from the atmosphere the 

 nitrogen which they require. It is found, however, 

 that certain classes of plants when they are grown 

 in a soil which is practically free from nitrogen 

 compounds to start with do take up nitrogen. One 

 explanation of this fact is that free atmospheric 

 nitrogen becomes converted in small quantity into 

 nitrogen compounds in the soil through the influ- 

 ence of micro-organisms present there. Another 

 explanation attributes the fixation of nitrogen to 

 micro organisms existing in, or on the surface of, 

 the plant. On the other hand, it has been con- 

 clusively shown that certain plants are unable to 

 exist unless appropriate compounds of nitrogen are 

 supplied to them in the soil in which they grow. 

 The main supply of the nitrogen of plants is cer- 

 tainly drawn from nitrogen compounds existing^ in 

 or artificially supplied to the soil. The most im- 

 portant of such nitrogen compounds are nitrates, 

 which are present in every fertile soil. These may 

 be produced by the decay in the soil of nitrogenous 

 animal or vegetable matters existing in it or sup- 

 plied to it as manure ; or they may oe added to it 

 ready formed as, for instance, in the form of 

 sodium nitrate, which is frequently applied as a 

 manure to soils poor in nitrates. 



Many attempts have l>een made to discover 

 methods by which the free nitrogen of the atmo- 

 sphere could be converted on- a manufacturing scale 

 into nitrogen compounds, but as yet no workable 

 and economical method has been discovered. There 

 are numerous methods for preparing pure or nearly 

 pure nitrogen. One method by which it is obtained 

 very nearly pure is to remove the oxygen as com- 

 pletely as possible from atmospheric air. This can 

 best be done by passing a current of air slowly 

 through a red-hot tube packed with copper turn- 

 ings or with spirals of copper wire gauze. The 

 oxygen of the air combines with the copper to 

 form an oxide of copper, whilst the nitrogen passes 

 on. Or the oxygen can be very completely removed 

 from a confined portion of moist air by suspending 

 in it pieces of phosphorus until the volume of the 

 gas ceases to diminish. Nitrogen can also be 

 prepared by passing a current of chlorine gas into 

 a solution of ammonia. In this method care must 

 lie taken always to use ammonia in excess, other- 

 wise chloride of nitrogen would be produced, which 

 is a dangerously explosive substance. A steady 

 current of nitrogen can be very easily obtained by 

 heating together strong solutions of ammonium 

 chloride and of potassium nitrite. Owing to the 

 great difficulty of forming compounds from free 

 nitrogen, new compounds of nitrogen are generally 

 prepared from compounds already existing. The 

 most important of these compounds are the nitrates 

 which are found in the sou in very considerable 

 quantity in different parts of the world, and in 

 some places occur in large deposits, as in the 

 nitrite regions of South America. From these 

 nitrates Nitric Acid (q. v. ) and other compounds of 

 nitrogen are derived. Several compounds of nitro- 

 gen with oxygen are known. 



Nitrous Oxide, or laughing-gas, N 2 O, was dis- 

 covered in 1772 by Priestley, who obtained it by 

 the action on nitric oxide of easily oxidisable sub- 

 stances. It is now prepared by the action of heat 

 on ammonium nitrate : 



NH 4 N0 3 = N,O + 2H 2 O. 



It is a colourless gas, which can with moderate ease 

 be reduced to the liquid state, a pressure of thirty 



