COMPOUNDS ol' NITKO(JKN WITH IIVDIIoiiKN AND oXY<JK.\ 273 



As nitric acid is very easily decomposed with the separation of 

 oxygen, it was for a long time supposed that it was not capable of 

 forming the corresponding nitric anhydride, N 2 O 5 ; but first Deville, 

 and then Weber and others, discovered the methods of its formation. 

 Deville obtained nitric anhydride by decomposing silver nitrate by 

 chlorine under the influence of a moderate heat. Chlorine acts on the 

 al M ve salt at a temperature of 95 (2AgNO 3 + CL 2 = 2 AgCl + N 2 O 5 4- O), 

 and when once the reaction is started it continues by itself without 

 further heating. Brown fumes are given off, which are condensed in a 

 tube surrounded by a freezing-mixture. A portion condenses in this 

 tube and a portion remains in a gaseous state. The latter contains 

 free oxygen. A crystalline mass and a liquid substance are obtained in 

 the tube ; the liquid is poured off, and a current of dry carbonic acid 

 gas is passed through the apparatus in order to remove all traces of 

 volatile substances (liquid oxides of nitrogen) adhering to the crystals 

 of nitric anhydride. These form a voluminous mass of rhombic crystals 

 (density 1'64), which sometimes are of rather large size ; they melt at 

 about 30 and distil at about 47. In distilling, a portion of the sub- 

 stance is decomposed. With water these crystals give nitric acid. 

 Nitric anhydride is also obtained by the action of phosphoric anhydride, 

 P 2 O 5 , on cold pure nitric acid (below 0). During the very careful dis- 

 tillation of equal parts by weight of these two substances a portion 

 of the acid decomposes, giving a liquid compound, H 2 O,2N 2 O r> 

 =N 2 O 5 ,2H]Sr0 3 , whilst the greater part of the nitric acid gives the 

 anhydride according to the equation 2NHO 3 + P 2 O 3 =2PHO 3 + N 2 O 5 . 

 On heating, and sometimes even spontaneously with explosion, nitric 

 anhydride decomposes into nitric peroxide and oxygen, N 2 O 5 

 =N 2 O 4 + O. 



Nitrogen peroxide, N 2 O 4 , and nitrogen dioxide, N0 2 , express one 

 and the same composition, but they should be distinguished like ordinary 

 oxygen and ozone, although in this case their mutual conversion is 

 more easily accomplished, even by vaporisation ; also, O 3 loses heat in 

 passing into O 2 , whilst N 2 O 4 absorbs heat in' forming NO 2 . 



Nitric acid in acting on tin and on many organic substances (for 

 example, starch) gives brown vapours, consisting of a mixture of N 2 O 3 

 and NO 2 . A purer product is obtained by the decomposition of lead 

 nitrate by heat, Pb(NO 3 ) 2 ==2NO 2 + O + PbO, when non-volatile lead 



a large quantity causes a wound and entirely eats away the membranes of the body. 

 The membranes of plants are eaten into with the greatest ease by strong nitric acid in 

 just the same manner. One of the most durable blue vegetable dyes which is employed 

 in dyeing tissues is indigo, yet it is easily converted into a yellow substance by the 

 action of nitric acid, and small traces of free nitric acid may be recognised by this means. 

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