NITBOBENZOL 419 



of soda, is more readily removed by solution from glass retorts, -when a range of these 

 sot in a gallery furnace is the apparatus employed. Nitric acid of specific gravity 1-47 

 may be obtained colourless ; but by further concentration a portion of it is decomposed, 

 whereby some nitrous acid is produced, which gives it a straw-yellow tinge. At this 

 strength it exhales white or orange fumes, which have a peculiar, though not very dis- 

 agreeable smell; and even when largely diluted with water it tastes extremely sour. 

 The greatest density at which it can be obtained is 1'51 or perhaps T52, at 60Fahr., 

 in which state, or even when much weaker, it powerfully corrodes all animal, vegetable, 

 and most metallic bodies. When slightly diluted, it is applied, with many precautions, 

 to silk and woollen stuffs, to stain them of a bright yellow hue. 



In the anhydrous state, this body consists of 26'15 parts by weight of nitrogen, and 

 73'85 of oxygen ; or of 2 volumes of the first gas and 5 volumes of the second. 



When of specific gravity 1'5, the acid boils at about 210 Fahr. ; of 1'45, it boils at 

 about 240 ; of 1'42, it boils at 253 ; and of 1-40, at 246 F. If an acid stronger than 

 T420 be distilled in a retort, it gradually becomes weaker; and if weaker than T42, 

 it gradually becomes stronger, till it assumes that standard density. Acid of specific 

 gravity l'48o has no more action upon tin than water has, though when either stronger 

 or weaker it oxidises it rapidly, and evolves fumes of nitrous gas with explosive vio- 

 lence. In two papers upon nitric acid, published by Dr. Ure in the fourth and sixth 

 volumes of the 'Journal of Science' (1818 and 1819), he investigated the chemical 

 relations of these phenomena. Acid of 1 -420 consists of 1 atom of dry acid and 4 of 

 water ; acid of 1'485, of 1 atom of dry acid and 2 of water ; the latter compound 

 possesses a stable equilibrium as to chemical agency ; the former as to calorific. Acid 

 of specific gravity 1-334, consisting of 7 atoms of water and 1 of dry acid, resists the 

 decomposing agency of light. Nitric acid acts with great energy upon most combus- 

 tible substances, simple or compound, giving up oxygen to them, and resolving itself 

 into nitrous gas, or even nitrogen. Such is the result of its action upon hydrogen, 

 phosphorus, sulphur, charcoal, sugar, gum, starch, silver, mercury, copper, iron, tin, 

 and most other metals. 



Nitric acid is never obtained as the waste product of any chemical operation. 

 Its manufacture is invariably the primary object of the process by which it is pro- 

 cured. 



It has been proposed to decompose nitrate of soda by the action of boracic acid, so 

 as to produce biborate of soda, or borax, and thus render the nitric acid a secondary 

 product. The success of this process depends, however, upon a circumstance of a 

 somewhat curious kind. Strong nitric acid is much more volatile than weak acid ; 

 and hence it is more easily expelled from its combination with soda in a concentrated 

 than in a diluted form. Now, boracic acid has 3 atoms of water in its crystallised 

 condition ; therefore, if we take 2 atoms of this acid, we have 6 atoms of water to 

 unite with the 1 atom of nitric acid capable of being disengaged from nitrate of soda ; 

 whereas this quantity of nitric acid needs at most but 2 atoms. The secret, therefore, 

 is to dry the boracic acid in the first instance, so as to get rid of the surplus water ; 

 and this is easily done at a temperature of 212 Fahr., at which two-thirds of the 

 water readily leave the boracic acid, and thus afford a mono-hydrated compound, 

 2 atoms of which contain precisely the amount of water needed for 1 atom of nitric 

 acid, and also of the boracic acid requisite for the production of the biborate of soda. 

 There are some peculiarities connected with the application of the necessary tempera- 

 ture, but they are of less importance. The biborate of soda is afterwards dissolved in 

 hot water, and crystallised. 



Anhydrous nitric acid, known to modern chemists as nitric anhydride, nitric oxide, 

 or nitrogen pentoxide, was discovered in 1849 by M. Deville, who isolated it by the 

 action of dry chlorine gas on nitrate of silver, heated at first to about 200 Fahr., and 

 then reduced in temperature to about 150. The anhydride, or radical of the acid, is 

 expelled, and condenses in transparent, brilliant, colourless crystals, which are ex- 

 tremely unstable, and readily dissolve in water, with production of ordinary nitric 

 acid. 



CTXTRXTES. Salts formed by the combination of nitrous acid with the metals, 

 earths, or alkalis. See Watts's ' Dictionary of Chemistry." 



roiTKOBEursoz,. Azobenzol. C 12 H 5 (N0 4 ) [C 6 H 5 (NO 2 )]. It is important 

 in the arts, both as a source of aniline for the manufacture of dye-colours, and on 

 account of its use for flavouring, as a substitute for oil of bitter almonds, which it 

 closely resembles in flavour when pure, and over which it has the advantage of not 

 being poisonous. 



_ It is prepared from benzol (which see) by adding it, drop by drop, into hot, fuming 

 nitric acid ; the nitrobenzol separates on dilution with water in the form of a yellowish 

 oil, which may be purified by washing with water alone, or a solution of carbonate of 

 soda. It has a density of 1-209 at 60 Fahr. (15'5 Cent.), and just above the freezing- 



EB 2 



