C_{6}H_{2}|(NO_{2})_{3}
|OH.

which shows its formation from phenol (C_{6}H_{5}OH.), three hydrogen atoms being displaced by the NO_{2} group.  The equation of its formation from phenol is as follows:—­

C_{6}H_{5}.OH + 3HNO_{3} = C_{6}H_{2}(NO_{2})_{3}OH + 3H_{2}O.

According to Berthelot, its heat of formation from its elements equals 49.1 calories, and its heat of total combustion by free oxygen is equal to +618.4 cals.  It hardly contains more than half the oxygen necessary for its complete combustion.

2C_{6}H_{2}(NO_{2})_{3}OH + O_{10} = 12CO_{2} + 3H_{2} + 3N_{2}.

The percentage composition of picric acid is—­Nitrogen, 18.34; oxygen, 49.22; hydrogen, 1.00; and carbon, 31.44, equal to 60.26 per cent. of NO_{2}.  The products of decomposition are carbonic acid, carbonic oxide, carbon, hydrogen, and nitrogen, and the heat liberated, according to Berthelot, would be 130.6 cals., or 570 cals. per kilogramme.  The reduced volume of the gases would be 190 litres per equivalent, or 829 litres per kilogramme.  To obtain a total combustion of picric acid it is necessary to mix with it an oxidising agent, such as a nitrate, chlorate, &c.  It has been proposed to mix picric acid (10 parts) with sodium nitrate (10 parts) and potassium bichromate (8.3 parts).  These proportions would furnish a third of oxygen in excess of the necessary proportion.

Picric acid was not considered to be an explosive, properly so called, for a long time after its discovery, but the disastrous accident which occurred at Manchester (vide Gov.  Rep.  No.  LXXXI., by Colonel (now Sir V.D.) Majendie, C.B.), and some experiments made by Dr Dupre and Colonel Majendie to ascertain the cause of the accident, conclusively proved that this view was wrong.  The experiments of Berthelot (Bull. de la Soc.  Chim. de Paris, xlix., p. 456) on the explosive decomposition of picric acid are also deserving of attention in this connection.  If a small quantity of picric acid be heated in a moderate fire, in a crucible, or even in an open test tube, it will melt (at 120 deg.  C. commercial acid), then give off vapours which catch fire upon contact with air, and burn with a sooty flame, without exploding.  If the burning liquid be poured out upon a cold slab, it will soon go out.  A small quantity carefully heated in a tube, closed at one end, can even be completely volatilised without apparent decomposition.  It is thus obvious that picric acid is much less explosive than the nitric ethers, such as nitro-glycerol and nitro-cellulose, and very considerably less explosive than the nitrogen compounds and fulminates.