The most accurate method of investigation for the purpose of separating chlorine from combination with organic matters is to cut out the stained portions, weigh them, and burn them up in a combustion-tube, the front portion of the tube being filled with caustic lime known to be free from chlorides; a similar experiment must be made with the unstained portions. In this way a considerable difference may often be found; and it is not impossible, in some instances, to thus detect, after the lapse of many years, that certain stains have been produced by a chlorine-holding substance. III.--Nitric Acid. § 79. =General Properties.=--Nitric acid--commonly known in England as _aqua fortis_, chemically as _nitric acid_, _hydric nitrate_, or _nitric monohydrate_--is a mono-hydrate of nitrogen pentoxide (N₂O₅), two equivalents, or 126 parts, of nitric acid containing 108 of N₂O₅, and 18 of H₂O. Anhydrous nitric acid, or nitrogen pentoxide, can be obtained by passing, with special precautions, dry chlorine over silver nitrate; the products are free oxygen and nitrogen pentoxide, according to the following equation:-- Silver Chlorine. Silver Nitrogen Oxygen. Nitrate. Chloride. Pentoxide. Ag₂O,N₂O₅ + 2Cl = 2AgCl + N₂O₅ + O By surrounding the receiver with a freezing mixture, the acid is condensed in crystals, which dissolve in water, with emission of much heat, forming nitric acid. Sometimes the crystals, though kept in sealed tubes, decompose, and the tube, from the pressure of the liberated gases, bursts with a dangerous explosion. Pure nitric acid has a specific gravity of 1·52, and boils at 98°. Dr. Ure examined the boiling point and other properties of nitric acid very fully. An acid of 1·5 specific gravity boils at 98·8°; of specific gravity 1·45, at 115·5°; specific gravity 1·40, at 118·8°; of specific gravity 1·42, at 122·8°. The acid of specific gravity 1·42 is the standard acid of the British Pharmacopœia. It can always be obtained by distilling either strong or moderately weak nitric acid; for, on the one hand, the acid on distillation gets weaker until the gravity of 1·42 is reached, or, on the other, it becomes stronger. There is little doubt that acid of 1·42 gravity is a definite hydrate, consisting of 1 atom of dry acid and 4 atoms of water; it corresponds to 75 per cent.[102] of the liquid acid HNO₃. There are also at least two other hydrates known--one an acid of 1·485 specific gravity, corresponding to 1 atom of dry acid and 2 of water, and an acid of specific gravity 1·334, corresponding to 1 atom of dry acid and 7 atoms of water. [102] The British Pharmacopœia states that the 1·42 acid equals 70 per cent. of HNO₃; but this is not in accordance with Ure’s Tables, nor with the facts.