Now the gas water contains carbonate of ammonia and sulphide of ammonium, and when any acid strong enough to decompose these substances is put into the liquid, the carbonic acid and sulphuretted hydrogen being volatile are driven off, and the acid combines with the ammonia to form a salt. For example, when muriatic acid is put into the liquid, it drives off the volatile gases and combines with the ammonia in solution to form muriate of ammonia, which is dissolved in water and evaporated till it crystallises; then it is vaporized and sublimed to free it from impurities. When ammonia and muriatic acid are separately vaporized, the two colourless transparent vapours, when mixed, combine into solid muriate of ammonia, a result so unexpected that as Mr. Playfair justly observes, it could only have been taught by experiment. About 4,000 tons of muriate of ammonia are annually made from gas water in England for soldering, and for making alum. Sulphate of ammonia to the extent of 5,000 tons is annually made by adding oil of vitriol to the liquid. It is also used for making alum, as well as for manure; it supplies our grain with nitrogen, an important article of vegetable food. To these may be added 2,000 tons of carbonate of ammonia, so that a substance that was considered to be good for nothing yields 11,000 tons of valuable materials, but even this quantity forms only part of the enormous amount annually consumed in the manufactures of Great Britain. Coal tar is of complicated nature, containing a variety of substances, many of which are more or less volatile. When it is distilled by sending a current of steam through it, the steam collects the volatile parts, condenses them into naphtha; the first product is condensed steam or water with naphtha swimming on its surface, the next product is dead oil, and the remainder is pitch. By the aid of the crude naphtha thus produced, Indian rubber is dissolved and waterproof clothes are made. When purified by sulphuric acid, it forms a substance like tar which is thrown away, and the remaining products when clarified are acid oils and neutral hydro-carbons. The carbolic and cressylic acids are the most important of these acid oils. The carbolic acid, which has the property of arresting the putrefaction and decay of organic matter, consists of 12 equivalents of carbon, 6 of hydrogen, and 2 of oxygen. The cressylic acid only differs from the preceding by having two more equivalents of hydrogen and two of oxygen in its chemical composition. Creosote is a mixture of these two acids. Those vast beams of wood that are driven as piles into the sand or mud at the bottom of the sea, as well as the timbers that form marine superstructures, are saturated with it to a certain depth to preserve them from the attacks of marine insects, especially Limnoria terebrans, an isopod crustacean, which is so destructive in some of our harbours. The wood is deprived of its air by heat and the creosote easily enters.