By the combustion of bodies, substances are generated that are either gaseous or solid, whence arises the variety of products. Of aëriform fluids, some are coloured, as nitrous acid vapour, (nitrous gas and oxygen), chlorine, and the protoxide and deutoxide of chlorine. The first is red, the rest yellowish-green, or yellowish. Some relight a taper, provided the wick remain ignited, as oxygen gas, protoxide of azote, and the oxides of chlorine. Others produce _white vapours_ in the air, as muriatic acid, fluoboric, fluosilicic, and hydriodic. The inflammable gases, which take fire in the air by contact of the lighted taper, are hydrogen, hydroguret, and bihydroguret of carbon, carbonic oxide, prussine or cyanogen, called also carburet of azote, and phosphuretted, sulphuretted, arsenuretted, telluretted, and potassuretted hydrogen. Other gases are acid, and redden litmus, which, for that reason, are called acid gases, such as nitrous, sulphurous, muriatic, fluoboric, hydriodic, fluosilicic, chlorocarbonic, and carbonic acids; the oxides of chlorine, sulphuretted hydrogen, telluretted hydrogen, and carburet of azote. Some gases are destitute of smell, as oxygen, azote and its protoxide, and carbonic acid; while others have a strong and characteristic odour, as ammoniacal gas. Some gases are very soluble in water, and others but slightly soluble, such as fluoric, fluosilicic, carbonic, sulphurous, and muriatic acids, and ammoniacal gas. Alkaline solutions absorb some gases, as nitrous, sulphurous, muriatic, fluoboric, carbonic, hydriodic, fluosilicic, chlorine, chlorocarbonic, and the two oxides of chlorine, sulphuretted hydrogen, telluretted hydrogen, and ammonia. Alkaline gases are ammonia, and potassuretted hydrogen. The character of gases is well defined. The compound gas of phosphorus and hydrogen takes fire spontaneously in the atmosphere, burning with a brilliant white flame; but there is another gas formed of the same substances, that does not inflame spontaneously, but is inflammable, called subphosphuretted hydrogen. This gas has a strong smell of garlic or phosphorus, and is luminous in the dark. It may be this peculiar combination, which gives rise to the _ignes fatui_; but the permanent ignes fatui, observed in volcanic countries, are said to be the slow combustion of sulphur, forming sulphurous acid gas. Sir H. Davy found, that phosphuretted hydrogen produced a flash of light when admitted into the best vacuum that could be made by an excellent pump of Nairn's construction.