43. The alkalies at present known are three in number; _potash_ (205), _soda_ (200), and _ammonia_ (207). Of these the two former, although till lately they have been considered simple substances, have been shown by Sir H. Davy to have metallic bases. SIMPLE COMBUSTIBLES. 44. By this term we are to understand all those mineral substances, capable of combustion, which have not been discovered to consist of more than a single component part. They are four in number; _hydrogen_, _sulphur_, _phosphorus_, and _carbon_. 45. _Hydrogen_, as its name imports, is a principal constituent part of water; for, singular as it may appear, that well─known fluid is formed by a combination of two species of air or gas, called hydrogen and oxygen (21), and in the proportion of about fifteen parts of the former and eighty─five parts of the latter. This gas had formerly the denomination of _inflammable air_, and has long been known in mines under the name of _fire─damp_. It is about twelve times lighter than atmospheric air. When pure it soon destroys such animals, and extinguishes all such flaming substances, as are immersed in it. Mixed with atmospheric air, it explodes with great violence on the application of any ignited body. 46. _Sulphur_ is a simple combustible substance, of yellow colour, which is found pure, or native, in several parts of the world, and is sufficiently familiar to us under the name of _brimstone_ (211). It strongly attracts oxygen (21), and is thereby converted into sulphuric acid (24). It frequently occurs in combination with mineral substances, such as arsenic, antimony, copper, and other metallic ores. 47. _Phosphorus_ is a combustible substance which, when pure, somewhat resembles bees’─wax both in colour and consistence; and, when exposed to the air under the usual temperature of our atmosphere, is luminous in the dark, and has a smell somewhat resembling that of garlic. It is so combustible that, when melted, it should be kept under water, as it cannot be exposed to the air during this process without great risk of catching fire. This substance is not known in a native state; and the whole of what is used in philosophy and commerce is obtained by different artificial processes. In union with oxygen (21) it becomes converted into an acid, called _phosphoric acid_ (25), and, under this form, in conjunction with lime, it constitutes the bones of men and animals. The greater part of the phosphorus of the shops is obtained from bones.