30. _Nitric Acid_ is a compound of oxygen and azote, or nitrogen, in the proportion of twenty─five parts, by weight, of the latter to seventy─five of the former. It is one of the constituent parts of nitre, or saltpetre (206); and, in a pure state, is transparent and colourless, like water. By the action of light, however, it soon becomes yellow; and, if exposed to the air, it emits yellow fumes, which even tinge the air of the same colour. To the taste it is extremely acid. It dyes the skin a yellow colour, which is very difficult to be removed; and it is so corrosive as to destroy almost every substance into which it penetrates. If poured upon oils, it sets them on fire. With various bases it forms compounds called _nitrats_. This acid, which hitherto has never otherwise been obtained than mixed with water, is chiefly known in commerce by the name of _aqua fortis_ (206). EARTHS. 31. The solid contents of the globe are composed of several elementary substances, amongst which have been enumerated no fewer than nine different kinds of earth: 1. Silex. 2. Alumine. 3. Zircon. 4. Glucine. 5. Yttria. 6. Barytes. 7. Strontian. 8. Lime. 9. Magnesia. These, when freed from foreign admixture, are, for the most part, of white colour, not soluble in water, not combustible, and do not exceed four times the weight of water. 32. The whole of these earths have, till lately, been considered simple and uncombined substances; but, by the discoveries of Sir Humphrey Davy, it has been ascertained that four of them have a metallic basis, and are in fact metallic oxides, or compounds consisting of a metal united with oxygen (21). These, which have the same affinity with their respective bases as rust has to iron, are silex, lime, barytes, and alumine. Until, however, some further light be thrown upon their nature and constitution, they must continue to hold their former situation of simple earths. 33. _Silex_, or _Siliceous Earth_, is the basis of all substances known by the name of quartz and silex (76). In a state of nature it has never been found pure; but, in combination with other substances, it abounds in almost every country of the globe. Common flint (90) contains ninety─seven parts in a hundred of silex: it consequently has given its name to this earth, _silex_ being the Latin word for flint. When purified it is a white powder, the particles of which are harsh to the touch, as if they consisted of very minute grains of sand. It is not quite three times as heavy as water, and has neither taste nor smell. Water will not dissolve it, nor any kind of acid, except fluoric. Sir H. Davy has discovered it to have a metallic basis, to which he has given the name of _silicium_.