What is the most important compound of chlorine?] This is sand, the base of flint. It is necessary for the growth of all plants, as it gives them much of their strength. In connection with an alkali it constitutes the hard shining surface of corn stalks, straw, etc. Silica unites with the alkalies and forms compounds, such as _silicate of potash_, _silicate of soda, etc._, which are soluble in water, and therefore available to plants. If we roughen a corn stalk with sand-paper we may sharpen a knife upon it. This is owing to the hard particles of silica which it contains. Window glass is silicate of potash, rendered insoluble by additions of arsenic and litharge. Liebig tells us that some persons discovered, between Manheim and Heidelberg in Germany, a mass of melted glass where a hay-stack had been struck by lightning. They supposed it to be a meteor, but chemical analysis showed that it was only the compound of silica and potash which served to strengthen the grass. There is always _enough_ silica in the soil, but it is often necessary to add an alkali to render it available. When grain, etc., lodge or fall down from their own weight, it is altogether probable that they are unable to obtain from the soil a sufficient supply of the soluble silicates, and some form of alkali should be added to the soil to unite with the sand and render it soluble. CHLORINE. [Of what use is chloride of lime? What is oxide of iron? What is the difference between the _per_oxide and the _prot_oxide of iron?] _Chlorine_ is an important ingredient of vegetable ashes, and is often required to restore the balance to the soil. It is not found alone in nature, but is always in combination with other substances. Its most important compound is with sodium, forming _chloride of sodium_ (or common salt). Sodium is the base of soda, and common salt is usually the best source from which to obtain both soda and chlorine. Chlorine unites with lime and forms _chloride of lime_, which is much used to absorb the unpleasant odors of decaying matters, and in this character it is of use in the treatment of manures. OXIDE OF IRON. _Oxide of iron_, one of the constituents of ashes, is common iron rust. _Iron_ itself is naturally of a grayish color, but when exposed to the atmosphere, it readily absorbs oxygen and forms a reddish compound. It is in this form that it usually exists in nature, and many soils as well as the red sandstones are colored by it. It is seldom, if ever, necessary to apply this as a manure, there being usually enough of it in the soil.