_Compounds._ We do not find these elements merely mixed with other elements to form a soil. They unite in definite proportions by weight to form chemical compounds. As conditions change, many of these compounds undergo change, giving up one element, or group of elements, and uniting with another element or group from a different compound. Heat, moisture and the action of bacteria are factors in promoting the changes. There is no more restless activity than may be found among the elements composing a productive soil. _Calcium_ is an element which will unite with oxygen and carbon dioxide to form a compound known as calcium carbonate. The chemist's symbol for calcium is Ca. _Calcium Oxide_ is a compound left after the burning of limestone, and is known as fresh burned lime, or quick lime. Its formula is CaO. It contains, when pure, 40 parts of calcium and 16 parts oxygen by weight. _Carbon Dioxide_ is a compound whose formula is CO2. _Calcium Carbonate_, known also as carbonate of lime, has a definite composition, containing, when pure, 56 parts CaO and 44 parts CO2. It is known to the chemist as CaCO3, and forms practically all of very pure limestones. Impure limestones contain some earthy materials that became mixed with the lime carbonate when the rock was being formed. _Calcium Hydroxide_ is a compound made by permitting calcium oxide to combine with water, and is known as lime hydrate. It contains 56 parts by weight CaO and 18 parts water, and has the formula Ca(OH)2. _Magnesium_ is an element, and is found in magnesium carbonate, a compound that is effective in correcting soil acidity. _Magnesian Limestone._ Magnesium carbonate is usually found in combination with calcium carbonate, and when about 47 per cent of the total carbonates is magnesium carbonate, the limestone is known as dolomite. _Ground Limestone_ is the stone pulverized so that it can be distributed. It is carbonate of lime (CaCO3), or a combination of calcium and magnesium carbonate, and in a way has a right to be designated as "lime," but such use leads to confusion. _Fresh Burned Lime_. Calcium oxide (CaO) formerly was accurately designated as "lime," but the words "fresh burned" are often prefixed to prevent confusion with lime carbonate or the hydrate. It is known as "lump lime," "caustic lime" and "stone lime." _Ground or Pulverized Lime_. Fresh burned lime may be ground fine, so that it can be spread on land without slaking. This product should not be confused with ground limestone or hydrated lime. Fresh ground lime is worth nearly twice as much per ton as ground limestone, but some of the product on the market is far from pure. There is opportunity to grind up unburned and waste material with the caustic lime, and this form of lime usually contains some hydrated material. _Hydrated Lime_ is the compound formed by the action of water or steam on fresh burned lime.