MORTAR. Common mortar is the sub stance placed between the stones or bricks of a building to cement them together. Mortar is essentially composed of slacked lime and siliceous sand. The hardness which mortar acquires is owing to the gradual conversion of the lime into carbonate of lime, which takes place very slowly by the absorption of carbonic acid gas from the atmosphere. In this state it adheres very firmly to the particles of silica diffused through it, and both are strongly united with the material employed in the building. When limestones contain consider able portions of silica and alumina, they form what has been termed Hydraulic Lime, and the mortars made with them are called Hydraulic Mortars. Of these, Parker's cement is a well-known kind : it will set, as it is termed, or become solid, in a quarter of an hour, either in the air or under water. In France artificial hydraulic lime has been pro pared, and appears to answer the purpose ex tremely well.
A second application of the name Mortar is to a vessel in which substances are either reduced to fragments, pulverised, or dissolved, by beating or trituration with a pestle. Mor tars are made of cast-iron, stone-ware, glass, agate, flint, or porphyry, according to the use to which they are to be applied.
It is not very easy to shew why such should be the case ; but the same name has a third and wholly different signification, as applied to a gun or cannon. A mortar is a piece of ordnance which, compared with guns, is very short, and is employed to throw shells or carcasses at considerable elevations (generally at 43°). Mortars are either of iron or brass ; they rest upon solid beds, and the trunnions or cylinders upon which they turn, in giving the required elevation, are placed at the lower extremity of the piece. The calibres of mortars in the British service are 41, 51, 8, 10, and 13 inches. All these different kinds of mortars are used on land, and the two last are also employed in the navy ; but in this latter service the pieces are about 16 inches longer than the land-service pieces of the same calibre. The first two are sometimes called royal mortars. The Dutch engineer Coehorn invented small mortars for throwing grenades. They were capable of being carried about and served by one man; consequently they could be readily brought up to a conve nient spot, and rapidly fired.