[313] SCIENTIFIC MANUAL. 27 



Marls are usually composed of clay, with shells, more or 

 less crushed, imbedded in it. 



Their value depends, principally, upon the per cent, of 

 carbonate of lime which they contain. Some, however, 

 contain potash and phosphoric acid. An application of 

 marl to land, in conjunction with coarse manure, or green 

 vegetable matter, materially improves, both the mechanical 

 condition and chemical constitution of light and heavy soils, 



Carbonate of lime is, to some degree, soluble in water 

 which contains carbonic acid, and, hence, in localities where 

 these limestones exist, there are many springs whose 

 waters, passing over the rock, dissolve some of the lime- 

 stone, and are known as " limestone springs." Thislime^ 

 stone water, when evaporated to dryness, leaves a deposit 

 of lime. This is seen in kettles and boilers, in which this 

 water is heated, as an incrustation on the bottoms and 

 sides. In boilers, connected with steam engines, this has 

 has to be guarded against, since the crust thus formed be- 

 comes, eventually, thick enough to prevent access of the 

 water to the iron, which becomes red-hot, or burns, and, in 

 either case, results in the bursting of the boiler. 



In limestone caves, the water, saturated with lime, drips 

 through the rock above, and, by exposure to air, soon 

 evaporates, leaving a small deposit of lime. This process 

 is continued by successive drops of water saturated with 

 lime, each leaving its small deposit, until, after a long pe- 

 riod, a conical mass is formed, with its base attached to the 

 roof of the cave. This is called a stalactite. When the 

 water falls to the floor, and builds up from below, the 

 rounded mass is called a stalagmite. The two often grow 

 toward each other, and unite into a column. 



Lime forms the usual salts, with acids ; with sulphuric 

 acid it forms sulphate of lime, or gypsum. This exists in 

 nature in various forms. The massive variety is known as 

 alabaster ; is very soft, and can be cut into rings, boxes 

 and ornaments. 



