SOILS. 23 



an acid with this metal. Some of these salts are found 

 in soils and in the ashes of plants. 



40. Sodium is a soft metal very much like po- 

 tassium in appearance and in properties. It also 

 must be kept under naphtha, but will not take fire 

 on water, unless it be held in one place, or the water 

 be warmed. It spins around over the water, decom- 

 posing it, and forming caustic soda (22). It forms a 

 large number of salts. Common salt is sodium chlo- 

 ride, and is found in all salt springs, in the ocean, and 

 in soils, and the ashes of plants. Caustic soda and. 

 caustic potassa, or potash, are called alkalies. They 

 destroy the flesh, neutralize acids, and color red lit- 

 mus-paper blue (23). In these respects they are 

 similar to ammonia. They are used to make soap : 

 caustic potash to make soft soap, and caustic soda to 

 make hard soap. 



41. Calcium is a metal very hard to separate 

 from its compounds. With oxygen it forms com- 

 mon, unslaked lime. Marble, and limestone, and 

 chalk, are calcium carbonates, which on being heat- 

 ed lose carbon dioxide, and are changed into lime. 

 Gypsum, or land plaster, is calcium sulphate, a valu- 

 able fertilizer. Spring and well water very often 

 contain these salts in solution. 



42. Magnesium and aluminium are hard, white 

 metals, the former a constituent of dolomite, or mag- 

 nesian limestone, and some other rocks ; the latter is 

 found in all clay and slate rocks. The metal mag- 

 nesium burns with great brilliancy and is sometimes 



