166 BIOLOGY OF PLANTS. 



their organs in a vigorous state of growth, and enable them 

 to attain their highest perfection. 



Sect. 4. Definitions. — Source and Assimilation of the inor- 

 ganic Constituents of Plants. 



Potash or potassa (KO=47.15) is composed of the metal po- 

 tassium and oxygen, one equivalent of each ; of course its com- 

 bining immber is 8-f-39=47. This substance is well known. 

 It is found in all plants. It is a solid, easily soluble in water, 

 caustic to the taste, eminently alkaline in all it properties and 

 relations. 



Carbonates of potassa are known to us under the name of pot 

 and pearl-ashes, and saleratiis. The nitrate of potassa is known 

 as nitre and salt-petre. All the salts of this alkali are useful 

 substances. 



Soda (NaO.31.3) is an alkali similar to potassa. It is com- 

 posed of 8 parts of oxygen, and 23.3 of the metal sodium ; hence 

 its equivalent is 31.3 and its symbol is NaO. It is a white or 

 gray solid, very soluble in water, caustic to the taste, and, com- 

 bined with acids, forms a large class of salts. 



The nitrate of soda, called cubic nitre, is similar in its chemical 

 properties to nitrate of potassa. The carbonate of soda is well 

 known, as the substance used for soda powders. The sulphate 

 of soda is the well known substance Glauber'' s salts. 



Common salt is a chloride of sodium, but when it is dissolved 

 in water, or when the chlorine is removed, the metal sodium 

 immediately combines with oxygen, if water is present, and 

 forms soda. The chlorine unites with the hydrogen of the wa- 

 ter, and forms muriatic acid; these may then combine, and form 

 hydrochlorate of soda. 



Magnesia (MgO.20.7) is a white powder, of an earthy appear- 

 ance, known in the shops as calcined magnesia. It is comi)osed 

 of a peculiar metal, magnesium, 12.7 parts by weight, and 8 

 parts of oxygen. Its symbol is MgO. It is very infusible and 

 slightly soluble in water, rc({uiring 5142 times its weight of wa- 

 ter, at ()0° F., and 36,000 of boiling water to dissolve it. AVhen 

 exposed to the air, it absorbs carbonic acid and is converted into 

 tlw carbonate of magnesia, also a white powder, very insoluble 

 in water. Phosphate of magnesia is a comi)ound of phosphoric 

 acid and magnesia, and has not been fully examined. Sul- 

 phate of magnesia is the common Epsom salts. 



