138 AGRICULTURAL CHEMISTRY 



slaked ? Why ? ( 2 ) Did any noticeable change take place in vol- 

 ume during slaking ? (3) What is lime water? (4) What is the 

 object of forcing respired air through the lime water of the test- 

 tube? (5) Write the reaction which took place. (6) Write the 

 reaction when HC1 was added to Ca(OH) 2 . 



175. Glass. Glass is a double silicate of calcium and 

 sodium, produced by fusing pure sand, sodium carbonate 

 and lime. When potassium carbonate is substituted for 

 the sodium salt, Bohemian or hard glass is prepared. 

 Other kinds and varieties of glass are made by intro- 

 ducing other substances and giving different mechanical 

 treatment to the material during its preparation. 



176. Occurrence of Magnesia. This element does not 

 occur as extensively in nature as calcium which it resem- 

 bles in many respects. It is found associated mainly 

 with calcium in the mineral dolomite, a double carbonate 

 of calcium and magnesium. Magnesium is separated 

 from its compounds more readily than calcium. It is 

 found in both plant and animal substances as is calcium. 

 In some plants and in some parts of the plant, as in the 

 seeds of grains, it is found more abundantly than calcium. 

 It is generally considered one of the essential elements of 

 plant food. The compounds of magnesium resemble 

 those of calcium in many respects, but differ materially 

 from the calcium salts in both chemical and physical 

 properties. 



177. flagnesium Salts. Magnesium carbonate and 

 magnesium sulfate (Epsom's salt) are among the most 

 common of the magnesium compounds. Magnesium 

 chlorid, MgCl 2 , and MgSO 4 are found as double salts in 



