46 Keport op the Acting Director and Chemist of the 



elements. Silicon combined with oxygen forms a compound com- 

 monly called silica. Quartz and sand are nearly pure silica. 



Silicon combined with oxygen and several of the metallic elements, 

 such as sodium, potassium, calcium, etc., forms compounds which 

 are called silicates. The feldspars are silicates ; clay is a silicate. 

 Ordinary glass consists of a mixture of silicates. 



Calcium. — Occurence. — The metal calcium is always found in 

 nature combined with other elements. The compounds of calcium 

 are ordinarily known as lime coiiipounds, because calcium, when 

 combined with oxygen to form a compound known as calcium oxide, 

 was called lime, and this calcium oxide or lime was supposed to be 

 present in calcium compounds. 



Calcium combined with oxygen and carbon forms a compound 

 which is known as calciwm carhonate or carhonate of lime. Lime- 

 stone, marble, chalk, eggshells and coral consist of calcium carbonate 

 or carbonate of lime. Calcium and sulphuric acid form a compound 

 known as calcium sulphate or sulphate of lime ; gypsum and plas- 

 ter of Paris are familiar forms of calcium sulphate ; it is often called 

 simply ''' plaster.''' 



Calcium combined with oxygen, as already noted, forms calcium 

 oxide, which is commonly known as lime or quichlime. This is 

 made by burning some form of calcium carbonate, as limestone, 

 oyster shells, coral I'ock, etc. ; the carbon dioxide (carbonic acid) is 

 driven off by the heat, and calcium oxide or quicklime remains. 



When quicklime is exposed to the air, it slowly absorbs moisture 

 and carbon dioxide and is changed back into calcium carbonate. 

 When quicklime is changed into calcium carbonate (carbonate of 

 lime), the lime is said to be ai/r-slacked. 



Potassium. — Occurrence. — The metal potassium is never found 

 uncombiued in nature. It is a constituent of many minerals. The 

 decomposition of these minerals give rise to the presence of potas- 

 sium compounds everywhere in the soil. It is taken up by plants ; 

 and when vegetable material is burned, the potassium remains 

 behind, chiefly as potassium carbonate. When wood-ash is treated 

 with water, or " leached," the potassium carbonate is dissolved out, 

 forming " lye," and this, evaporated to dryness, leaves impure potas- 

 sium carbonate, which is commercially known && potash. 



In using the tQYxn potash in connection with fertilizers, potassium 

 oxide is always meant. The compounds of potassium are commonly 



