THE ELEMENTS OF PLANT FOOD 65 



largest source of borax is from deposits left in dried-up lakes in 

 California. Borax is used in cleaning, in soldering, and as an anti- 

 septic. 



ALUMINIUM (Al) 



Description and Occurrence. The metal aluminium 

 is not found free in nature, but is prepared by passing 

 an electric current through a bath of melted cryolite in 

 which aluminium oxide is dissolved. It is a remark- 

 ably light metal, very tenacious and ductile, and takes 

 a bright polish. It is used for cooking utensils and be- 

 cause of the fact that it does not form poisonous com- 

 pounds with foods it is particularly well adapted for 

 such purposes. It is used also to increase the hard- 

 ness and strength of brass and other metals. Alumin- 

 ium is not an essential element of plant food, though 

 it is found in most plants in small quantities. 



Aluminium Oxide (A1 2 O 3 ), alumina. This com- 

 pound is widely distributed, occurring in ruby, sapphire, 

 and corundum. Its crystals are very hard, so that 

 they are sometimes used as substitutes for the diamond 

 in cutting glass and in abrading hard substances. 

 Emery is an impure variety of alumina. The ruby 

 and sapphire are pure aluminium oxide tinted with a 

 trace of some other mineral. 



Feldspar is a double silicate of alumina and an alkali 

 (K, Na) or an alkaline earth (Ca) or both. The feld- 

 spars form a large part of the great rock masses of the 

 earth. When they are decomposed by weathering the 

 carbonates of the alkalies and alkaline earths are formed, 

 together with clay (H 4 Al2Si 2 O9). Thus potassium car- 

 bonate becomes available for the use of plants, while 

 clay beds are formed of the clay through the action of 

 M. & H. AG. 5 



