146 AGRICULTURAL CHEMISTRY 



amounts is found in plant substances although it takes no 

 part as plant food. Aluminum is not easily isolated from 

 its compounds. It can be produced by treatment of its 

 chlorid with sodium but is now quite extensively pre- 

 pared by electrolysis. When pure, it is not as readily 

 oxidized or acted upon by acids as is iron. Aluminum 

 forms a large number of compounds and also alloys with 

 many of the metals. 



185. Alums. In industrial operations, alum is used 

 the most extensively of any of the compounds of alumi- 

 num. An alum is a double sulfate of aluminum. It has 

 the general composition of MAl(SOJ 2 i2H 2 O in which M 

 represents any bivalent metal as potassium. The Al can 

 also be replaced by a trivalent element. Alum is exten- 

 sively used in the tanning of leather, manufacture of 

 paper, and in the coloring of cloth as the basis of the 

 mordant or material for making the dye permanent. 

 Alum is also used occasionally in the preparation of 

 baking-powders. 



Experiment 32. Add a few drops of alum solution to a test-tube 

 containing 5 cc. of water, and then add a few drops of tincture of 

 logwood and 2 cc. (NH 4 ) 2 CO 3 . Observe the result. Mix about 

 2 grams of flour in a dish with water containing a few drops of 

 alum. Add a few drops of logwood and the same amount of am- 

 monium carbonate solution ; mix well, and observe the result. Re- 

 peat the test, using a baking-powder, and test for the presence or 

 absence of alum. In the presence of alum, a blue color is always 

 obtained with tincture of logwood and ammonium carbonate solu- 

 tion. 



Experiment 33. To a solution of egg albumin, add a few drops 

 of alum solution and observe the result. 



