72 LABOR A TOR Y EXERCISES 



lead"), and potassium dichromate; new, unbleached cotton cloth; 

 white woolen yarn; cochineal (powdered) and picric acid. If the dyes 

 are furnished in solution, less material will be wasted. 



a. Into a bottle or flask put about half a teaspoojiful of 

 powdered cochineal and 100 cu. cm. of water. Shake the 

 mixture from time to time during 10 minutes; then let it settle. 



Prepare a mordant of aluminum acetate as follows : 



To half a test tube full of lead acetate solution in a beaker or bottle 

 add about 10 cu. cm. of alum solution. A white precipitate of lead 

 sulphate is formed at once. Let this settle, and add more of the alum 

 solution (about 1 cu. cm. at a time) until no more precipitate is formed. 

 Let the precipitate settle completely; then pour the clear solution of 

 aluminum acetate into another dish. Throw away the white pre- 

 cipitate. 



b. From a piece of new, unbleached cotton cloth cut off 

 5 strips about 2.5 cm. (1 in.) wide and 10 cm. (4 in.) long. 

 Wash the strips with soap and water, and rinse them. 



Put 50 cu. cm. of the cochineal solution into a beaker or 

 porcelain dish, heat it to gentle boiling, and leave a strip of the 

 cotton cloth in it for 2 minutes. Remove the cloth, rinse it 

 thoroughly, and let it dry. Is the color fast? 



c. Soak the second piece of cloth in the aluminum acetate 

 solution for 2 minutes, remove it, and let it dry for 5 minutes. 

 Then boil it in the cochineal solution for 2 minutes. Remove 

 the dyed cloth, rinse it thoroughly, and dry it. Is the color 

 fast now, or not? Paste the cloth in your note book, and label 

 it. Save the mordant for e. 



d. Dissolve about 2 grams of picric acid in 20 cu. cm. of hot 

 water; add 3 drops of dilute sulphuric acid. Put into the hot 

 solution a piece of white woolen yarn and a strip of the cotton 

 cloth. Heat the solution to boiling for 2 minutes; then remove 

 the cloth and the yarn, and rinse them thoroughly. Which 

 one is dyed permanently? Its color? 



