hot water. Add equal portions of this solution to portions of the colloidal solutions of antimony sulfide and of ferric oxide. With these solutions repeat the experiments of § 158. Does the presence of the agar interfere with coagulation? It is often the case that the presence of organic colloids interferes with precipitation tests in chemical analysis. Such colloids are then called protective colloids.

160. Precipitation of one colloid by another. Sometimes one colloid will act upon another, causing mutual precipitation. Treat some of the colloidal solution of iron oxide with an equal volume of the antimony sulfide, taking care that no free hydrogen sulfide is present. Does a precipitate form? Filter some of the solution. Is the filtrate red like the iron, or orange like the antimony?

161. Adsorption. a. Fold a small filter paper to fit a funnel and place upon it a layer of bone black 2 or 3 cm. thick. Wet it thoroughly with water and let it drain. Prepare 50 cc. of a solution decidedly colored by blue litmus, congo red, or some other dye. Slowly filter the colored solution through the bone black, repeating several times. Is the color removed?

b. Repeat the experiment, using a dilute solution of ammonia instead of the dye. Can you detect ammonia in the filtrate? How does it affect red litmus?

162. Emulsions. a. In a 250-cc. flask pour 5 cc. of a 5 per cent solution of hard soap in water. Add benzene in successive portions as follows: 5, 10, 20, 20, 20, 20, 30, 50 cc., shaking the stoppered flask vigorously after each addition. A stiff emulsion should be obtained consisting of about 97 per cent benzene emulsified in 3 per cent of water by a very little soap.

b. Shake together 20 cc. of raw linseed oil and 20 cc. of water, adding the water in small portions at a time. The oil always contains enough rosin to act as the emulsifying agent.

163. Gels. a. Break up some dry gelatin or agar into very small pieces and place them in a test tube to a depth of about 2 cm. Add 10 cc. of water and heat nearly to boiling until a clear solution is obtained. Immerse the test tubes in ice water

for a time. What change do you note? Now put the tube in hot water. Does the gel liquefy? Once more cool. Does the gel form again? What is such a gel called?

b. To 10 cc. of water glass of density 1.1 add 10 drops of concentrated sulfuric acid. Shake the solution thoroughly and set aside if necessary until the next period. Tap the test tube gently and note the vibrations of the firm gel of silicic acid.

c. Immerse the tube containing the gel in hot water. Does the gel liquefy? With a glass rod dig out a little of the gel and see if you can dissolve it in water. What is such a gel called?

CHAPTER XXVII

GENERAL METHODS FOR PREPARATION OF COMPOUNDS

164. General methods. a. Chlorides. How can you prepare zinc chloride from zinc ? from zinc carbonate? from zinc oxide (R)? How can you prepare calcium chloride from calcium carbonate? What chlorides are insoluble? Prepare small amounts of each (R) and note their physical properties.

b. Sulfides. Describe the different ways in which sulfides have been prepared in the laboratory. Devise a method for preparing the following insoluble sulfides: lead sulfide, silver sulfide, antimony sulfide, zinc sulfide, manganese sulfide. Prepare (hood) small amounts of each in test tubes (R).

c. Nitrates. How can you prepare copper nitrate from copper? from copper carbonate? from copper hydroxide (R)? Are any of the nitrates insoluble? What is the effect of heat on copper nitrate? on lead nitrate? on ammonium nitrate ?

d. Nitrites. Recall the preparation of sodium nitrite (R). e. Sulfates. The following sulfates have been prepared in previous exercises: zinc sulfate, sodium sulfate, copper sulfate, iron sulfate. Write the equations for the reactions involved in the preparation of each. What sulfates are insoluble? Prepare small amounts of each, noting the color and writing the equations for the reactions involved in each case.

f. Sulfites. Recall the method used in the preparation of sodium sulfite. How can you distinguish between sulfates and sulfites?

g. Carbonates. What carbonates are soluble? Give a general method for the preparation of the soluble carbonates; give a general method for the preparation of the insoluble carbonates. Prepare calcium carbonate by two different