of a salt of aluminium. Does a precipitate form? How could you test for zinc and aluminium in the presence of each other? How test for magnesium and aluminium?

b. Obtain about 1g. of some dry compound containing aluminium, and heat it on charcoal with the blowpipe (R). Moisten the residue with a drop or two of a solution of cobalt nitrate and heat it once more. Note the color of the residue. This constitutes a test for compounds containing aluminium.

c. Repeat the experiment, substituting a compound of zinc for that of aluminium. The green product is called Rinmann's green.

d. Mix a little dry alum and sodium bicarbonate and rub them together. Pour a little water on the mixture and note the result.

181. Double salts. a. Alums. What is a double salt? What is an alum? Calculate the weight of aluminium sulfate (remember that it is a hydrated salt) and of ammonium sulfate required for the preparation of 25 g. of crystallized alum. Dissolve these separately in hot water so that the combined volume will be about 75 cc. Unite the hot solutions and set the product aside to crystallize. Can you make out the form of the crystals? Test the reaction to litmus paper of a solution of a few pure crystals.

b. Carnallite. What is the formula of carnallite? Calculate the weight of the individual salts necessary to make 25 g. of crystallized carnallite. Which of these are hydrated? Weigh out the required amount of magnesium chloride and about one third more than the required amount of potassium chloride, mix the salts, and dissolve them in the least possible volume of hot water. Allow the solution to cool, decant the mother liquor, and wash the crystals with a very little cold water. See if you can detect both magnesium and potassium in the crystals. Is this a double salt or a complex salt?

CHAPTER XXIV

THE IRON FAMILY

182. Reactions of the ferrous ion. a. Place about 1 g. of iron in a beaker and cover the material with water. Iron by alcohol dissolves the fastest, but tacks or clean turnings will do. Add dilute hydrochloric acid, a small portion at a time, so as to keep up a brisk evolution of gas (R). Note the odor. It is chiefly due to phosphine; how do you account for the presence of this substance? Note the choking effect of the gas when breathed. Hold a nonluminous flame over the beaker for a moment. How do you account for the flashes of light? Before the iron has all dissolved, filter and at once make the following tests, using 1 or 2 cc. for each.

b. Try the action of a solution of ammonium sulfocyanate (NH CNS). Pass hydrogen sulfide into a little of the solution (R). Test the action of ammonium sulfide (R). How do you account for the difference in the results of the last two experiments? Add sodium hydroxide to some of the solution (R). What is the color of the precipitate? What change occurs after the contents of the test tube have stood exposed to air (R)?

183. Ferrous ammonium sulfate. Dissolve 10 g. of iron in dilute sulfuric acid and filter from the insoluble residue (what is it?). What weight of ferrous sulfate should be in the filtrate? Dilute it to about 50 cc. What would be an equimolecular weight of ammonium sulfate? Weigh out this amount and add it to the hot solution of ferrous sulfate. Set the beaker aside for the crystallization of the salt. Filter off the crystals, wash them with a very little cold water, and spread them on filter paper to dry. What is the formula?