iron precipitate in citric acid, and electrolyse the solution according to the directions given upon page 79. The filtrate, containing the zinc, manganese, nickel and cobalt, together with a little barium salt, is carefully treated with just sufficient dilute sulphuric acid to remove the barium. After filtering, electrolyse the filtrate in a platinum dish, connected with the anode of a battery, yielding 3-5 c.c. of electrolytic gas per minute. A weighed piece of platinum foil will answer for the cathode. The manganese is deposited as dioxide (p. 76); the other metals remain dissolved and can only be separated by one of the usual gravimetric methods. This course proved quite satisfactory in the analysis of the mineral franklinite, where, after having obtained the iron and manganese as described, the zinc was also determined electrolytically in the liquid poured off from the manganese deposit. If the solution containing these two metals be very slightly acid with sulphuric acid, they can be precipitated simultaneously-the zinc at the cathode, and manganese dioxide at the anode.

Iron can be further separated, in citrate solution, from aluminium, chromium and titanium (p. 79). The deposition should occur in a cold solution, with a current, liberating 12 c.c. electrolytic gas per minute. This method will also serve to separate iron from chromium and phosphoric acid. Classen separates iron, cobalt, nickel and zinc from manganese, chromium and aluminium by electrolysing their hot oxalate solu

tion in the presence of a large excess of ammonium oxalate. The first four are deposited as metals, while the manganese dioxide upon the anode has carried with it some chromium, and should be dissolved, and the manganese reprecipitated by sodium hydroxide. and sodium hypochlorite (p. 104). The main solution from the metals is digested until the excess of ammonia is expelled, when the aluminium hydroxide is filtered off; the chromate solution, then reduced with hydrogen sulphide in the presence of an acid, is precipitated with ammonium hydroxide. In all cases where it is necessary to add oxalic acid to redissolve the aluminium hydroxide, or manganese dioxide, the acid should be introduced without the interruption of the current. When phosphoric acid is present with the iron and manganese it will be found in the liquid (oxalate) from which the metals have been previously precipitated, and may be removed as ammonium. magnesium phosphate.

Little can be said relative to the separation of the rarer metals; further investigation is required in this direction.

Quite recent experiments, made in this laboratory, show that mercury and palladium can be separated if present together in a solution containing not less than 3 grams of potassium cyanide for 0.2-0.4 gram of the metals. The current necessary here may vary from

0.08 c.c.-0.22 c.c. electrolytic gas per minute. Notwithstanding that cadmium and silver are easily precipitated from their double cyanide solutions, it is impossible to separate them from palladium. In fact, they appear to hasten the deposition of the latter metal.

Mercury, silver and cadmium, furthermore, can be separated from solutions containing excess of alkaline cyanide together with tungstic and molybdic acids, without carrying down any of the latter. The conditions most favorable for these separations are perfectly similar to those just given above (pp. 96, 98, 101) for the separation of these metals from arsenic (Am. Ch. Jr., 12, p. 428).

3. OXIDATIONS BY MEANS OF THE ELECTRIC CURRENT.

When natural sulphides, e. g., chalcopyrite, marcasite, etc., are exposed to the action of a strong current in the presence of a sufficient quantity of potassium hydroxide their sulphur will be quickly and fully oxidized to sulphuric acid (Jr. Fr. Ins., April, 1889, Ber., 22, 1019). The metals (iron, copper, etc.) originally present in the mineral separate as oxides and metal on dissolving the fused alkaline mass in water. This method of oxidation eliminates many other disagreeable features of the old methods. Its rapidity

and accuracy entitle it to the following brief description:

Place about 20 grams of caustic potash in a nickel crucible 14 inches high and 13% inches wide. Apply heat from a Bunsen burner until the water has been almost entirely expelled, when the flame is lowered so that the temperature is just sufficient to retain the alkali in a liquid condition. The crucible is next connected with the negative pole of a battery, and the sulphide to be oxidized is placed upon the fused alkali. As some natural sulphides part with a portion of their sulphur at a comparatively low temperature, it is advisable to allow the alkali to cool so far that a scum forms over its surface, before adding the weighed mineral.

The heavy platinum wire, attached to the anode, extends a short distance below the surface of the

fused mass. When the current passes a lively action ensues, accompanied with some spattering. To prevent loss from this source, always place a perforated watch-crystal over the crucible. After the current has acted for 10-20 minutes interrupt it. When the crucible and its contents are cold place them in about 200 c.c. of water, to dissolve out the excess of alkali and alkaline sulphate. Filter. Invariably examine the residue for sulphur, by dissolving it in nitric acid and then testing with barium chloride. The alkaline filtrate is carefully acidulated with hydrochloric acid, and after digesting for some time, is precipitated with

a boiling solution of barium chloride. When the hydrochloric acid is first added, care should be taken to observe whether hydrogen sulphide or sulphur dioxide is liberated. If the oxidation is incomplete sulphur also makes its appearance as a white turbidity. The caustic potash employed in these oxidations should always be examined for sulphur and other impurities. As it is difficult to obtain alkali perfectly free from sulphur compounds, a weighed portion should be taken and its quantity of sulphur deducted from that actually found in the analysis.

The arrangement of apparatus employed in the oxidations just outlined is represented in Fig. 25. The crucible A is supported by a stout copper wire bent as indicated, and held in position by a binding screw attached to the base of a filter stand. The arm of the latter carries a second binding screw holding the platinum anode in position. While the platinum rod is generally the positive electrode, it is best to make it the negative pole for at least a part of the time during which the current acts. This is advisable because in many of the decompositions metals are precipitated upon the sides of the crucibles, and can readily enclose unattacked sulphide, so that by reversing the current (the poles) any precipitated metal will be detached, and the enclosed sulphide be again brought into the field of oxidation. Cinnabar is a sulphide which has a tendency to mass together, and it could only be decomposed and its sulphur thor