duced to the metallic state, and to be dissipated as vapor.

On

the other hand, it may also occur that the precipitate is not thoroughly washed, and this may give rise to a higher result in figures than theoretically calculated. In such cases the residue will be found to possess an alkaline reaction.

b. By precipitation as sulphuret of zinc.

Ammonia is added to the solution of the salt under examination, until the precipitate which forms at first is re-dissolved; sulphuret of ammonium is then added in excess, and the precipitate which forms is allowed to subside; the supernatant fluid is then passed through the filter, and the precipitate is subsequently likewise placed upon the latter, and edulcorated with water impregnated with sulphuret of ammonium ; - the filter whilst still moist is, together with the precipitate, introduced into a beaker-glass, and drenched with concentrated hydrochloric acid slightly in excess; -the solution is placed on a moderately warm spot until it smells no longer of sulphuretted hydrogen; it is then diluted with a small portion of water, filtered, and the filter carefully washed with hot water; the solution of chloride of zinc, obtained in this manner, is finally precipitated, &c., as directed sub. a.

From a solution of acetate of zinc, this metal may be preci pitated completely, or very nearly so, by means of sulphuretted hydrogen gas, even though the acetic acid be present in excess, provided always no other acid be present. The precipitated

sulphuret of zinc is, in this instance, washed with water, impregnated with sulphuretted hydrogen, and, for the rest, treated exactly like the sulphuret of zinc, precipitated by means of hydrosulphuret of ammonia. (Compare experiments, sub. No. 60).

c. By direct ignition.

The salt to be examined is introduced into a platinum crucible; the latter is then covered and exposed to a gentle heat at first, which is to be increased gradually to the very highest degree of intensity, and to be maintained until the residue ceases to suffer the slightest diminution of weight.

§ 81.

a. Solution.

1. PROTOXIDE OF MANGANESE.

Many of the salts of the protoxide of manganese are soluble in water. The pure protoxide, as well as those of its salts which are insoluble in water, dissolve in hydrochloric acid. The higher oxides of manganese dissolve likewise in this acid. Chlorine is evolved in this solution, and the fluid, after having been subjected to the application of heat, contains protochloride of manganese. b. Determination.

Manganese is weighed either as protosulphate, or as manganoso-manganic oxide. Into the latter form it is converted either by precipitation as hydrated protoxide, or, as protocarbo nate, and subsequent ignition,-(the application of the latter method is in some instances preceded by precipitation as sulphuret of manganese)—or, finally, by direct ignition.

2. PROTOSULPHATE OF MANGANESE.

All the oxides of manganese, and likewise all its salts with volatile acids, provided no non-volatile

substance be present.

The method 1 d. is the most simple and accurate, and is therefore preferred to all others, wherever it is admissible. The method 2, is convenient and expeditious, but its results are not absolutely accurate; the method 1 c. 18 applied only in cases where the application of none of the other methods is admissible. 1 a. is generally preferred to 1 b. in cases where the choice is permitted. If the solution of manganese contains sugar or any similar nonvolatile organic substance, neither 1 a. nor 1 b. are applicable, and recourse must be had to 1 c. Protophosphate of manganese and protoborate of manganese are treated either according to the direction of 1 b. or to those of 1 c. The proportion of manganese in the silicate of this substance, is determined according to the directions given in 1 a. after the separation of the silicic acid. (§ 106.) Protochromate of manganese is treated like the corresponding salt of zinc. (§ 99.)

1. Determination as manganoso-manganic oxide.

a. By precipitation as protocarbonate of manganese.

The solution of the salt under examination is heated to 212°, and is then mixed with carbonate of soda in excess, and the application of heat is still continued for some time-(ebullition is not necessary); the fluid is then filtered off from the precipitate formed, and the latter is washed, dried, and ignited, as directed § 32. The lid is to be placed loose on the crucible, and the heat should be maintained at a high degree of intensity, until the weight of the residue ceases to vary, even to the slightest extent. Should the solution contain ammoniacal salts, it must be precipitated in a flask in the manner indicated at § 80 a. (Determination of zinc by precipitation as carbonate.) For the proper

ties of the precipitate and residue, vide § 52. This method, if properly executed, yields very accurate results.

b. By precipitation as hydrated protoxide of manganese.

The method is the same as applied sub. a., with this difference, that the precipitation is effected by means of solution of caustic potass instead of carbonate of soda. If phosphoric acid is present, or boracic acid, the precipitation is to be effected in a flask, and the precipitated fluid kept boiling for some time, with an excess of potass, since this will completely decompose the protophosphate or protoborate of manganese. Properties of the precipitate, vide § 52.

c. By precipitation as sulphuret of manganese.

The solution of the salt under examination is mixed with sal ammoniac, and subsequently with ammonia until the latter predominates; the yellow sulphuret of ammonium is then added in excess; the precipitate formed is allowed to subside, and the supernatant fluid is passed through the filter; the precipitate is finally placed upon the latter, and carefully washed with water containing a small portion of yellow sulphuret of ammonium in admixture. The moist precipitate and filter are then introduced into a beakerglass, and drenched with dilute hydrochloric acid; the mixture is heated until it ceases to smell of sulphuretted hydrogen; the solution is then filtered, and both the filters are carefully washed; the filtrate is precipitated as directed sub. a.

d. By direct ignition.

The compound of manganese under examination is introduced into a platinum crucible, which is kept closely covered and exposed to a gentle heat at first; subsequently the lid is taken off and replaced loosely upon the crucible, and the latter is then exposed to the most intense heat until the weight of the residue ceases to vary. The conversion of the higher oxides of manganese into manganoso-manganic oxide requires a more intense heat and a more protracted application of this than the conversion of the protoxide and peroxide. When salts with organic acids

are under examination, it is indispensable to ascertain whether all the charcoal has been completely consumed during ignition; should the contrary turn out to be the case, the residue should be either re-dissolved in hydrochloric acid, and precipitated, &c., as directed sub. a., or it ought to be repeatedly evaporated with nitric acid until all the charcoal is oxidized. This method, if properly executed, yields very accurate results. The ignition with salts of manganese with organic acids generally causes a minute portion of the salt to volatilize; the results in such cases are therefore not strictly accurate.

2. Determination as protosulphate of manganese.

The same method as described § 77, 1. (Magnesia.) For the properties of the residue, vide § 52. Great care should be taken to ignite the residue very feebly, or else a small portion of sulphuric acid is likely to escape.

Many of the salts of protoxide of nickel are soluble in water. The pure protoxide and those of its salts which are insoluble in water, dissolve, without exception, in hydrochloric acid; metallic nickel dissolves slowly, with evolution of hydrogen gas, when heated with dilute hydrochloric acid or sulphuric acid; in nitric acid it dissolves with great readiness. Sulphuret of nickel is but sparingly soluble in hydrochloric acid, but it dissolves readily in aqua regia. Peroxide of nickel dissolves as a protochloride with evolution of chlorine, when heated with hydrochloric acid.

b. Determination.

Protoxide of nickel is invariably weighed as such. (§ 53.) The compounds of nickel are converted into the pure protoxide, either by precipitation as hydrated protoxide,-preceded, in some instances, by precipitation as sulphuret of nickel,—and subsequent ignition, or by direct ignition.