cipitate is dissolved in hydrochloric acid, heated to boiling, sulphite of sodium added, and then excess of caustic soda, and the whole boiled till the precipitate is black and pulverulent. This black precipitate (ferrosoferric oxide) is filtered off, dissolved in hydrochloric acid, nitric acid added and the whole boiled, and the iron precipitated by ammonia (2. a). From the acidulated filtrate the aluminium is precipitated as above. The alumina and ferric oxide, after weighing, should be treated again with hydrochloric acid, as they frequently contain small quantities of silica, the weight of which must be deducted, and added to that of the silica previously obtained. Similarly, the purity of the silica obtained in the analysis of a silicate should always be tested by carbonate of sodium or hydrofluoric acid: pure silica dissolves entirely in the former, and when dissolved in the latter leaves no residue on evaporation.

In silicate analyses, calculate the silicon and metals separately: the oxygen is determined by difference.

24. Prehnite. (Silicate of calcium and aluminium, containing water of crystallisation.)

The same portion in which the water has been determined by ignition is decomposed by hydrochloric acid, and the silica and aluminium separated as in 23. Air should be excluded as far as possible during the filtration of the alumina, which must be precipitated by sulphide of ammonium: from the filtrate the calcium is precipitated as oxalate (4. a). As the alumina thus separated generally contains both silica and lime, it must, after weighing, be redissolved in hydrochloric acid, evaporated to dryness, and the above process of separation repeated: the weights of silica and calcium obtained being deducted from that of the alumina and added to the former amounts. If iron be present it is separated as in 23. Stilbite (a mineral containing the same elements as prehnite) is analysed in the same manner, excepting that (as in mesotype and most other zeolites) the portion decomposed by hydrochloric acid must not have been ignited.

25. Olivine. (Silicate of iron and magnesium: contains also traces of nickel and manganese, frequently of copper and tin.)

The mineral is decomposed and the silica separated as in 23. The filtrate is saturated with hydrosulphuric acid, which precipitates copper and tin as sulphides: the filtrate from which is concentrated by evaporation, boiled with chlorate of potassium, and the iron precipitated by succinate of ammonium or carbonate of barium (11). The nickel and manganese are then precipitated by sulphide of ammonium, excess of which must be avoided: the precipitate is allowed to subside completely, filtered off, washed with very dilute sulphide of ammonium, and treated on the filter with very dilute hydrochloric acid, which dissolves only the sulphide of manganese. The sulphide of nickel is ignited in the air and weighed as oxide (16): the manganese is precipitated by carbonate of sodium at a boiling heat, ignited, and weighed as Mn604 (11). Finally the magnesium is precipitated by phosphate of sodium (5. a).

26. Felspar. (Silicate of aluminium and potassium).

a. Silicon and Aluminium.-The mineral is very finely powdered, dried at about 200°, and intimately mixed in a platinum crucible with 4 times its weight of a mixture of carbonates of potassium and sodium, and fused for half an hour at a strong red heat. When cool the crucible is placed in a beaker, digested with very dilute hydrochloric acid, the beaker being covered with a glass plate to prevent loss by spurting. When the fused mass is completely removed from the crucible, the contents of the beaker are transferred to a porcelain basin, evaporated to dryness with excess of hydrochloric acid, and the silicon, aluminium (and iron, if present), determined as in 23. If the felspar contain calcium (as labradorite, anorthite), the aluminium is precipitated by sulphide of ammonium, and the calcium from the filtrate by oxalate of ammonium (4. a).

b. Potassium.-A second portion of the powdered mineral is decomposed by hydrofluoric acid in a platinum dish, either by digesting it with the liquid acid, or by exposing it, moistened with

dilute sulphuric acid, to the vapours of the acid evolved from fluorspar and sulphuric acid in an appropriate leaden vessel (p.54). When the decomposition is complete, concentrated sulphuric acid is cautiously added, the whole evaporated to dryness, and heated till all the fluorine and silicon are volatilised, and the excess of sulphuric acid expelled. The residue is moistened with concentrated sulphuric acid, and water added, in which it should dissolve completely, and the aluminium and iron separated as above. The filtrate is evaporated to dryness, heated to expel the ammoniacal salt, a fragment of carbonate of ammonium being placed in the hot crucible to prevent the formation of an acid sulphate; and the residual normal sulphate of potassium weighed. (SO4K2: K2:: 87: 39.) If sodium be present also, the mixed sulphates are converted into chlorides by repeated ignition with chloride of ammonium until their weight is constant, and the metals separated as in 8.

Another method, which, like the preceding one, will serve either for the complete analysis of the felspar, or for the estimation of its alkaline metals only, is to treat the mineral as in (a), substituting carbonate of barium for the alkaline carbonates: or it may be fused in a silver crucible with hydrate of barium. After the separation of the silica, the barium is removed by dilute sulphuric acid (excess of which must be avoided), and the analysis proceeded with as above. It is essential to the success of this method that the mineral be reduced to a perfectly impalpable powder, that the mixture with the flux be as intimate as possible, and that the fusion be effected at an intense white heat. Whenever a sufficient quantity of the mineral is at hand, it is preferable to analyse it in two distinct portions, as directed above.

27. Glass. (Silicate of calcium and potassium or sodium, frequently also of lead; often contains iron, manganese, aluminium, and magnesium).

Glass is analysed in the same manner as felspar. One portion is taken for the determination of all the elements except the alkaline metals, and fused with alkaline carbonates. After the

separation of the silica, the lead is removed by hydrosulphuric or sulphuric acid: chlorine-water is added to the filtrate and then ammonia, which precipitates iron, manganese, aluminium, and perhaps magnesium. The calcium is then precipitated by oxalate of ammonium (4. a), and the filtrate tested for magnesium by phosphate of sodium.

A second portion is decomposed by hydrofluoric acid, or by fusion with carbonate of barium, for the determination of the alkaline metals (26).

28. Augite, Hornblende, Garnet, Idocrase, Epidot. (Silicates of iron, manganese, aluminium, calcium, and magnesium.)

The finely powdered mineral is fused with 4 parts alkaline carbonates, the fused mass treated with hydrochloric acid, with addition of a few drops of nitric acid, and the silica separated as in 26. The filtrate is treated with chlorine-water and ammonia, which precipitates iron, manganese, aluminium, and perhaps magnesium: after which the calcium is precipitated by oxalate of ammonium, and the magnesium by phosphate of sodium. The precipitate by ammonia is dissolved in hydrochloric acid, heated to boiling in order to convert all the manganese into protochloride, largely diluted, and gradually neutralised by carbonate of sodium, with constant stirring: the iron and aluminium are thus precipitated, filtered off, and separated as in 23. From the filtrate, which contains the manganese and magnesium as acid carbonates, the manganese is precipitated by hypochlorite of sodium, as in 11: and the magnesium, after concentration, by phosphate of sodium. Carbonate of barium may also be employed to precipitate the iron and aluminium.

29. Bone-earth. (Normal phosphates of calcium and magnesium, carbonate and fluoride of calcium.)

In order to estimate the amount of bone-earth, or ash, contained in bones, a portion of the bone is carefully cleaned, powdered, digested with water, and dried at 150°. A weighed portion of the

* Garnet, idocrase, and epidot are completely decomposed by hydrochloric acid after ignition.

powder is then ignited until it is perfectly white, when it consists of nothing but bone-earth. Another portion of the dry powder is taken for the determination of the carbonic acid by loss (4. b).

A portion of the perfectly white earth is dissolved in hydrochloric acid, heated for some time to expel all the carbonic acid, saturated with ammonia, the precipitate redissolved in as little hydrochloric acid as possible, and acetate of sodium added. Traces. of iron are generally thus precipitated as phosphate, and must be filtered off and estimated: but if a more abundant white precipitate is formed, dilute hydrochloric acid is added till it is entirely redissolved. From the solution the calcium is then precipitated by oxalate of potassium (4. a): the filtrate is saturated with ammonia, which precipitates all the magnesium (together with the amount of phosphoric acid corresponding to it) as phosphate; this is filtered off, ignited, and weighed, and the rest of the phosphoric acid precipitated from the filtrate by sulphate of magnesium, chloride of ammonium, and ammonia (6. a).

Another method is to heat the bone-earth with moderately strong nitric acid, together with a weighed quantity of pure metallic tin, from two to three times the weight of the earth. All the phosphoric acid in the earth combines with the tin, forming an insoluble compound, which is filtered off after dilution, dried, ignited, and weighed. The excess of weight of this residue over that of the tin employed, represents the phosphorus and oxygen of the phosphoric acid. The calcium and magnesium are determined in the filtrate as usual. A more accurate method of estimating the phosphoric acid is by transferring the insoluble phosphate of tin, washed by decantation, to a platinum dish, dissolving it in the smallest possible quantity of caustic potash, saturating the solution with hydrosulphuric acid, adding sulphide of ammonium, and acidulating feebly with acetic acid. The whole is then transferred to a weighed flask of about 1 litre capacity, diluted with water, and the full flask weighed. After about twelve or sixteen hours' standing, the clear solution is passed through a filter, concentrated, and the phosphoric acid precipitated as phosphate of magnesium and ammonium. The flask is then weighed again, in order to determine the weight of the liquid