solution in sulphuric acid and titration with standard potassium permanganate. Report the result as percentage of calcium oxide (CaO).

Mg.-Concentrate the combined filtrates from the calcium precipitate to about 150 c.cs. by gently boiling in a beaker. Transfer to a porcelain evaporating basin and evaporate to dryness, and then cautiously heated over a naked bunsen to expel the ammonia salts. When cool, dissolve the residue in 5E. HCI (about 20 c.cs.). Heat and filter through a small paper, washing out the dish and then the paper with hot water, and bringing the filtrate up to about 150 c.cs.

When cold (cool under the tap), carefully add about 40 c.cs. E. Na2HPO4. After adding the phosphate, stir with a glass rod (preferably of thin glass tube, closed and grounded at one end), and avoid touching the sides of the beaker with the rod, or the crystals of MgNH,PO will adhere firmly to the beaker, and cause subsequent trouble. Cold and frequent agitation assist the precipitation, and if convenient the beaker may be set in a dish of iced water or in an ice chest. Allow to stand for six to twelve hours to complete the precipitation. After standing, test if the precipitation is complete or not by removing about 5 c.c. in a test tube, and adding about 5 c.c. magnesia mixture (made by dissolving 1 gm. MgSO, and 1 gm. NH4Cl in 8 c.cs. H2O, and adding 3 c.cs. 20 E. NH,HO. If a precipitate forms, it shows that sufficient Na,HPO, has been added to the liquid; but if no precipitate forms, add 10 c.cs. more of the E. Na,HPO. Stir and let stand a few hours. When precipitation is complete, filter. Wash with dilute NH HO (1 part 20 E. NH HO to 3 parts H2O) till no reaction is obtained for chlorides (acidify a few drops of the washings with pure HNO, and test with AgNO3). Dry the filter and precipitate. Transfer the precipitate to a crucible (preferably platinum), and incinerate the filter on a platinum wire. If, as sometimes happens, difficulty is met with in incinerating the paper, it should, before ignition, be moistened with 5 E. NH.NO, solution, dried, and then incinerated.

Transfer the filter ash to the crucible, and heat it and the contents cautiously with a bunsen, using a small flame. When no more ammonia fumes (detect by smell or acid on a glass rod) come off, raise the heat to a dull red for ten minutes, and then heat to a bright red over the blast for another ten minutes. Remove in the desiccator, cool and weigh. Repeat the ignition under the blast till the weight is constant within the required limits. If the precipitate is discoloured or black, moisten with a drop or two of strong HNO, and re-ignite. After deduction of the ash, calculate from the weight of Mg,PO, the percentage of magnesium oxide in the mineral.

Summarise the combined results, adding up the percentages of H2O, CO2, SiO2, FeO, Al,O,, CaO, and MgO. Run a duplicate, repeating any doubtful estimations, and check the total results.

(d) THE ANALYSIS (partial) OF AN INSOLUBLE SILICATE.

For analysis the student may procure a sample of a felspar such as 'orthoclase,' or a sample of one of the more common igneous rocks such as granite or syenite.

The complete analysis of a rock involves the determination of approximately ten to twenty-five components as follows:-SiO2, Al,O, Fe2OŽ, FeO, MgO, CHO, Na,O, K,O, H2O, and small quantities of certain of the followingTiO2, ZrO2, P2O5, CO2, SO3, Cl, S, C, Cr2O, V2O, NiO, COO, CuO, MnO, Sro, BaO, Li,O.

It is only intended here to present instructions for a partial analysis of a complex silicate, as the complete analysis is beyond the student's present capabilities, and should not be attempted till the end of his third year.

As

a partial analysis, though accurately conducted, cannot represent the true composition of the silicate, some brief notes will be added, after summarising the methods used, indicating where error will probably occur, and what may be the extent of such error. When the degree and probability of error are approximately known, the partial analysis becomes to a certain extent reliable; but if the analysis is required for exacting mineralogical or petrological work, a partial analysis is of no great value, and must be replaced by a complete analysis based on the lines laid down by Dr Hillebrand in his paper entitled "Some Principles and Methods of Rock Analysis," and published as Bulletin No. 176, U.S. Geol. Survey. With the purest reagents, a liberal supply of platinum vessels, a clean laboratory, and proper facilities for working, he requires that the results of a complete analysis when summed up shall range between 99.75 and 100.50. The student who wishes an ideal of high-class analytical work is strongly advised to refer to the bulletin mentioned. The high degree of accuracy therein demanded can be obtained only by continued practice and a thorough knowledge of theoretical chemistry, more especially as applied to the properties (chiefly the solubility) of the various components of the rock or other silicate, and of the precipitates obtained in the analysis.

Method of Analysis. In the partial analysis the following components will be estimated, subject to the errors to be mentioned :-SiO2, Al2O3, Fe2O3, FeO, CaO, MgO, NaO, KO, and H2O.

The moisture (hygroscopic) is first estimated by drying a weighed quantity of the finely ground ore at 110° C. Then the powder is ignited at a red heat, and the result reported as "loss on ignition" (chiefly combined water). The powder is then fused with Na2CO3, or Na,CO,K¿CO, and the fused mass dissolved in dilute HCl, evaporated to dryness, taken up with dilute HCl, the SiO, filtered off and estimated gravimetrically.

In the solution the Al2O3, Fe as Fe,O,, CaO, and MgO are estimated as usual. The FeO is estimated by dissolving a fresh portion of the powder in HF and dilute H2SO, in an atmosphere of CO2, and titrating direct with KMnO, the ferrous iron alone being estimated by this method.

The Na,O and K2O are separated from their silicates (J. Lawrence Smith's method), heating to a red heat a mixture of one part rock, three parts CaCO3, and one half part NH Cl. On digesting the mass in hot water the solution contains CaCl, NaCl, KCl, and Ca(OH)2. The lime and magnesia are separated, and the joint chlorides are then estimated, after which they are dissolved, and the potassium precipitated as K,PtCl, and from this the KO is calculated. Na2O is then estimated by difference, the calculation being given later on.

Probable Errors. The ignition loss should not be reported as H2O. It will include any CO2 and SO2. Also any trace of FeS, present will be oxidised to Fe2O, thereby introducing slight error.

Again, if the TiO, be not separately estimated, part of it is included with the SiO2, and most of it with the Al,O. The TiO, may amount to 2% or 3%, though on the average it is, along with P2O5, about 8% (as estimated by Prof. F. W. Clarke). Also any P2O is estimated along with the Al,Og. Therefore

the Al,O may be overestimated by 2 or 3%; on the average, ·8%.

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Further, BaO and SrO are nearly always present, and to the extent sometimes of over 1%, the error falling on the CaO and MgO.

Other errors due to the non-estimation of SO, Cl, MnO, etc., will also occur, though these errors are, as a rule, not so serious as those first mentioned. Briefly, with a summation of results between 99.5 and 101, there may be an error of several per cent. even in single estimations.

Of course the true value of the error cannot be determined without further

analyses, therefore the student must accept the results obtained with the more or less vague qualifications stated.

The Analysis.-The first step in the analysis is the preparation of the sample, which is obtained in the usual manner. A sample of about 10 gms. is then reduced in a large diamond mortar. About 5 gms. of the coarse powder is sampled, and then ground to an impalpable powder in a large agate mortar (10 cm. diameter is a convenient size). About 25 gm. at a time is transferred to the agate and ground till, on rubbing a little of the powder between the tips of the fingers, not the least grit is felt, and it rubs off smoothly and impalpably. This process is tedious, but an extra half-hour here will save subsequent trouble on fusion and other operations.

Moisture (Hygroscopic).-Weigh out Î gm. of the finely powdered silicate as follows:-Counterbalance with the weights a large platinum crucible (25 c.cs. or more). Place an additional 1 gm. weight on the pan with the weights, then proceed to add to the crucible approximately 1 gm. of the powder, either adjusting to the exact weight or weighing exactly with the rider (the latter saves time). At the same time weigh out two lots of 5 gm. and 1 gm. respectively. Transfer to watch glasses, cover with funnels, and reserve for the determination of the alkalies and ferrous iron.

Transfer the platinum crucible and contents to the oven at 110° C. Heat for one hour. Remove, cool, and weigh. Reheat for fifteen minutes. Again weigh, and repeat the heating till constant to the usual limits. resulting difference in weight as percentage of moisture.

Report the

Ignition Loss.-Transfer the crucible to a suitable triangle (either platinum wire or pipeclay wrapped in platinum foil) on a tripod. Ignite for fifteen minutes over the bunsen at a red heat, and then for ten minutes over the blast. Remove as usual. Cool and weigh. Blast again for another ten minutes. Again weigh, and repeat till constant as required. Report the difference caused by this operation as percentage of "Ignition Loss " Ignition Loss" (mostly combined water).

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SiO2.-Transfer the contents of the crucible to a small glass mortar. Add 6 gms. pure anhydrous Na,CO (Merck's guaranteed), and mix thoroughly and very intimately by grinding with the pestle. Transfer the mixture to the crucible, which is then placed on the triangle, and the lid placed on at an angle, so that the contents will be visible during fusion. The Na2CO3 used should be afterwards checked by running a blank fusion on the carbonate alone, and estimating the SiO, Al,Og, and CaO in the flux.

Proceed at first to heat the crucible with a bunsen, and when it shows signs of melting round the edges apply the blast, directing the flame at an angle against the side and bottom. Regulate the blast so that the charge does not boil over. Continue heating till the contents of the crucible are in a state of quiet fusion, the charge having subsided into a clear liquid at the bottom of the crucible. Remove the flame, and when nearly solidifying, grasp the triangle in the forceps and give the crucible a gentle motion to swirl the contents up the sides. Continue the motion till the mass solidifies. A bluish green coloured mass indicates manganese, which however may be present even when this indication is absent.

Transfer the crucible and lid to a 15 cm. porcelain evaporating dish. Cover with an inverted glass funnel. Dissolve the melt by pouring down outer edge of the funnel excess of 5E. HCl. When the mass seems completely detached from the crucible, rinse the funnel into the dish, and remove the crucible and lid with the aid of a glass rod. The crucible is raised up the side of the basin, washed well outside with hot water, then taken in the fingers and the interior rinsed out; the lid is treated similarly. If the

fusion has been complete there should be no gritty particles at the bottom of the dish.

The SiO, in solution must now be separated by evaporation. According to Dr Hillebrand this is best done as follows. Evaporate the solution till almost dry. Break up the gelatinous mass with a glass or platinum rod to facilitate drying, and continue till just dry. Take up with about 100 c.cs. E. HCl. Filter and wash with hot water till free from chlorides. Reserve the precipitate and evaporate the filtrate to dryness in an evaporating dish. Again take up with 100 c.cs. E. HCl. Filter and wash as before. The remainder of the SiO is thus separated out. Reserve the filtrate.

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Dry the two filters and precipitates and ignite them in a weighed platinum crucible, first at a dull red, and then for about twenty minutes over the blast. As a check, weigh the empty crucible after the last weighing, and note any change in the weight of the crucible.

Add

As a further check, transfer the precipitate back to the crucible. about 10 c.cs. strong HF and 3 or 4 drops 36E. H2SO4. (The HF can be obtained pure in ceresine bottles; if impure, a blank must be run for residue.) Evaporate in a fume cupboard and add fresh portions of HF as the evaporation goes on. Evaporate to dryness. Ignite and weigh. Repeat the process till constant. The loss in weight is SiO2.

For the present the student may report the total percentage of SiO2 and residue, that is, the weight of the first precipitate as SiO2, etc.

Al2O(FeO3), etc.-The joint Al,O,Fe,Og precipitate will be determined as usual, and the Al2O, estimated by difference, throwing the burden of error on the alumina.

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If manganese is present precipitate the iron and alumina by the Basic Acetate method, but, if absent, by the Ammonia precipitation.

To the filtrate from the SiO2, NH,HO and NH4Cl are added as usual to precipitate the iron and alumina. The precipitate is filtered off, dissolved and reprecipitated. Combine the filtrates and reserve the combined filtrate.

Dry the precipitate and ignite separately the filter and precipitate. Combine and blast for five or ten minutes. Weigh and repeat till constant. The iron present is now determined as follows:-To the precipitate in the platinum crucible add about 5 gms. KHSO, and fuse gently over a good rose-bunsen for 2 to 4 hours. When the precipitate is dissolved, remove and swirl the mass when cooling. Take up the melt in a 200 c.c. beaker with about 100 c.cs. hot E. H2SO4. When dissolved, filter and wash well. Reduce the filtrate by a brisk current of HS, heating to boiling meanwhile. Whilst this is going on fit up a 'Kipp' to generate CO2, and lead its delivery tube into a flask which is to be set under a funnel to catch the filtrate from the HS solution. Continue passing HS for a short time after boiling. Filter quickly into the flask, washing quickly. Remove the flask and 'Kipp' (still passing CO2), and heat the flask to boiling to expel HS. When expelled, cool under the tap (still passing CO2) and when cool titrate with KMn2OË. Calculate the result to Fe2O3. From this will have to be subtracted the quantity of Fe,O, equivalent to the ferrous iron (FeO) in the portion analysed (see following). The difference is the FeO present. The Al,O, etc. is calculated by deducting the total Fe,O, (from the titration) from the weight of the joint oxide precipitate. Report the result as percentage of Al,Og, etc.

The Fe2O, cannot be reported until the FeO determination has been made. CaO, etc., MgO, etc.-In the following precipitation a small quantity of Sro and traces of BaO will probably be estimated with the CaO.

the

Evaporate the combined filtrate from the iron and alumina precipitation to dryness and heat to expel the excess of ammonia salts. Take up residue with about 20 c.cs. E. HCl and 100 c.cs. water. Filter if necessary, and estimate the calcium as instructed in the analysis of dolomite, precipitating twice and reserving the filtrate for the estimation of the magnesium. Report the result in percentage of CaO, etc.

The magnesium in the filtrate is then estimated as before by precipitation with Na,HPO4, and the result reported as percentage of MgO, etc. Hillebrand recommends that the phosphate solution (NaNH,HPO4) should be added to the acid solution of magnesium, and that then ÑÍHO should be added in slight excess. To facilitate the ignition he recommends Mastbaum's method of applying the full flame to the precipitate wrapped in its paper (in the crucible), and when most of the carbon is burned off the mass is moistened with 3 or 4 drops of 16E. HNO3, evaporated carefully, heated with the full burner for a few minutes and then with the blast for half a minute.

FeO.-Transfer the 1 gm. portion (previously reserved) of powder to a large platinum crucible, moisten with 36E. H2SO4, and place in a suitable sized hole on the water bath. Arrange a Kipp' to deliver CO2. Round the

crucible place a cylinder of tin about 8 cm. high, and open at both ends. Lead the CO2 delivery tube to within 2 cm. of the top of the crucible. Raise the cylinder. Pour into the crucible about 20 c.cs. HF. Enclose with the cylinder. Pass a steady stream of CO2. Heat the water in the bath, and lightly cover the top of the cylinder with a small piece of lead foil. When boiling commences the gas current may be slackened. Continue the heating for about 40 minutes. Remove the flame, and cool the crucible by quickly transferring the crucible and fittings to a porcelain basin containing a little cold water.

When cool, empty the containing 50 c.cs. water. with standard K,Mn,Og. percentage of FeO.

contents of the crucible into a 200 c.c. beaker Wash out the crucible, and titrate the solution Calculate the result to FeO, and report as

Calculate the percentage of FeO to Fe2O3, and deduct from the percentage of FeO previously found, and the result is the percentage of Fe, in the rock.

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Na2O, K2O.-The third portion of the finely ground rock is now taken and carefully and intimately ground with 5 gm. pure NH4Cl (prepared by neutralising pure HCl with NH,HO, evaporating to dryness and recrystallizing). When thoroughly mixed, add 4 gms. pure CaCO (prepared by dissolving and precipitating the CaCO, from pure crystalline calcite). Continue grinding till a thorough mixture is obtained. Transfer to a covered platinum crucible (20 to 30 c.c.).

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Through a piece of asbestos board cut a hole so that the crucible will project down through it by about two-fifths its length. Set on a tripod, and under it bring a quadruple bunsen burner. Heat at first with a low

soon as an odour of

flame about 3 cm. away from the crucible, and as ammonia ceases increase the heat so that the bottom of the crucible is at a dull red heat. Continue at this temperature for 50 minutes.

Remove the crucible lid and contents. Detach the sintered cake into a 100 c.c. beaker, softening the mass with water if necessary. Wash out the crucible and lid with a little hot water. Heat on the hot plate, breaking up any lumps with a glass rod. Filter and wash by decantation till free from chlorides. (The residue should completely dissolve on testing with excess of HCl.)