a weighed quantity to full redness so long as it loses weight, weighing the residue, and noting it as Cr ̧Ó ̧.

The pink salt is Cr2(SO4)3.

Exp. 10. Compare the reaction between sulphuric acid and chromium trioxide with the reaction between the same acid and sulphur trioxide.

Dissolve about 10 grams of chromium trioxide in 11⁄2 parts by weight of concentrated sulphuric acid and 3 parts of water; place the liquid in a beaker surrounded by cold water, and add alcohol drop by drop until the liquid is green; then add a considerable quantity of alcohol; collect the crystals which form and press them between filter paper, and prove, as far as can be done by qualitative reactions, that the salt is a sulphate of chromium.

Now dissolve sulphur trioxide in concentrated sulphuric acid until the liquid fumes strongly in the air, and set the liquid in an exsiccator over concentrated sulphuric acid. After some time colourless crystals separate; drain these from the liquor; examine their reactions qualitatively, and prove that the substance thus obtained is an acid.

The acid is disulphuric H,S,O7.

The results of Exps. 7 to 10 shew that SO, and CrO, are acidic oxides; that CrO, also interacts with acids to form salts the compositions of which shew that they correspond to the oxide Cr2O; but that SO, does not exhibit any basic functions.

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Exp. 11. Prepare chromic chloride and compare it with sulphur chloride. Dissolve a quantity of moist Cr2O ̧ . xH ̧O (8. Exp. 9) in hot concentrated hydrochloric acid; evaporate until the liquid is thick, and then cool; press the solid which separates between paper; transfer it to a tube of hard glass and heat to about 250° in a stream of dry chlorine until a lilaccoloured, hard, solid remains. Now raise the temperature to full redness, maintaining a rapid stream of chlorine; let the end of the tube pass into a small weighed bottle; continue to heat strongly until some of the chromic chloride formed is sublimed into the bottle; remove the bottle and at once close it with a good stopper.

The compound thus formed is chromic chloride, Cr2Cl.

Compare the composition as determined by your analyses,

the method of formation, and the reactions towards water and acids, of chromic chloride with those of sulphur chloride (s. Exp. 5, Chap. I. Part II.).

III. The elements Iron and Copper belong to different groups. The following experiments are designed to illustrate some of the differences between these metals.

Exp. 12. Prepare oxides of the two metals, and determine their compositions. Strongly heat a few grams of finely divided copper in a stream of oxygen so long as any visible change occurs; then heat the product for some time in the air, and allow to cool in an exsiccator.

iron.

Perform a similar experiment with very finely divided

Estimate the copper in a weighed portion of the copper oxide, either (1) by dissolving in hydrochloric acid, precipitating CuOH, by addition of potash, boiling, washing, heating, and weighing as CuO; or (2) by dissolving in nitric acid, adding ammonia, and titrating with a standardised solution of potassium cyanide.

Assuming the sole product of the reaction of oxygen with hot copper to be an oxide of copper, find the simplest formula which expresses the composition of this oxide (Cu = 63.2, 0 = 16).

Estimate the iron in a weighed portion of the iron oxide prepared, by dissolving in hydrochloric acid, reducing the ferric chloride so formed to ferrous chloride by means of stannous chloride, and titrating with a standardised solution of potassium dichromate. Assuming the sole product of the interaction of iron and oxygen at a high temperature to be an oxide of iron, find the simplest formula which expresses the composition of this oxide (Fe=56, O=16).

Exp. 13. Determine the temperatures at which reduction of the oxides CuO and Fe,O, by CO begins.

[Before beginning this Exp. read the paper by Wright and Luff in C. S. JOURNAL, TRANS. for 1878, p. 1.]

Prepare 10 or 15 grams of CuO, and about the same quantity of Fe,O,, by adding pure potash to CuSO,Aq and Fe,Cl Aq, respectively, boiling, washing the pps. thoroughly by hot water, drying and heating over a Bunsen-lamp. Place about 5 gram of the copper oxide and the ferric oxide thus

prepared in test tubes connected with potash bulbs containing clear, freshly prepared, baryta water; immerse the tubes in a paraffin bath; pass pure carbon monoxide over each oxide, and determine, in each case, (1) the temperature at which a faint turbidity is produced in the baryta water, i.e. the temperature at which the reduction of the metallic oxide by carbon monoxide begins, and (2) the temperature at which a considerable turbidity is produced by a few bubbles of the gas issuing from the tubes, i.e. the temperature at which the reducing action of the carbon monoxide is well marked.

For details of the method of procedure reference must be made to the paper referred to above.

The results of this Exp. shew that cupric oxide is reduced by carbon monoxide at a temperature considerably lower than that at which ferric oxide is reduced.

Exp. 14. Examine the reactions between steam and the metals copper and iron.

Place about 10 grams of the oxides CuO and Fe,O,, prepared in the last Exp., in tubes of hard glass, each of which is connected with an apparatus for preparing pure dry hydrogen. The hydrogen should be prepared by the reaction of pure dilute sulphuric acid with redistilled zinc, and should be purified from any hydrides of sulphur, carbon, phosphorus, and arsenic, by passage through solutions of lead acetate, and copper sulphate made strongly alkaline by potash. Heat the oxides in the hydrogen-stream until reduction to metal is complete. Then remove the hydrogen apparatus; replace it by a flask containing hot water; fit corks and exit tubes into the other ends of the tubes containing the metals; arrange the tubes so that each passes through a small iron box fitted with a thermometer; pass steam over each metal, and roughly compare the temperatures at which the steam is decomposed, with production of hydrogen, by the two metals.

Iron decomposes steam fairly rapidly at a low red heat; copper, only at a full red heat, and then slowly.

Exp. 15. Prepare sulphates of the two metals, determine their compositions, and compare the reaction of each with an oxidising agent.

Dissolve about 20-30 grams of cupric oxide in warm dilute sulphuric acid; evaporate to the crystallising point; pour off the mother liquor from the crystals which form on cooling;

recrystallise two or three times from water, and dry by pressing between filter paper.

Estimate the copper in a weighed quantity of these crystals. Also estimate the water by heating to 210°-220° and weighing the dehydrated copper sulphate which remains. Before estimating the water prove that the crystals do not evolve sulphuric acid when heated over a Bunsen-lamp.

Prepare crystals of ferrous sulphate by adding iron filings little by little to warm dilute sulphuric acid until no more iron is dissolved, then adding a little more iron, boiling for a few minutes, filtering into a beaker which has been rinsed out with a little concentrated sulphuric acid (this dissolves any basic sulphate that is formed during evaporation), adding alcohol to the warm liquid, stirring well, collecting the pp. on a filter, washing with alcohol, spreading out on filter paper, and exposing to the air of a moderately warm room until the smell of alcohol is quite gone. Preserve the crystals in a well stoppered bottle.

Estimate iron in a weighed quantity of the crystals by dissolving in water, adding sulphuric acid, and at once titrating with standardised permanganate solution. Heat some of the crystals over a Bunsen-lamp and prove that sulphuric acid is evolved. The water cannot therefore be estimated by determining the loss of weight which the crystals undergo when heated. Mix a weighed quantity of the crystals with 2 or 3 times their weight of dry pure lead monoxide; place the mixture in a short tube of hard glass, and cover it with a layer of lead monoxide; weigh the tube and its contents; heat to about 300° until moisture is no longer given off; cool in an exsiccator, and weigh again. Repeat the heating until the weight of the tube is constant. (The lead oxide combines with the sulphuric acid produced by heating, and forms lead sulphatePbSO-which is unchanged at the temperature of the experiment.)

Find the simplest formulae which express the compositions of the two sulphates you have prepared (Fe = 56, Cu = 63.2, O 16, S=32.)

The sulphates are CuSO4.5H2O, and FeSO4.7H2O, respectively.

Now roughly weigh out a few grams of the copper sulphate you have prepared; dissolve in water; add about as much concentrated sulphuric acid as the weight of the copper sulphate taken; boil, and add concentrated nitric acid drop by

M. P. C.

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drop; evaporate, and crystallise; recrystallise from water; dry the crystals obtained, and determine copper in them. These crystals contain the same percentage of copper as those with which you began the Exp.; their appearance and prominent physical properties are also the same as those of the original crystals.

Copper sulphate CuSO,.5H,O is unchanged by the action. of oxidising agents.

To a solution in water of 10 or 15 grams of the ferrous sulphate you have prepared add about the weight of the salt of concentrated sulphuric acid; then heat to boiling, and add concentrated nitric acid drop by drop until the colour of the liquid is a clear reddish yellow; now evaporate until the liquid gets thickish; then allow to cool, and pour off the liquid from the semi-solid mass which remains; heat the residue over a very low Bunsen-flame until it is quite dry, and sulphuric acid is no longer evolved.

The appearance, and comparative insolubility in water, of the solid thus produced shew that it is not ferrous sulphate. Dissolve an accurately weighed quantity of this solid (about 5 gram) in dilute sulphuric acid; add a few pieces of pure zinc, and allow the reaction to proceed until the liquid is almost colourless, and all zinc is dissolved; then determine the iron by titration with permanganate. Assuming the salt to be anhydrous, and to be a sulphate of iron, calculate its composition, and find the simplest formula which will express this composition.

The salt prepared by the interaction of nitric and sulphuric acids with ferrous sulphate is ferric sulphate, Fe2(SO4): the nitric acid supplies

oxygen;

2 (FeO. SO3)+0+SO2 · H2O=Fe2O3.3SO3+H2O.

Before beginning Part III. the student should work out a number of fairly difficult quantitative analyses; he should also go through a systematic course of preparations of organic compounds; and a course of gas-analysis. [He may also, if time permit, perform a few qualitative analyses of complex mixtures of salts including the salts of the rare elements.]

While the student is performing the experiments described in Part III. he should also be working at practical physics; in his work in the physical laboratory he should pay particular attention to determinations of (1) refractive indices of organic compounds, (2) specific rotatory powers of organic compounds, (3) wave-lengths and also mapping spectra, (4) quantities of heat, (5) electrical conductivities of solutions of acids and salts, (6) electrolytic depositions of different metals, (7) crystallographic forms.