soften the wax, which may then be rubbed off, and observe that where the wax was scratched the glass is etched, while the part protected by wax has not been attacked.

Avoid breathing the fumes, as the gas is very irritating to the lungs, and acts powerfully on the skin, producing painful wounds.

48 Preparation of sulphur dioxide.

When sulphuric acid acts upon copper, copper sulpeate, sulphur dioxide, and water are produced. Thus :

Cu + 2 H2SO4 = CuSO4 + SO2 + 2 H2O. 63.5+ 196 = 159'5 +64 + 36. Place some copper turnings in a flask of about half a litre in capacity, provided with a funnel and conducting tube bent twice at right angles. Pour enough strong sulphuric acid down the funnel tube to cover the copper, and apply heat, collecting the gas, which is heavier than air, by downward displacement (see Fig. 10). Fill two jars with the gas, and then allow it to pass, first into a test tube containing nitric acid, and then into one containing a solution of potassium chromate. Observe in the first case the brown vapours of oxides of nitrogen which are given off, and in the second the change of colour from yellow to green. In both cases the sulphur dioxide has acted as a reducing agent, reducing the nitric acid to a lower stage of oxidation, and the potassium chromate to a chromium salt of a green colour; while in each case the sulphur dioxide is oxidized to sulphuric acid, as may be seen

by adding barium chloride, which will precipitate white barium sulphate, insoluble in hydrochloric acid. 49. Properties of sulphur dioxide.

(a) Sulphur dioxide does not support combustion, and reddens litmus solution.

Place a lighted taper in a jar of the gas and observe that it is at once extinguished. Then add a solution of litmus, and observe the bright red colour produced.

(6) Sulphur dioxide is extremely soluble in water.

Invert a jar of the gas in the pneumatic trough, and observe the rapid rise of water inside the jar, produced by the absorption of the gas.

50. Preparation of sulphuretted hydrogen.

When sulphuric acid acts upon ferrous sulphide, ferrous sulphate and sulphuretted hydrogen are produced. Thus :

FeS + H2SO4 = FeSO4 + H2S.

Place a few pieces of ferrous sulphide in a small flask provided with a funnel and conducting tube, the latter of which passes air-tight into a small flask containing water (to collect any impurities which may pass over), and having a second tube fitted into it and bent so as to pass into a flask containing water (see figure). Now cover the ferrous sulphide with a layer of water, and add a few drops of strong sulphuric acid: observe the effervescence which soon begins, and the disagreeable and characteristic smell which the water in the flask soon possesses from

When the water smells then decant off the acid

the solution of the gas in it. strongly, remove the flask;

from the ferrous sulphide, wash it two or three times with water, retaining it in the flask so that it may be used again by simply adding fresh acid.

51. Properties of sulphuretted hydrogen.

Sulphuretted hydrogen precipitates the solutions of salts of certain metals in an acid solution, others in an alkaline solution, and does not, under any circumstances, precipitate the remainder; thus, solutions of copper salts are precipitated in an acid solution, solutions of iron salts in an alkaline one, and solutions of sodium salts are not precipitated at all. Thus:

Copper sulphate and sulphuretted hydrogen yield copper sulphide and sulphuric acid :

CuSO4 + H,S= CuS + H2SO4.

Iron sulphate and potash and sulphuretted hydrogen yield iron sulphide, potassium sulphate, and water. FeSO4 + 2 KHO + H2S

=

=

FeS + K2SO4 + 2 H2O. 88 +1742 + 36.

152 + 112'2 +34 Place in a test glass a solution of copper sulphate, in another a solution of iron sulphate, and in a third a solution of sodium chloride; to each add a few drops of hydrochloric acid, and then a little sulphuretted hydrogen water. Observe the black precipitate of copper sulphide in the first glass, and no precipitate in the other two glasses. To each of these add a little potash solution, and observe the black precipitate of ferrous sulphide in the one case, and the absence of a precipitate in the other.

52. Preparation of sodium hydrate (caustic soda). When caustic lime is added to a solution of sodium bicarbonate, caustic soda and calcium carbonate are produced. Thus :

Dissolve about 40 grams of sodium bicarbonate in about half a litre of hot water. Then weigh out 30 grams of quick lime, slake it with water, and when it is thoroughly slaked, stir it up with more water so as to obtain a milky fluid having lime in suspension. Add this to the hot solution of sodium bicarbonate, and boil for a few minutes. Withdraw the lamp, allow the precipitate to subside, and observe if a small portion of the clear liquid effervesce when hydrochloric acid is added to it. If so, there is still some sodium

bicarbonate unconverted into caustic soda, and more lime must, therefore, be added. If, on the other hand, no effervescence occurs, the decomposition is complete, and the clear liquid is then evaporated in a clean iron or silver dish to dryness. The resulting white substance is sodium hydrate (NaHO) or caustic soda.

53. Properties of sodium hydrate.

Sodium hydrate is a powerful alkali, and turns red litmus solution blue. When hydrochloric acid is added to it, it is neutralized, and sodium chloride (common salt) formed. (See 39.)

Sodium hydrate and hydrochloric acid yield sodium chloride and water:

Dissolve some of the caustic soda obtained in 52, in water, and add to it a solution of reddened litmus, and observe the change in colour from red to blue. Dissolve a second portion in water, and add to it hydrochloric acid by degrees, until a drop of the liquid taken out on a glass rod ceases to colour litmus paper blue. On evaporating the liquid thus obtained to a small bulk, sodium chloride will separate out.