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 + H2S = CuS + H2SO.

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 + 1122 + 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.

PART II.

BLOW-PIPE ANALYSIS.-PRELIMINARY

EXAMINATION.

54. Blow-pipe reactions. How to use the blow-pipe.*

Close the holes at the foot of the Bunsen lamp (Fig. 13) so as to exclude air, and thus obtain a luminous flame. Now place the nozzle of the blow-pipe in the centre of the flame, and blow gently through the tube: observe that the flame produced is blue and corresponds to the non-luminous flame of the Bunsen lamp. This is the oxidizing or outer flame of the blow-pipe.

Now hold the nozzle of the blow-pipe just outside the luminous gas flame, and blow gently: observe that the flame is partly yellow. This is the reducing or inner flame of the blow-pipe (Fig. 14).

The oxidizing flame is used when a substance has to be oxidized, the reducing flame when a body has to be reduced, e.g. from a salt to the metallic state.

*The student should be shown, once for all, the different uses of the blow-pipe, and then be allowed to practise on several different sub stances.

EXAMPLES.

1. Reduction. Mix together in a mortar equal small quantities of dry sodium bicarbonate and silver nitrate. Place a portion in a little hollow scooped out of a

a

α

FIG. 13.

FIG. 14.

sound piece of charcoal, and heat in the reducing flame of the blow-pipe. Observe the bright metallic bead of silver obtained, dissolve it in nitric acid, and precipitate it as chloride with a few drops of HCI.

2. Oxidation.-(a) Make a small loop on the end ot a piece of platinum wire, heat it, and dip it while hot in some sodium bicarbonate, so as to cause a small quantity to adhere to the wire; now heat it with the blow-pipe flame until it is fused.* Then place on it a minute quantity of any manganese compound, and heat again in the oxidizing flame of the blow-pipe; by

In a similar way, beads are also made with borax or microcosmic salt, instead of sodium bicarbonate.