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centimetres beneath the surface of water; in the event of the water being sucked back, it rises into the neck of the retort, and falls into the bulb, allowing air to pass up.

218. A solution of hydrobromic acid may be conveniently made by the following synthetical process, viz. by passing a mixture of hydrogen and bromine vapour over a red-hot spiral of platinum wire. For this purpose a glass tube, about 18 centimetres long and 2 centimetres wide, is fitted at each end with an india-rubber cork, carrying a short straight piece of narrow tube. Through each cork a stout iron wire is passed. These wires are joined together by a short spiral of platinum wire, about 20 millimetres long. To put the apparatus together, one of the corks is inserted, with the wires attached (see fig. 72),

FIG. 72.

and the projecting wire pushed into a pin-hole previously made in the other cork. When the wire is through the cork, the projecting end can be held by a pair of pliers, and the cork drawn along to its place in the glass tube. One end of the apparatus is connected to a small wash-bottle containing a quantity of bromine, through which a stream of hydrogen can be made to bubble. The other end may be attached to a Woulf's bottle containing water, or simply to a delivery tube dipping into a beaker of water. Hydrogen is first slowly passed through the apparatus for one or two minutes, to sweep out the air, and the little platinum spiral is then heated to bright redness by means of an electric current. Complete combination of the hydrogen and bromine at once takes place in contact with the hot wire, and perfectly colourless hydrobromic acid gas passes out from the tube, along with an excess of hydrogen. The supply of bromine vapour may be regulated by standing the bottle containing the bromine in warm water. About 60°C. is a convenient temperature, and it will be seen

that under these circumstances there is very little hydrogen escaping. So long as even a slight excess of hydrogen is

passing (which is.

readily seen by the escape of bubbles through the water in the absorbing vessel), the issuing hydrobro

mic acid gas will re

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main perfectly colourless and free from bromine vapour; it is therefore not necessary to scrub the gas through a tube containing red phosphorus. When the reaction is going pretty rapidly, a lambent flame occasionally makes its appearance in the neighbourhood of the platinum wire, but it shows no tendency to strike back along the narrow tube into the bromine bottle. If desired, however, the additional precaution may be taken of plugging this fine tube with a little piece of glass wool, which renders any inconvenience from this cause quite impossible.

219. Hydrobromic acid in dilute solution may be prepared by passing a stream of sulphuretted hydrogen through bromine water, and filtering the solution to remove the precipitated sulphur.

220. To show the solubility of hydrobromic acid in water. A cylinder is filled with the gas by displacement, and covered with a thick glass plate. On removing the plate under water, the water will rush into the cylinder and completely fill it.

221. To show the action of chlorine upon hydrobromic acid. Two cylinders, one filled with gaseous hydrobromic acid gas, and the other with chlorine, are placed mouth to mouth, and the plates withdrawn. When the gases mix, the brown vapour of bromine makes its appearance, and on inverting the cylinders, in order to mix the gases thoroughly, the action will be complete.

IODINE

222. Preparation from potassium iodide. 2KI+MnO2 +2H2SO,=K2SO, + MnSO, + 2H2O+I. This experiment is conducted in the manner described for the preparation of bromine. In this case it is not necessary to cool the receiver.

223. To show the colour of iodine vapour. A few crystals are dropped into a large clean flask, which is warmed by a Bunsen. A smaller flask may be heated all over, and placed in the beam of the electric or lime light, and a few crystals of iodine introduced, when the violet colour of the vapour will be manifest on the screen.

224. To show the dichroism of iodine vapour. A small quantity of iodine, about 25 gram, is put into a test tube, 15 x 2 centimetres, and the tube drawn out and hermetically sealed. The tube is hung by a wire in a horizontal position in front of the lamp, and a narrow ray of light from a slit passed through it, an image of the slit being focussed upon the screen. On heating the tube the iodine vapourises, and the reddish violet colour will be seen. If the heat be continued, the colour loses its red tinge, and becomes a deep blue. Care must be taken to avoid having an excess of iodine in the tube, or before the blue becomes visible the quantity of iodine vapour present may render the tube practically opaque to the beam of light.

225. To show the action of iodine upon mercury. A small quantity of mercury and iodine are placed in a test tube and heated together. Combination takes place, and the iodide of mercury sublimes in the form of red and yellow crystals.

226. To show the action of iodine upon phosphorus. A few crystals of iodine are brought in contact with a fragment of phosphorus upon a block of wood. The phosphorus at once inflames.

227. To show that phosphorus and iodine do not react if the temperature is below that at which iodine gives off vapour. A fragment of phosphorus and a few crystals of iodine in separate test tubes are cooled by being placed in a freezing mixture of ice and calcium chloride. The cooled substances may then be mixed together and no combination takes place. If the tube containing the two elements be removed from the freezing mixture and allowed to become warm, combination takes place.

228. The action of iodine upon starch may be shown by adding a few drops of a dilute solution of iodine, either in water or potassium iodide, to a quantity of dilute starch emulsion contained in a large glass vessel.

The starch solution is prepared by mixing a few grams of clean white starch with a small quantity of cold water, to the consistency of a thin cream, and then rapidly pouring upon it a large excess of boiling distilled water. The operation is best performed in a beaker capable of holding forty or fifty times ast much water as was employed to mix with the starch in the cold.

229. The blue colour obtained in the above reaction is discharged on the addition of a few drops of chlorine water.

230. To show the effect of heat upon the blue compound. When the solution is heated, the blue colour gradually disappears, but returns again with somewhat less intensity when the solution is cooled. For this experiment the starch solution should be largely diluted, and a small quantity of it filtered. To a few cubic centimetres of the clear solution a slight excess of a dilute solution of iodine in potassium iodide is added. If the blue so obtained be too dark, the mixture is diluted with water until the colour is of the requisite intensity. If a small quantity of this blue solution be gently heated in a test tube by being dipped in water at about 90°C., the colour will be rapidly discharged, and the solution become perfectly colourless. Upon dipping the test-tube into cold water the blue colour is once more restored. It will be found that if there be an excess of starch present in

the solution, the colour that returns is far less intense than is the case when the iodine is in excess.

This experiment may be shown upon the screen, the hot water being contained in a glass cell with parallel sides.

HYDRIODIC ACID. HI

231. Preparation by the action of water upon phosphorus iodide. P+51 +4H2O=H ̧PO̟ ̧+5HI. This may be done in a similar apparatus to that used for hydrobromic acid. A mixture of iodine and red phosphorus is placed in the dry flask, and water slowly dropped upon the mixture from the dropping funnel. The gas may be collected by displacement.

232. To prepare an aqueous solution of hydriodic acid see Hydrobromic Acid, Exp. 217.

233. To show the solubility of hydriodic acid in water see Hydrobromic Acid.

234. To show the action of chlorine upon hydriodic acid. Cylinders of the two gases are placed mouth to mouth and the covers removed. As the gases mix, iodine is liberated.

235. To show the effect of heat upon hydriodic acid. A flask filled with gaseous hydriodic acid is closed with a cork carrying two stout copper wires, which are joined at their lower ends by means of a loop of platinum wire soldered to each. An electric current is passed through the wire of sufficient strength to raise its temperature to dull redness, when the gas is decomposed and iodine vapour makes its appearance.

OXIDES AND OXYACIDS OF IODINE

236. Preparation of iodic acid by the action of nitric acid upon iodine. 3HNO2+I=H20+ N2O3 + NO2 +HIO3. Two or three grams of iodine are placed in a flask, and about 40 c.c. of

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