IODINE.— Iodides,

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passing the gas through a solution of iodine inalcohol; but it is accompanied by a product ofdisagreeable odour arising from the action of thesulphydric acid on the alcohol. Naumait operatesat first on a small scale, until the reaction is accel-erated by the solubility of the iodine in the hydriodieacid produced. The liquid is then cooled, andiodine and ■water are added from time to time asthe action slackens. In this manner a very rapidstream of hydrogen sulphide is completely absorbed,and in a short time large quantities of hydriodieacid are obtained of specific gravity 1-56.

Hydriodie acid, prepared according to the firstof these methods, is a colourless gas, soluble inwater, and capable of strongly reddening litmus;it has a very pungent odour, somewhat similar tohydrochloric acid. When allowed to escape intothe air , it produces dense white vapours, which arevery suffocating when inhaled. Its specific gravityis 4-4429 ; it is neither a combustible body nor asupporter of combustion. It may be liquefied bypressure, and solidifies at —51° C. (—59'8° Fahr.).In this state it is perfectly transparent and colour-less, but intersected with fissures like ice. The ten-sion of the vapour at 0° is 2-9 atmospheres. Steam,sulphurous acid, chlorine, bromine, and a few otherbodies, are capable of decomposing hydriodie acidwith the separation of iodine. The gas is composedof equal volumes of iodine vapour and hydrogen,without condensation. In the liquid state hydriodieacid is colourless, and possesses an odour similarto that of the gas. Upon exposure it graduallysuffers decomposition, and on prolonged contactwith the air oxidation causes the liberation of theiodine, which separates in fine lamellar crystals.

Sulphuric acid, when concentrated, sets free theiodine, forming sulphurous acid and water. It is alsodecomposed by nitric acid, chlorine, and bromine.

This acid is capable of decomposing the oxides ofmost of the metals affording metallic iodides andwater. The following table shows the relativestrength of aqueous hydriodie acid at differentspecific gravities:—■

Specific Gravity at 59° Per Cent, of

Fahr. (IS® C.). Acid.

1-708 . 51-9

1-551 . 47-2

1-442 . 39-2

1-297 . 30-3

1-175 . 18-5

1-053 . 5-9

Oxygen Acids. —Iodine forms, with oxygen, com-pounds of various degrees of oxidation. The best

known are iodic acid (III0 3 ) and periodic acid

(III0 4 ). These form salts with the metallic bases,called iodates and periodates, respectively.

With sulphur, phosphorus, chlorine, and bromine,iodine produces several compounds which are veryinteresting to the chemical student.

Iodide of Nitrogen. —The combination of iodinewith nitrogen is remarkable, in being analogousto the chloride of nitrogen, although its explosiveproperties are much less violent. When iodineis digested with a strong solution of pure ammonia,

this compound is precipitated as black powder. Thepowder should be washed on a filter with water, toremove the excess of ammonia and the iodide ofammonium which forms. Another method is todissolve the iodine in alcohol, and add excess ofstrong solution of ammonia. After briskly agitating,the mixture is diluted with water, upon which theiodide of nitrogen precipitates, and may be washedin the cold. The compound is dried carefully uponbibulous paper at the ordinary temperature. Inperforming this operation, care should be taken todivide the mass into small portions, which oughtto be placed at a distance from one another, inorder to preclude the possibility of an explosionof any considerable quantity of the salt.

It may be desiccated with safety in a receiver filledwith ammoniacal gas.

The slightest agitation will decompose the iodideof nitrogen with a violent explosion, and the evolu-tion of a violet light. The feather of a quill drawnover the powder is sufficient to produce this effect,and if contained in a dish in moderate quantity, theforce is such as to shatter it in pieces. Even ifrubbed strongly under water, an explosion takesplace ; but the causes which determine this changein the air are incapable to effect the decompositionwhen it is mixed with large quantities of water. Itis not liable to explode by mere contact with oilsand fats, like the chloride of nitrogen. When moistit decomposes slowly in the air, evolving nitrogen,iodic and hydriodie acids. It differs in composition,but all the analyses point to the constitution ofan iodamide. NIII 2 and NII 2 I have been described.

Iodide of Ammonia (Iodammoninm, Nllgl).—Whendry ammoniacal gas and iodine are brought in con-tact combination takes place, and a heavy, blackish-brown, very tenacious liquid is produced. Thecompound is easily decomposed in part when heated,ammonia being set free ; the chief portion of theiodine salt, however, sublimes in violet-colouredvapours. It indicates the presence of ammonia andiodine.

Iodide of Ammonium, NII 4 I.—The compound whichis met with in commerce, and used chiefly in photo-graphy, is prepared by neutralizing a solution ofammonia or carbonate of ammonium with hydriodieacid, and evaporating the liquid in a water bath.When the evaporation is conducted carefully underthe bell jar of an air pump, the salt crystallizes incubes and is colourless; but if exposed to the air,it readily deliquesces, and turns brown, owing to aportion of the iodine being set free. It may be pre-pared likewise by bringing ammoniacal and hydriodieacid gases in contact, or by decomposing iodide ofiron by carbonate of ammonia, filtering off theferruginous precipitate, and evaporating the solutiontill the iodide crystallizes.

The iodide of ammonium thus prepared is a verydeliquescent salt, very soluble in -water and alcohol.By keeping, even in diffused daylight when out ofcontact with air, it decomposes, turning brownish-yellow, ammonia being evolved.

Iodide of Potassium. —When potassium and iodine