THE HALOGENS.] CHEMISTRY 491 acid, but in this case hydrochloric acid is first produced by the action of the sulphuric acid on the salt Nad + H 2 S0 4 = HC1 + NaHS0 4 The hydrochloric acid then enters into reaction with the manganese dioxide, as explained above, but the manga nese dichloride is converted into manganese sulphate and hydrochloric acid by the sulphuric acid. The following equation therefore expresses the final result, but does not take into account that there are several stages in the re action 2XaCl Sodium chloride. 3H 2 S0 4 Sulphuric acid. = C1 2 Chlorine. 20H 2 Water. MnS0 4 Manganese sulphate. MnO 2 Manganese d oxide. 2NaHS0 4 . Sodium hydrogen sulphate. Bromine and iodine are obtained in a precisely similar manner by treating the liquid containing the bromide or iodide with manganese dioxide and sulphuric acid. Another process for the manufacture of chlorine has been recently introduced by Deacon, which consists in passing a mixture of hydrochloric acid gas and air through a heated brickwork chamber filled with pieces of firebrick or marble which have been soaked in a solution of copper sulphate and dried. The hydrogen of the hydrochloric acid forms water with the oxygen of the air, and chlorine is liberated, but the precise nature of the changes by which this is effected is not known. The reaction is at its maxi mum at a temperature of about 500 C., which is much below that at which oxygen alone acts upon hydrochloric acid to form water and chlorine ; the copper sulphate is but little affected, although a certain amount is always transformed into chloride. There can be no doubt, however, that the decomposition is the result of recurrent action, and another somewhat similar method of obtaining chlorine may be quoted in illustration. Thus, when a mixture of silica and sodium chloride is heated to redness in a current of oxygen, chlorine is evolved, and the chloride is converted into silicate 2Si0 2 Silica. + 4XaCl Sodium chloride. 0, = 2Xa 2 Si0 3 - Oxygen. Sodium silicate. 2C1 2 ; Chlorine. but on passing hydrochloric acid gas over the silicate it is re-converted into the chloride and silica Na 2 Si0 3 + 2HC1 = SiO 2 + 2XaCl + OH., Sodium silicate. Hydrochloric acid. Silica. Sodium chloride. Water. Hence, by passing hydrochloric acid together with air over the mixture of silica and chloride, a continuous evolution of chlorine is obtained, reaction in the one direction having no sooner taken place than reaction in the opposite direc tion sets in. There can be little doubt that the produc tion of chlorine by Deacon s process is, in a similar manner, the result of a series of changes in opposite directions ; the nature of which, however, remains to be discovered. Chlorine is very readily condensed by a pressure of about 4 atmospheres at 15 C., or by cold alone at about - 50 C., into a yellow mobile liquid of specific gravity about 1 - 33, but it has not been solidified. Bromine has at C. the specific gravity 3 i 88 ; it boils at 59 5 C., yielding a dense red vapour, and solidifies at - 24 5 C. Iodine melts at 107 C., and boils at about 175 C., yielding a magnificent purple vapour. Chlorine gas and the vapour of bromine and iodine have a pungent, irritating, disagreeable odour, and are irrespirable. Water at 10 C. dissolves about 2 5 times its bulk of chlorine, and about 3 per cent, of bromine, but iodine is very sparingly soluble in water. When chlorine is passed into a mixture of crushed ice and water a crystalline hydrate, C1 2 + 10H 2 O, is produced, and a similar compound of bromine, Br 2 +10H 2 O, may be obtained, but iodine does not furnish a hydrate. The halogens furnish with hydrogen compounds in which a single atom of hydrogen is united with a single atom of the halogen, viz. : Hydrogen fluoride or hydrofluoric acid, HF ,, chloride ,, hydrochloric ,, HC1 ,, bromide,, hydrobromic ,, HBr iodide ,, hydriodic ,, HI The conditions under which hydrogen and chlorine enter into reaction have already been described, and we have seen that no change of volume occurs in the formation of hydrogen chloride from its elements. A mixture of hy drogen and bromine vapour does not explode on the application of flame, but hydrobromic acid is slowly formed when the mixture is heated ; hydriodic acid is also produced when iodine is heated in hydrogen. Gore has shown that hydrofluoric acid may be prepared by heating silver fluoride in an atmosphere of hydrogen 2AgF + H 2 = 2HF + 2Ag; Silver fluoride. Hydrogen. Hydrofluoric acid. Silver. and he finds that one volume of hydrogen in uniting with flu orine produces two volumes of hydrofluoric acid gas, thus proving that the constitution of hydrogen fluoride is similar to that of hydrogen chloride, bromide, and iodide. Hydrogen chloride or hydrochloric acid is usually prepared by gently heating a mixture of sodium chloride or common salt and concentrated sulphuric acid, diluted with a small quantity of water Nad + H 2 SO 4 = HC1 + NaHS0 4 . Sodium chloride. Sulphuric acid. Hydrogen chloride. Sodism hydrogen sulphc.te Hydrogen bromide and iodide cannot be prepared in this manner from metallic bromides and iodides, as they are partially decomposed by concentrated sulphuric acid, bro mine or iodine being liberated; this decomposition takea place much more readily with hydriodic acid than with hydrobromic acid 2HI + H 2 S0 4 = I 2 + S0 2 + 20H 2 . Hydrogen Iodide. Sulphuric acid. Iodine. Sulphur dioxide. Water. Hydrobromic and hydriodic acid are therefore usually prepared by the action of water on the compounds of pliosphorus with bromine and iodine ; thus PBr 3H 2 = 3HBr PH 3 3 Phosphorus tiibromide. Water. Hydrogen bromide. Phosphorous add. PI, + 3H 2 = SHI + PH.O. . 3^3 Phosphorua ti iiodide. Water. Hydrogen iodide. Phosphorous acid. Hydrogen fluoride is procured by gently heating finely powdered calcium fluoride or fluor-spar with concentrated sulphuric acid in leaden or platinum vessels, or by strongly heating potassium hydrogen fluoride in a platinum re tort CaF 2 + H 2 S0 4 = 2HF + CaS0 4 Calcium fluoride. Sulphuric acid. Hydrogen fluoride. Calcium sulphate. HKF, HF + KF. Potassium hydrogen fluoride. Hydrogen fluoride. Potassium fluoride. The compounds of hydrogen with the halogens, or, as they are frequently termed, the haloid acids, are not only similar in composition, but they resemble each other very closely in properties. Thus, hydrogen chloride, bromide, and iodide at ordinary atmospheric temperatures are colourless transparent gases, which fume strongly in moist air, whilst hydrogen fluoride is a highly volatile colourless liquid, of specific gravity 988 at 12 7 C., which boils at 19 0> 4C. ; its vapour fumes strongly in moist air. At a temperature of 10 C., under a pressure of 40 atmospheres, hydrogen chloride is condensed to a colourless liquid of specific gravity 1 "27, but it has never been solidified; hydro gen bromide and iodide are still more readily liquefied, and both may be caused to solidify, the former at - 73 C., and the latter at -51C. The circumstances which attend the formation of hydro
gen chloride, bromide, and iodide from their elements, and