Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/502

Rh 490 CHEMISTRY [THE HALOGENS. ditions of a chemical change and its amount have been made by Harcourt and Esson. The experiments consisted iu adding successive equal portions of sodium hyposulphite to a solution containing hydrogen dioxide, hydriodic acid, and a little starch. By this reagent the iodine which is continually being liberated by the action of the dioxide on the hydriodic acid is instantly reconverted into iodide, so that the liquid, though it contain starch, and though iodine is being formed in it, remains quite colourless as long as any hyposulphite is present. But when the last trace of hyposulphite has been removed by the action of the iodine, the portion of iodine next formed remains free, and the liquid becomes suddenly blue. The addition of another small portion of hyposulphite again removes the colour, and until all the hyposulphite is decomposed the solution remains colourless, and then again becomes suddenly blue. The intervals at which the blue colour appeared wore carefully noted, and the amount of hydro gen dioxide decomposed being known from the amount of hyposulphite employed, the quantity of dioxide decom posed in a given time was thus determined. The observed results are given in the following table : Amount of Dioxide. Time from the beginning in minutes. Chemical change in each interval. Interval in minutes. 20-95 o-oo 19-95 4-57 1 4-57 18-95 9-37 1 4-80 17-95 14-5 1 5-13 16-95 19-87 1 5-37 15-95 25-57 1 5-70 14-95 31-63 1 6-11 13-95 38-20 1 6 52 12-95 45-23 1 7 03 11-95 52-82 1 7-59 10-95 61-12 1 8-30 9-95 70-15 1 9-03 8-95 80-08 1 9-93 7-95 91-27 1 11-19 6-95 103-88 1 12-61 5-95 118-50 1 14-62 4-95 135-85 1 17-35 3-95 157-00 1 21-15 2-95 181-53 1 27-53 1-95 223-45 1 38-92 95 291-13 1 66-73 The general conclusion deducible from these experi ments is, that the amount of change at any moment varies directly with the amount of dioxide present in the solution ; iu accordance with this law, the quantities of dioxide at the end of a series of times taken in arithmetical progression are themselves in geometrical progression. This law of chemical action has been corroborated by the investigation of other reactions, and it is probably of very general appli cation. When hydrogen dioxide solution is mixed with a con centrated solution of barium hydroxide, crystalline hy- drated barium dioxide, BaO 2 + 6H 2 O, separates H.0 + BaOH = Ba0. + 2H/) . . 22 Ba(OH) 2 = . 2 Hydrogen dioxide. Barium hydroxide. Barium dioxide. Water. In a similar manner, peroxides of many metals are preci pitated on the addition of their salts to a solution of hydrogen dioxide. Hydrogen dioxide, it will be evident, differs remarkably from hydrogen monoxide or water. Its instability, and its tendency to enter into reaction with other bodies with separation of oxygen, appear to be explained by the fact that its decomposition into water and oxygen is attended with the development of a very considerable amount of heat. The behaviour of ozone and hydrogen dioxide, in fact, strikingly illustrates one of the most important laws of chemical action, viz., that those decompositions and reactions which are attended with the development of heat always take place more readily than those which require an absorption of heat, and they take place the more readily the greater the amount of heat which is liberated. The heat developed by the decomposition of ozone arid hydrogen dioxide, we have seen, is to be traced to the same cause, being due, it can scarcely be doubted, to the combination of the oxygen atoms. FLUORINE CHLORINE BROMINE IODINE. Fluorine, Symbol, F ; Atomic vt., 19 1 ; Molecular vt, (?) Chlorine, Cl ; 35 36 ; 7072. Bromine, Br ; ,, 7975 ; ,, 159-50. Iodine, I ; 126 53 ; 253 07 These four elements form with metals compounds analogous to sea salt, and from this circumstance the name halogens, or salt-producers (from a/s, sea-salt), has bee-n applied to them, their compounds with other radicles being frequently termed haloid compounds. They are always classed together on account of their close analogy in pro perties, but there are numerous and very important dis tinctions between them. The element fluorine is not known in the free state, all attempts to isolate it having failed on account of the im possibility of obtaining vessels which can withstand its action. Chlorine is a gas of a greenish yellow colour, whilst bromine, at ordinary atmospheric temperatures, is a mobile red liquid, so deep in colour as to be almost opaque, and iodine is a black, crystalline, and very brittle solid, which exhibits metallic lustre. None of these elements are ever met with in the free state, but their compounds are very widely distributed, and they are to be detected in most rocks and soils, in spring and sea-water, and in the ashes of plants and animals. Fluorine occurs most abundantly in combination with calcium as fluor-spar, and chlorine in combination with sodium as ordinary salt, large deposits of which exist iu various parts of the globe ; considerable deposits of bromine in combination with potassium have within recent years been discovered in Stassfurt, but no abundant source of iodine has hitherto been discovered. Chlorine was discovered by Scheelein 1774, and was so named on account of its colour (from ^Xwpos, green), but its elementary nature was first established by Davy in 1810. Bromine was first described in 1826 by Balard, who obtained it from bittern, the mother liquor of sea- water, after the less soluble salts have been extracted by evaporation and crystallization ; it was named on account of its irritating odour (from /3pw/xos, a stench). Iodine was discovered by Courtois in 1811, in the waste liquors from the manufacture of sodium carbonate from the ashes of sea-weed ; it received its name from the beautiful violet colour of its vapour (loeiSi^, violet-coloured). Chlorine is usually prepared, both in the laboratory and on the large scale, by gently heating a concentrated solu tion of hydrochloric acid with manganese dioxide; the reaction appears to take place in two stages, the first con sisting in the formation of the manganese chloride corre sponding to manganese dioxide Mn0 2 + 4HC1 = MnCl f ^ + 2H 2 O ; Water. Manganese dioxide. Hydrochloric acid. Manganese tetrfc- chloride. but this compound is so unstable that it breaks up into chlorine and a lower chloride of manganese MnCl 4 Manganese tetra- chloride. = Cl 2 + Chlorine. Manganese dichloride. It may be procured directly from salt by acting on a mixture of salt and manganese dioxide with sulphuric