HYPOSULPHITES 877 



then be taken with great care, as the differences will always be small. The 

 reading of the wet bulb will give the temperature of evaporation, and from that and 

 the temperature of the ordinary air has to be deduced the dew-point. 



HYMENJEA COURBARIIi. A tree growing in South America, from which 

 the resin anime exudes. See ANIME. 



HYOSCYAirenrE. A crystalline alkaloid obtained from the Henbane (Hyoscya- 

 mus niger}. It is also found in the thorn-apple (Datura Stramonium], and is therefore 

 sometimes called Daturine. 



HYPERSTHENE. A foliated mineral resembling diallage, having a pearly or 

 metallic lustre on the cleavage planes. It is a silicate of protoxide of iron and mag- 

 nesia ; found at St. Paul's Island, Labrador, and elsewhere. 



H YPERSTHENITE, or Hypersthene Rock. A rock resembling gabbro, composed 

 of labrador-felspar and hypersthene. 



HYPOCHXiORXC ACID. CIO 4 (C1 2 O 4 ) At. wt. 67'5. When finely-powdered 

 chlorate of potash is gradually mixed into a paste with strong sulphuric acid, and 

 heated in a bath of alcohol and water, a yellow gas is disengaged, which ia this hypo- 

 chloric acid, or the peroxide of chlorine. Although of much interest as a chemical 

 compound, it has no use in the arts. See Watts's ' Dictionary of Chemistry.' 



HYPOCHIiOROTTS ACID. CIO (C1 2 O) At. wt. 43'5. This acid is best 

 obtained by diffusing red oxide of mercury finely divided through twelve times its 

 weight of water, which is introduced into a bottle containing chlorine, and agitated 

 until the gas is absorbed. An oxychloride of mercury is formed, which is removed 

 by subsidence. The weak fluid obtained is 'put into a flask, and heated in a water- 

 bath, when the evolved gas is collected in a smaller portion of water, which becomes a 

 pure solution of hypochlorous acid. 



The salts are termed hypochlorites. See CHLORINE and BLEACHING. 



HYPOSUXiPHATES. Saline compounds formed by the union of hyposulphuric 

 acid with bases. 



HYPOSULPHITES. Saline compounds formed by the union of hyposulphurous 

 acid with bases. 



Hyposulphate of Soda. The salts of the hyposulphuric acid are obtained from the 

 hyposulphate of manganese, which is itself thus prepared : finely-divided binoxide of 

 manganese is suspended in water, artificially cooled, and a stream of sulphurous acid 

 passed through it. The binoxide gives up half its oxygen, becoming protoxide, 

 which unites with the hyposulphuric acid which is formed, producing the soluble 

 hyposulphate of manganese, which is separated from the excess of binoxide by filtra- 

 tion. 



The following equation represents the reaction : 



Mn0 2 + 2S0 2 = MnO,S 2 O s . 



If the temperature were allowed to rise, sulphuric acid would be formed, and not 

 hyposulphuric : 



Mn0 2 +S0 2 = MnO,S0 3 . 



The hyposulphuric acid, unlike the hyposulphurous acid, may be obtained in the 

 free state, and its solution permits even of being evaporated in vacua, until it acquires 

 the density of 1*347 ; but if carried further, it is decomposed into sulphuric and 

 sulphurous acids. 



The acid is obtained in the free state by adding baryta water to the hyposulphate 

 of manganese ; the soluble hyposulphate of baryta, filtered from the oxide of man- 

 ganese, and precipitated exactly by the cautious addition of sulphuric acid, and fil- 

 tered from the precipitate of sulphate of baryta, yields the pure solution of the acid, 

 which may be evaporated in vacuo, as above stated. 



It has no odour, but a very sour taste. 



The hyposulphate of soda may be made directly from the manganese salt or. from 

 the free acid. 



All the hyposulphates are soluble ; they have not as yet met with any commercial 

 application. 



Hyposulphite of Soda. This salt, now so extensively used for photographic pur- 

 poses, was first introduced by Sir J. Herschel. It may easily be prepared by the 

 following process : viz. by transmitting through a solution of sulphide of sodium 

 (prepared by fusing together in a covered crucible equal weights of carbonate of soda, 

 and flowers of sulphur), a stream of sulphurous acid until it ceases to be absorbed ; the 

 liquid is then filtered and evaporated, when the hyposulphite of soda, NaO,S' : 2 + 5HO 

 (*ra 2 0,S 2 2 + 5H 2 0) crystallises out. 



Another and perhaps better process consists in digesting a solution of sulphite of 

 soda on flowers of sulphur. The sulphur gradually dissolves, forming a colourless 



