CHLOROUS ACID. 373 



Chlorous acid or peroxide of chlorine, Cl O 4 . This acid can- 

 not be obtained in a state of purity, without considerable dan- 

 ger. Gay-Lussac recommends, in preparing it, to mix chlorate 

 of potash into the state of a paste with sulphuric acid previously 

 diluted with half its weight of water and cooled, and to distil 

 the mixture in a small retort by a water-bath. It comes off as 

 a gas, of a yellow colour considerably deeper than chlorine, 

 which must be collected over mercury. It is composed of 2 

 volumes of chlorine with 4 volumes of oxygen, condensed into 

 4 volumes, which gives it a density of 2337-5. This gas is de- 

 composed gradually by light, but between 200 and 212 its 

 elements separate in an instantaneous manner, with the disen- 

 gagement of light and a violent explosion which breaks the ves- 

 sels. Water dissolves 7 volumes of this gas, and the gas itself 

 may be liquefied by pressure. It bleaches damp litmus paper, 

 without first reddening it, and is absorbed by alkaline solutions 

 with the formation according to Martens of a peculiar class 

 of decolorizing salts.* These, however, readily pass into chlo- 

 rates and chlorides, like the hypochlorites of Balard, when their 

 solutions are heated. 



Chlorous acid has a violent action upon combustibles, kindling 

 phosphorus, sulphur, sugar and other combustible substances in 

 contact with which it is evolved. Its action upon phosphorus 

 may be shewn by throwing a drachm or two 

 FIG 4] f cr y sta l nze d chlorate of potash into a deep 

 foot-glass or ale-glass (see figure 41) filled with 

 cold water, to the bottom of which the salt falls 

 without much loss by solution. Oil of vitriol 

 is then conducted to the salt, in a small stream, 

 from a tube funnel, the lower end of which has 

 been drawn out into a jet with a minute open- 

 ing. A gas of a lively yellow colour is evolved 

 with slight concussions, and immediately dis- 

 solved by the water, to which it imparts the 

 same colour. If while this is occurring a piece 

 of phosphorus be thrown into the glass, it 

 is ignited by every bubble of gas, and a brilliant combustion 

 is produced under the water, forming a beautiful experiment 

 wholly without danger. If a few grains of chlorate of potash in 



* An. de h. ct de Ph. t. 61, p. 293. 



