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XVIII. On lodo-arsenious Acid. 

 By William Wallace^ Ph.D., F.C.S., Glasgow*. 



THE results of a recent investigation on the compound of 

 arsenious acid with chloride of arsenic, to which I gave 

 the name of chloro-arsenious acid f^ have induced me to prosecute 

 the inquiry whether similar combinations could be formed con- 

 taining bromine and iodine. I have succeeded in obtaining a 

 bromo-arsenious acid having a I'emarkably close resemblance to 

 the chlorine compound ; but as my experiments on this inter- 

 esting body are not yet completed, 1 reserve a description of them 

 for a future communication. 



The iodine compound differs very much from those containing 

 chlorine and bromine. I find that it has already been described 

 as an arsenite of teriodide of arsenic by Plisson, and also by 

 Serullas and Hottot f ; but neither of these authorities have 

 given any analysis of it, nor do they appear to have understood 

 its true nature. 



Iodide of arsenic is readily obtained by heating iodine with an 

 excess of powdered metallic arsenic, and distilling or subliming 

 the compound. It forms, as thus prepared, a brick-red cry- 

 stalline mass ; but when distilled in an atmosphere of hydrogen, 

 it is obtained of a yellowish-red colour, while a small portion of 

 arsenic is set free, arising from the presence of traces of arsenious 

 acid in the iodide. 



The iodide requires for solution at the boiling temperature 

 3'32 parts of water. On boihng down this liquid, beautiful 

 red-coloured crystals, consisting of pure and anhydrous iodide 

 of arsenic, are obtained. 



When, however, the solution is allowed to cool slowly, thin 

 pearly scales gradually separate : this compound is the subject 

 of the present communication. It cannot be washed with water 

 without partial decomposition, and is best dried by pressure be- 

 tween folds of bibulous paper. When freed from the adhering 

 red mother-liquor it is quite colourless, but it acquires a slight 

 yellow tint on exposure to the air, owing to the separation of a 

 minute quantity of iodine. On being strongly heated, it yields 

 a sublimate consisting chiefly of teriodide of arsenic, while grey- 

 coloured arsenious acid remains behind. The compound con- 

 tains chemically combined water, which it loses completely over 

 oil of vitriol. It cannot be dissolved in water without under- 

 going decomposition. 



* Communicated by the Author. 



t "On Chloro-arsenious Acid and some of its Compounds," Phil. Mag. 

 November 1858. 



t Vide Graelin's ' Handbook,' vol. iv. 



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