GO Iodide of Polas^uua, us a Test fur Arseutc. 



ill the suspected solution, tliey would be sepaj-ated by the carbonated 

 alkali employed to dissolve the arsenious acid. Neither docs it ap- 

 pear, from the experiments made, that coffee, tea, milk, and the 

 other liquid articles of food have much effect in retarding the opera- 

 tion. But we cannot, in all cases, trust to simple precipitation when 

 iodine alone is employed. This substance, for example, will produce 

 the effect when added to a solution of coffee quite free from arsenic. 

 It is not desired, however, to recommend even the iodide, for the 

 detection of arsenic in complicated cases, and on this account, I shall 

 refrain from offering any further remarks, but pass on to consider the 

 white compound in a manner purely chemical. 



Properties. — ^In several respects it resembles arsenious acid, par- 

 ticularly in its solubility and precipitation ; boiling v/ater, for instance, 

 dissolves a^jout 5.3 per cent, and deposits nearly one lialf upon cool- 

 ing. So also, when it is separated from weak solutions, it adheres I 

 with great tenacity to the vessels, resembling a white enamel in ap- 

 pearance. But arsenic begins to sublime at a temperature a lltde 

 above 300*^, whereas this powder requires a beat equal to 550^ F- 

 When exposed to a heat of about GOO^, it undergoes decomposition, 

 arsenical fumes being given off abundantly, and towards the end of 

 the process, particularly when the temperature is more elevated, io- 

 dine is liberated very freely. The degree of its decomposition by 

 heat alone is very variable, the minimum and ma:cimum losses being 

 30 and 70 per cent. In close tubes there is no reduction ; the pro- 

 ducts being arsenious acid, iodine and a yellow matter which v/as con- 

 sidered as the iodide of arsenic ; but as the substance bears a high 

 temperature without decomposition, metallic arsenic may be collected 

 even in small quantities, by rubbing up with it charcoal powder. 

 Black fluK is entirely unnecessary. 



Comj)osition. — As this precipitate appeared at fust to be a double 

 salt, composed of iodide of potassium and arsenlte of the alkali em- 

 ployed, several attempts were made to determine its composition by 

 using definite amounts of either of these salts, and adding tlie other 

 as long as there was any precipitation. Analysis w^as also subse- 

 quently performed upon the white matter precipitated ; but in all 

 such cases the results were unsatisfactory, owing, as it subsequently 

 appeared, to the variable amount of the arsenic in tlic different arsen- 

 ates employed. Notwithstanding the want of uniformity, it appears 

 advisable to indicate the method pursued. 



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