60 Iodide of Potassium., as a Test for Arsenic. 



in the suspected solution, they would be separated by the carbonated 

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

 pear, from the experiments made, that oofFee, 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 water, for instance, 

 dissolves about 5.3 per cent, and deposits nearly one half upon cool- 

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

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

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

 above 300°, whereas this powder requires a heat equal to 550° F. 

 When exposed to a heat of about 600°, it undergoes decomposition, 

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

 die 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 maximum 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 was 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 flux is entirely unnecessary. 



Composition. — As this precipitate appeared at first to be a double 

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

 ployed, several attempts were made to determine its composition by 

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

 as long as there was any precipitation. Analysis was 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 the different arsen- 

 ites employed. Notwithstanding the want of uniformity, it appears 

 advisable to indicate llie method pursued. 



