1839.] On the detection of Arsenical Poisons, fyc. 149 



By repeating Marsh's process on a mixture containing tartarized an- 

 timony, it will be seen that the gas evolved burns with nearly the same 

 color, and deposits a similar crust on the glass tube. 



On examining closely the distinguishing characters of this crust, it is 

 very possible for an experienced eye to distinguish it from one produced by 

 arsenic. The eye however must be experienced indeed, and that to a 

 degree which very few observers can be supposed to lay claim to. Again, 

 the sulphuretted hydrogen produces with crusts of arsenic and antimony 

 yellow stains so faintly differing in tint as to lend even a practised 

 experimentalist but little assistance in his research. The sulphate of 

 copper, again, gives only such indications as are too faint to be relied 

 on individually, though of some value as corroborating evidence. 



Nevertheless the silver test can be readily applied so as to give unques- 

 tionable evidence of the nature of the crust of metal and of oxide 

 obtained by Marsh's process. This may be accomplished by a method 

 which differs slightly from one pointed out by Mr. Thomson in the 

 paper alluded to. The tube on cooling should be moistened with a 

 solution of nitrate of silver in distilled water, and then held over the 

 mouth of a bottle containing strong ammonia, so that the vapor may 

 traverse the tube. If the crust be arsenical, it instantaneously assumes 

 a vivid canary color, owing to the formation of the arsenite of silver. 

 No approach to such an effect is produced by the antimonial com- 

 pounds, so that this test affords a simple, but most conclusive check on 

 Marsh's invaluable method 



It is right to repeat a precaution as to the zinc employed. That 

 found in the bazar often contains traces of arsenic, and should always 

 be tested itself by Marsh's process before being employed in pursuit of 

 any legal investigation. Secondly, the zinc by which arsenic has been 

 once detected should never be used again, as the surface often unites 

 with and retains as much of that metal as may falsify a further 

 experiment. 



