248 Medico-Legal Application of RiencK 's Test for Arsenic. 



not be mistaken for any other substance. But when the quantity 

 in solution does not exceed one part in fifty thousand of the fluid, 

 it will not fall until hydrochloric acid and heat are added — con- 

 ditions under which arsenious acid also is reduced. So that mer- 

 cury can be separated from a solution, except a small quantity, 

 without disturbing the arsenic ; but if the amount be very mi- 

 nute, both metals fall down, and the character of the stain is no 

 longer a test. The presence of corrosive sublimate is however 

 rarely to be expected, since it is the more active poison of the 

 two ; but the test under consideration may be used for the detec- 

 tion of mercury, as well as arsenic and antimony. Calomel may 

 be looked for in many instances ; and although it will not be 

 found in fluids, yet in those cases where the coats of the stomach 

 are macerated in dilute hydrochloric acid, for the solution of ar- 

 senious acid, it will also be dissolved and converted into corrosive 

 sublimate. But this is not so great an inconvenience as might 

 appear at first sight, for by sublimation the two substances are 

 entirely separated from each other. 



Introducing the copper strips coated with both metals into the 

 subliming tube, and heating, the mercury will rise in the metallic 

 form, and the arsenic as arsenious acid. The extremity of the 

 tube surrounding the copper will not be stained green, as already 

 stated, but be colored with the orange oxide of mercury. A mi- 

 croscopical examination of the sublimate will quickly decide the 

 question whether any arsenious acid be present ; for its octahe- 

 dral form and transparency contrast strongly with the spheres of 

 opaque mercury. In these cases it is best to dissolve the acid 

 away from the metal, by boiling the whole sublimate in pure 

 water and testing again, before an opinion is given. 



The results of the examination made on this subject, may be 

 condensed under the following heads. 



1st. In consequence of the occasional failure of Marsh's test, 

 as shown by Messrs. Danger and Flandin, and the length of time 

 necessary to carry on the process, when minute quantities of arse- 

 nic are present, it has become a desideratum to possess some more 

 certain means of collecting the poison out of solutions. The 

 process recommended by M. Riench is the best yet discovered ; 

 but it is not a good test for the metal, because many other sub- 

 stances produce deposits which resemble that of arsenic to a great 

 extent. But by subliming always from the precipitate collected, 

 the test is increased in value and certainty. 



