130 DETECTION OF ARSENIC. 



any other reason for his doubt, than the belief that sulphu- 

 retted hydrogen will not produce a precipitate with that acid, 

 and if this be his reason, in my opinion it is unfounded; for 

 we have the authority of Rose, perhaps the first analyst of the 

 day, for the fact that sulphuretted hydrogen does produce a 

 yellow precipitate with arsenic acid, and its salts, but to do 

 this requires more boiling and a longer time than for arsenious 

 acid or its salts. . This test, of all others, is most implicitly 

 relied on, and its action is so delicate as to produce a pre- 

 cipitate, if the arsenical compound be dissolved in a hundred 

 thousand parts of water. It is an essential aid in the reduc- 

 tion process presently to be described, and is certainly liable 

 to none of the objections by which each of the other tests 

 may be rendered null. 



The reduction process next claims attention. The tests 

 which have just been described, may be considered rather as 

 incidental, than indispensable, except the last, while we are 

 possessed of so certain a proof, as that about to be explained. 



The reduction process, has been emphatically called the 

 'experimentum crucis,' and it yields a result so eminently 

 decided in its character, being the metal itself in a state of 

 chrystallization generally, that it may be implicitly relied 

 upon by the operator, and when exhibited to a jury, must pro- 

 duce conviction in the mind of the most incredulous. The 

 small quantity of arsenic in any form which may be made 

 sensible by this process, is almost incredible to one who is not 

 familiar with the minute operations of chemistry. The charac- 

 teristics of arsenic are well marked, says Christison, in crusts 

 which weighed only a 286th part of a grain. Berzelius says, 

 that the 190th of a grain of oxide will yield a good crust, and 

 that he has prepared a crust from a quantity so small as to be 

 insensible to any of his balances. 



The manner in which this test is to be applied is, to take 

 the precipitate which has been thrown down by the sulphu- 

 retted hydrogen, and to mix it with black flux, or with charcoal 

 and carbonate of soda, and to introduce it into a small glass 

 tube, closed at one end. The dust must be carefully wiped 

 away from the sides of the tube, and any moisture which may 

 be condensed on the sides during the process, must also be 

 carefully removed during the operation. The heat of a spirit 



