132 DETECTION OF ARSENIC, 



rapidly, or the flux be scanty in proportion to the quantity of 

 sulphuret. Several objections have been made to this process, 

 but I think none which are irrefutable. It has been said that 

 a ring of charcoal finely divided may be taken for the arsenical 

 crust, that antimony forms a crust similar to it, that the action 

 of the flux on the glass in reducing its lead may be mistaken 

 for it, and finally that zinc may be sublimed in its metallic 

 state. But the three former can never deceive one who 

 examines with any care the crust which is produced, and the 

 full white heat required for the sublimation of zinc cannot be 

 obtained in a spirit lamp. The reduction of the lead in 

 the glass, occurs at the bottom of the tube, and cannot extend 

 beyond the actual contact of the charcoal, while the arsenical 

 crust, which only resembles it in its lustre, is generally formed 

 about half an inch above it. The value of this process is now 

 generally appreciated, and in the opinion of most chemists, is 

 alone sufficient to determine the presence of arsenic, how small 

 soever the quantity reserved for examination may be. It isy 

 however, of infinite importance that the preliminary operations 

 be conducted with great accuracy and nicety, where the quan- 

 tity of material which is brought to the chemist for examina- 

 tion is either very minute in quantity, or much involved by 

 admixture with organic matters. The presence of any empy- 

 reumatic matter in the reduction tube is a great source of em- 

 barrassment, and is positively interdicted. The tests essential 

 to the detection of arsenic having now been described in gene- 

 ral terms, it remains to mention the two or three others which 

 have been enumerated, which will be done in a cursory man- 

 ner; these tests having become almost obsolete, except the 

 small galvanic circle, which by the way is rather an elegant 

 experiment for the illustration of a lecture than a test, properly 

 so called, for the detection of so violent a poison. 



The alliaceous odour, is to be sure strongly marked, but it is 

 liable to so many exceptions, as to be scarcely worthy the name 

 of a test. It may serve the analytic mineralogist, whose analyses 

 are never embarrassed with organic matter, but cannot cer- 

 tainly be relied upon in cases where zinc, phosphorus, burn- 

 ing papers, and animal matter exhale an odour, so similar to 

 that of arsenic. 



The 7^educiion beticecn copper 2:)lates is now seldom used. 

 It is accomplished by laying the suspected matter on one of 



