An introduction to practical chemistry : including analysis . be found to have thecharacteristic odor of garlic. A few of these spots should be retained for furtherexamination, and for comparison with those of anti-mony (320). 316. Hold a short test-tube with the mouth down-wards, just above the apex of the flame (Fig. 72) for a Fig. 72. few moments, so as to col-lect some of the arseniousacid formed by the oxida-tion of the arsenic vapor,and reserve the tube forcomparison with antimony(323). 317. Marshs test as justdescribed is so extremelydelicate, that it is capableof detecting arsenic in a
An introduction to practical chemistry : including analysis . be found to have thecharacteristic odor of garlic. A few of these spots should be retained for furtherexamination, and for comparison with those of anti-mony (320). 316. Hold a short test-tube with the mouth down-wards, just above the apex of the flame (Fig. 72) for a Fig. 72. few moments, so as to col-lect some of the arseniousacid formed by the oxida-tion of the arsenic vapor,and reserve the tube forcomparison with antimony(323). 317. Marshs test as justdescribed is so extremelydelicate, that it is capableof detecting arsenic in a solution containing the mil-lionth of its weight of the acid, and may be consideredthe most conclusive test which we possess. It is how-ever liable to this objection, which is, in practice, easilyovercome. It is found that antimony, when present ina mixture of zinc and dilute sulphuric acid, combineswith the liberated hydrogen, precisely in the same wayas arsenic, forming an analogous compound called an-timoniuretted hydrogen (SbH3); which when, heated, is. 120 METALS BELONGING TO CLASS IV. decomposed, and the metallic antimony is at the sametime deposited. Hence it is extremely important thatwe should be able to distinguish accurately betweenthem, as otherwise we should not be sure whether thecrust produced by Marshs test were due to arsenicor antimony. One or two experiments are generallysufficient to enable us to do this. 318. For the purpose of comparison, empty the zincand sulphuric acid from the bottle used for the arsenicexperiments (313), and substitute fresh zinc and the gas has been coming off about five minutes(note to 313), pour in a few drops of a solution of thedouble tartrate of antimony and potash [KO^SbO^C^H^Ol0-\-2Aq) and apply heat as before at the point d. Acrust of antimony will be deposited at the heated point,and not, as in the case of arsenic, at a little distancefrom it; this is owing to the antimony being less vola-tile than arsenic, and it
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