The characters of the crust have been mentioned already under the head of fly-powder (p. 199). They are distinct even in crusts weighing only a 300th of a grain. A crust of this weight, a tenth of an inch broad and four times as long, may show characteristically all the physical characters of an arsenical sublimate a hundred times larger. The fallacies to which the test has been supposed to be liable (excluding at present that part of it which consists in the oxidation of the metal, and which renders it quite unimpeachable), are the following.— Dr. Paris says he has known an instance where a person, “by no means deficient in chemical address, mistook for it a deposit of charcoal,”[510] and I have known the same mistake happen in the hands of one of my pupils, a beginner in the study of medico-legal chemistry. The outer surface of a charcoal crust may be mistaken for arsenic by a careless person; but with ordinary care it is quite impossible to err if the inner surface be examined, for that of charcoal is brown, powdery, and perfectly dull.—It has been suggested to me and has been stated in print,[511] that the preparations of antimony yield by reduction a sublimate resembling closely an arsenical crust. But in consequence of repeated trials I am certain that no preparation of antimony, reduced either by charcoal or the black flux with the fullest red heat of the blowpipe will yield any metallic sublimate; and the same facts were observed by the late Dr. Turner.—It has even been said by Mr. Donovan that the action of the flux on glass which contains lead causes a stain similar to an arsenical crust.[512] If it be meant by this observation, that the lead contained in the glass usually gives that part of the tube which contains the flux a glimmering appearance and impairs its transparency, the author is correct: but it is impossible that a sublimate can be so formed.—Dr. Mitchell of Philadelphia in an elaborate paper on the process of reduction seems to consider the crust undistinguishable from that formed in similar circumstances by cinnabar.[513] Crusts of cinnabar, however, do not present the peculiar character possessed by the internal surface of arsenic.—Zinc, it is said, may be sublimed in its metallic state; but the sublimation of zinc requires a full white heat; which in the process for arsenic cannot be generated.—Tellurium, cadmium, and potassium sublime at a lower heat; but these metals are so exceedingly rare, that it is quite unnecessary to particularize the characters of their sublimates.—Lastly, it is said that a crust may be produced from arsenic contained in the glass of the tube. A few years ago MM. Ozanam and Idt of Lyons detected arsenic in the remains of a body which had been seven years interred; but subsequently M. Idt imagined he had discovered that the glass used in the analysis contained arsenic, and yielded it by the process of reduction. He accordingly retracted his original opinion; and the person