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 If a farther test should be desired, it is only necessary, as was first proposed by Dr. Turner of London, to chase the crust up and down the tube with the spirit-lamp flame till it is all oxidated, when little octaedral crystals of adamantine lustre are formed, on which, either with the naked eye or with the aid of a common lens, triangular facettes may be distinguished.

The niceties to be attended to in applying the preceding tests will be considered presently under the head of the next compound, the sesquioxide.

2. Of the Tests for Arsenious Acid.

Arsenious acid, the sesquoixide, or white oxide of arsenic, usually called white arsenic, or simply arsenic, is the most common and important of all the arsenical preparations.

It is met with in the shops in two forms,—as a snow-white gritty powder, and in solid masses generally opaque, but sometimes translucent. When newly sublimed it is in translucent or even almost transparent masses of a vitreous lustre, conchoidal fracture and sharp-edged. By keeping it becomes opaque and white. The nature of the change has not been determined; but some alteration is certainly effected, for Guibourt, who has examined both varieties with care, found that the opaque variety is more soluble in water than the other. He adds that the former is alkaline, the latter acid, in its action on litmus paper; but I have always found the opaque variety acid. The powder soon becomes analogous to the opaque variety of the oxide in mass.

The oxide of arsenic has a specific gravity of 3·729, according to the experiments of Dr. Ure,—of 3·529 when opaque, according to Mr. Alfred Taylor, and 3·798, when translucent. Very incorrect notions prevail as to its taste. It was long universally believed to be acrid, and is described to be so in many systematic works and express treatises; but in reality it has little or no taste at all. The reader will find some details on this point in a paper I published in the Edinburgh Medical and Surgical Journal. In the present work it is sufficient to observe, that I have repeatedly made the trial, and seen it made at my request by several scientific friends, and that, after continuing the experiment as long, and extending the poison along the tongue as far back, as we thought safe, all agreed that it had scarcely any taste at all,—perhaps towards the close a very faint sweetish taste. It appears to me that the experiments made on that occasion might have set at rest the question as to the taste of arsenic, and corrected an important error long committed by systematic authors in chemistry as well as medical jurisprudence. And accordingly in this country the truth is generally known. Professor Orfila, how-*