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confounded with any other metal. Of the various methods of reduction which have been proposed, only two deserve our attention, viz., the process of Marsh, or rather the improved method developed on the base of Marsh's original process, and the process of Fresenius and Babo. The process of Marsh is more simple and more easily executed than that of Fresenius and Babo, it requires less experienced and less cautious hands for its execution, it yields very various and characteristic proofs for the presence of the metal, and admits of an easy application of the various reagents for examining the nature of the metallic mirrors. But the process of Fresenius and Babo surpasses that of Marsh in this respect, that it excludes the possibility of confounding arsenic with antimony. The process of Marsh does not yield the whole of the arsenic, as we shall see hereafter, whereas the process of Fresenius and Babo affords the whole of it in the reduced state, but it is scarcely possible to entirely prevent the escape of a part of the metal. In this respect, therefore, the two methods may be considered equally good. According to my observations, the process of Marsh produces more distinct and characteristic evidence of the presence of very small quantities of arsenic than that of Fresenius and Babo. In the great majority of cases of poisoning, however, there will be sufficient material to employ both processes.

$ 17. We will first direct our attention to the process of Marsh. It is founded on the observation that hydrogen and arsenic, when meeting in the inoment of being set free from combination, that is, when in the nascent state, unite to arseneted hydrogen, from

which the arsenic may be obtained in different manners, ex. gr., by heat or by incomplete combustion ; and which, on being passed through solutions of salts of easily reducible nietallic oxides, yields its arsenic as arsenious acid. The different kinds of apparatus which have been proposed for the execution of the operation may all be replaced by the simple arrangement represented in Fig. 4.

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It consists of a gas-bottle (a), which may be substituted by a small Woulff's bottle or any other twonecked bottle, provided with a funnel-tube, another tube (b) bent at a right angle, a small drying-tube (c), and a reduction tube (d).

The bulb of the tube (b) serves to condense and collect the greater part of the particles of liquid which are carried off mechanically, or by evaporation, from the contents of the bottle. When a Woulff's or a twonecked bottle is used as gas-generating vessel, the bulb is more conveniently situated on the descending leg of the tube. The descending leg must be sufficiently large in diameter, and cut obliquely at the end, to facilitate the dropping back of the condensed liquid into the bottle.

The drying-tubė (c) contains some fused chloride of calcium, and sometimes a small piece of caustic potassa;* some loose cotton, introduced at both. ends of the tube, prevents the pieces from falling out. According to my experience, the reductiontube (d), made of glass, free from lead, is most conveniently one centimetre in diameter, the thickness of the glass being one and a half millimetres, and seven millimetres the diameter of the bore. Fig. 5.

represents a section in natural size. The tube is narrowly drawn out in several

places, as shown in Fig. 4, and there the - Fig. 5. arsenic is made to condense by heating the wider part .of the tube immediately preceding. If the tube is of the above-mentioned thickness the heating may be effected by means of an alcohollamp with Argand's burner, without causing the tube to bend, provided the ignited portion of tho tube to rest on the ring of the lamp, and the bent part to be supported. This is very important; for there is nothing more disagreeable and annoying

* In consequence of recent experiments I must insist on having the tube filled with hydrate of potassa alone, or hydrate of potassa in the first half of the tube, and chloride of calcium in the last. This is necessary, because every atom of sulphuric acid which is carried from the gas-bottle into the tube containing the chloride of calcium, sets some hydrochloric acid free, which, by its presence, prevents the formation of the arsenic-mirror. This precaution is absolutely necessary when a liquid containing hydrochloric acid is introduced into the gas-bottle.

than to be obliged to guard during the performance of the experiment against the softening and collapsing of the tube. The opening of the narrowly drawn out ascending leg of the tube must neither be too wide nor too narrow.

The mode of connecting the different tubes with each other depends, of course, upon their respective diameters. The most simple way is to take the drying-tube of the diameter of the reduction-tube, and to draw it out at the other end to the diameter of the delivery-tube (b). Two small tubes of vulcanized India-rubber are then sufficient to effect a close connexion ; they fit so well that an additional tying is quite superfluous. The India-rubber tubes, though fitting tightly, still are movable on the glass-tube, and thus admit of the ascending leg of the reductiontube being turned downwards.

$ 18. We now proceed to the process of reduction. Some pure zinc, granulated or in bars, and in not too small a quantity (several ounces), is introduced into the bottle; connection is then made between the different parts of the apparatus, and so much water poured through the funnel-tube, that its lower end dips into the liquid; some pure distilled sulphuric acid, previously mixed with three parts of water, is then added in small portions to produce a moderate evolution of hydrogen. If the mixture of sulphuric acid and water is applied after perfect cooling, and only small portions at a time are added, the temperature of the bottle · (a) will be but slightly raised—and this is of great importance, since under such circumstances only a gas is obtained which is not charged with too much watery vapor.

When all the atmospheric air may be supposed to have been expelled from the apparatus, the escaping gas is kindled, and the flame made to strike against a dish or a saucer of glazed white porcelain, which is held horizontally into the flame. If black spots, or only traces of black spots are deposited on the porcelain, the materials, the zinc or the sulphuric acid, are not free from arsenic, and are not fit to be used; even the apparatus has become useless, or, at least, requires to be very carefully cleaned. The corks must, at all events, be replaced by new ones.

If no spots, or traces of spots, are deposited on the porcelain, we cannot yet rest convinced of the purity of the materials, but must proceed to a second and more stringent examination. This is done by heating the reduction-tube before the first narrowing to redness, by means of a spirit-lamp with Argand's burner, and allowing the gas to pass through for at

least half an hour, an equal evolution of hydrogen · being sustained by occasional additions of acid. If

after this time not the slightest trace of a metallic mirror or coating is deposited in the tube beyond the heated portion, which is tested by holding the tube against a sheet of white paper, the materials may be used for our purposes, for they are either free from arsenic, or contain a quantity not sufficient to interfere with the results. But if a mirror or crust has been formed, purer materials must be looked for. It is a great advantage of the process of Marsh, that it admits of this easy mode of testing the purity of the necessary materials, and in the very same way that is followed to detect the presence of arsenic.

$ 19. After the purity of the materials has thus

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