buried allows us to conclude that this action, though slow, has been sufficient to produce such marked effects of oxidation. The same reasoning is applicable to many other phænomena, which are daily occurring before our eyes. It is for this reason, for instance, that we are obliged to attach the copper sheeting of our ships with copper nails, and not with nails made of iron, that the contact of two dissimilar metals may not give rise to an electrical action, which, by the decomposition of the water, would speedily determine the oxidation of the iron, the copper in this case being negative. ARTICLE V. Extraction of Selenium from the sulphureous Deposits left in the Manufacture of Sulphuric Acid from Pyrites. Translated from the German of M. Lewenau, by M. Robinet.* M. LEWENAU has presented a monograph on selenium to the Société de Pharmacie. Having been desired to extract whatever is interesting and new in the Memoir, I have been occupied in examining the work. It gives a complete history of the discovery, properties, and modes of obtaining selenium, condensing in one view all that is known of this substance, from the several accounts that have been published respecting it, since M. Berzelius discovered it in 1818. But independently of what he has borrowed from others, M. Lewenau's treatise contains observations which belong to himself alone, and have appeared nowhere else; they deserve the attention of chemists in general. M. Lewenau has been principally occupied with the methods of preparing selenium, and the following is the process he has adopted. I give it exactly as he has detailed it. "One pound of the deposit was introduced into a tubulated retort, of the capacity of four pints, taking care that none should adhere to the sides; the retort was placed on the sand-bath, and a large globular receiver, united by a Woulf's tube to a flask full of water, adapted to it. The apparatus being luted, the acid was introduced, in the proportion of eight pounds of muriatic acid, sp. gr. 1.200, to four pounds of nitric acid, sp. gr. 1.500. To avoid the effects of the violent action which suddenly takes place, only a fourth part of the acid was introduced at first, and carefully poured over the bottom of the retort by means of a funnel with a long neck. The mass immediately began to heat and swell up, and to give off a considerable quantity of red vapours. The liquid assumed a dark-grey colour, and * From the Journal de Pharmacie. the water in the Woulf's bottle soon became reddish-yellow. When the action of the acid had moderated, a pound and a half more was added; the same phænomena occurred again, and were followed by a fresh introduction of acid. Next, to complete the action of the acid, and get rid of the now inert liquid, it was distilled over into the receiver, with a gentle heat; the distillation was accompanied by the disengagement of a reddish-yellow gas; towards the end of the process, the neck of the retort was lined with small yellow stellated crystals, very probably a binary compound of selenic and muriatic acid, which disappeared on increasing the heat. When almost the whole of the exhausted liquid was thus separated, the remainder of the acid was introduced, in separate portions, as before. The action was always very violent on each addition of fresh acid, and it was necessary several times to change the water in the flask, as it became saturated with the acid vapours. At last, all the liquors were returned into the retort and redistilled. The insoluble residuum, and the sides of the retort, appearing of a deep-red colour, as if occasioned by pure precipitated selenium, the solubility of which in fuming nitric acid had been demonstrated by direct experiment, a pound and a half of that acid was introduced into the retort, and distilled with a gentle heat till no supernatant liquid remained, but without entirely reducing the residuum to dryness. Distilled water was affused over the residual mass at the bottom of the retort, made to boil, and the whole then poured out and filtered, and the residuum washed, till the washings passed off perfectly insipid. The filtered liquid had a light-yellow colour; that which had been distilled into the receiver was found to be slightly seleniferous. "In order to separate the selenium from the filtered liquor, in which it existed as selenic acid, without regard to the metals it might contain, sulphite of ammonia, recently prepared, was employed, which threw down the selenium, in the form of large flakes, of a cinnabar-red colour. The colour was proportionately brighter, as the quantities precipitated were smaller. The precipitation was instantaneous, and preceded by slight turbidness for a few moments, when a concentrated solution was acted on; but if the solution was diluted, precipitation did not ensue for some time, although a large excess of sulphite of ammonia were added, and, which is advantageous in all cases, the liquor, at first clear, became coloured (sometimes at the expiration of many hours), and at last turbid, and deposited selenium. In a certain state of dilution, the precipitate was black or dark-grey. The selenium thus obtained was washed with cold distilled water, till the washings ceased to precipitate muriate of barytes; five or six washings are commonly necessary; the selenium was then dried in the shade. "To obtain the small portion that might still remain in the solution from which the selenium was precipitated, it was evaporated to two-thirds of its bulk in a retort; by these means small grey spangles were obtained, possessing a metallic brilliancy, and friable between the fingers: they were metallic selenium. The concentrated liquor, mixed with sulphite of ammonia, afforded a fresh quantity, but this had a dirty-brown colour. The acid products of this distillation, collected in the receiver, not giving any precipitate, nor becoming turbid with sulphite of ammonia, bars of zinc were immersed in it; the liquor being previously divided into several portions, and diluted, to avoid a too violent action: selenium was thus deposited, in dark-grey flakes, with a brisk disengagement of hydrogen gas, of a peculiar odour. It is necessary to separate these flakes speedily from the liquor, or they soon disappear. The selenium thus obtained was washed and dried. It must be observed in this operation, that immediately after the precipitation of the selenium, the bars of zinc should be removed from the acid, otherwise there is danger that it may mix with the metallic particles; in that case, it is advantageous to wash them with water acidulated with sulphuric acid. Finally, it may happen that all the selenium may not be obtained by this method, for zinc does not appear to be capable of precipitating it wholly from the solution. "By the preceding process, one pound of the sulphureous deposit afforded, Gros. grs. « Red selenium, precipitated by sulphite of 8 ammonia .. Grs. troy. 14 = 484∙16 "If we compare M. Lewenau's process with those successively adopted by M. Berzelius, we find it very superior to any of them. In fact, by the old methods, we perform many useless operations for the purpose of freeing the solution of selenium from all foreign substances, in order to precipitate it in a pure state. This mode had serious inconveniences, in consequence of the difficulty of separating the sulphuric acid, and the metallic oxides, without at the same time their carrying with them a portion of the selenium. M. de Lewenau avoids this, and at the same time obtains a larger product by simplifying the operation." * The sulphureous deposit on which M. Lewenau made his experiments, was procured from a sulphuric acid manufactory in Hungary. M. Henri, jun. has repeated the process on the seleniferous sulphur from Fahlun, and obtained a much smaller proportion of selenium than was obtained by the author.-Note by M. Robinet. ARTICLE VI. On the Natural Arrangement of the Pulmonobranchous Mollusca. By John Edward Gray, MGS. (To the Editors of the Annals of Philosophy.) GENTLEMEN, July 5, 1824. BEING dissatisfied with the arrangement of the air-breathing mollusca proposed by Ferrussac in his showy work on the Land and Freshwater Shells, which he appears to have thought imperfect himself, as he has proposed a new one in one of his late numbers, which I consider to be more artificial, and consequently inferior to his former order, I have sent you a sketch of their circular disposition, which I had considered as the proper linear disposition of them before the publication of Mr. W. S.. Macleay's excellent views. The animals of this order of mollusca are at once distinguished by their closed pulmonary cavity, on the parietes of which the aerating vessels are reticulated. They all breathe free air; for those animals which live constantly in water rise to the surface to replenish their stock of that fluid so essential to life. They have no operculum, and this latter character at once distinguishes the shells of this family from the Cyclostomida, which appear to be the connecting family between them and the Turbinida. The true land animals of this group, as the slugs and snails,. which may be considered as the type, are characterized by their tentacula always being capable of being withdrawn into themselves, as the finger of a glove, and by their eyes always being placed on a pedicle capable of similar retraction; whereas those which are almost constantly found in or floating on the surface of water, as the Land-ears, the Pond and Shield-snails, are furnished with contractile tentacula and have their eyes usually sessile at the base of them. The groups of this order may, therefore, be thus characterized : 1. Tentaculis retractilibus, oculis pedicillatis. Terrestres. Mollusca gasteropoda... Mollusca trachelipoda, pallii marginibus in crassatis. LIMACIDE. HELICIDE. 2. Tentaculis contractilibus. Aquaticæ. Mollusca trachelopoda, pallii marginibus incrassatis, testæ labio multiplicato AURICULADA Mollusca trachelopoda, pallii marginibus tenui- LYMNEADE. The affinity between these families is so close that I shall not have much difficulty in pointing out their connexion one with another. Commencing with the Slugs; they are connected with the Snails by means of the genus Testacella, and indeed it is exceedingly difficult to draw the line of demarcation between the Limacide and the Helicide in the present imperfect state of our knowledge with regard to the animals of the latter family. But their shells may be known from those of all the other families by their mouth being closed, when the animal is at rest, by a peculiar membrane which is called an Epiphragma, or Poma. The Snails are connected to the Auriculada by means of a genus named by Ferrussac, Partula, which has the eyes sessile, as in the latter family, and also has the peculiarity of being ovo-viviparous therefore, we are thus led to the Auriculada, which are mostly aquatic, or at least found in marshes. I should certainly exclude from this family the genera Pyramidella and Tornatella, which Ferrussac has added to it, and place them in the family Turbinida, for the former has an operculum like the Trochi, formed of many gradually enlarging, and the latter like the Natica, formed of a few rapidly increasing whorles, and they both have the pectinobranchous animals of the latter group; but I would retain the genus Pedipes of Adanson, which is said to be marine, in this family, on account of its near affinity to the Auricula nitens of Lamarck (the Voluta triplicata of Donovan), which, like several of this family, is found in salt marshes, or estuaries, and I would also add to it the Voluta fluminea of Dr. Maton, which, by the peculiar form of its outer lip, may perhaps form a new genus. From this family, by the general similarity of the animals, the general habitat, and particularly by the peculiar form of the shell of Auricula Dombeyana, I proceed to the Lymneada, which are all truly aquatic, and usually called Pond Snails, and which, by the addition of the genus Planorbis to the divisions pointed out by me in Sowerby's Genera, will form a very complete circle. From thence, by means of the much shifted genus Ancylus, we are led to the Onchidiade which only differ from them in being destitute of any shell; and by means of the land section of this family, which Ferrussac has placed with the Limacida, we are led to return to that family, thus completing the circle, which, at another opportunity, I shall attempt further to illustrate. It is impossible, till more is known of the animals of the Snails, to point out distinctly the analogy between the genera of the |