The nature of kelp, and the mode of its manufacture, are now generally understood. It is a very impure carbonate of soda ; containing sulphate and muriate of soda, and also sulphuret of soda, with a portion of charcoal. In Scotland the manufacture is carried on chiefly in the months of Juiy and August. The kelp kiln is nothing but a round pit or basin dug in the sand or earth on the beach, and surround ed with a few loose stones. In the morning a fire is kin dled in this pit, generally by means of peat or turf. This fire is gradually led with sea-weed, in such a state of dry ness that it will merely burn. In tile course of eight or ten hours, the furnace is found to be nearly full of melted matter. Iron rakes are then drawn rapidly backward and forward through the mass in the furnace, in order to com pact it, or bring it into an equal state of fusion. It is then allowed to cool, when it is broken in pieces, and carried into a store-house, to remain till shipped.
The making of kelp from sea-weed was practised in France and England for more than half a century before the manufacture was introduced into Scotland. Mr James Fca of Whitehall in Stronsay was the first person in Ork ney who (about 1722) exported a cargo of kelp; he sailed with it himself to Newcastle ; and his success in the en terprizc soon aroused the attention of the Orcadians. At present, in coasting these islands, as well as the Hebrides, in the summer months, great volumes of smoke are every where to be seen rolling from the kelp furnaces, and the peculiar odour, probably arising chiefly from muriatic acid gas, is felt to a considerable distance. From 40,000/. to 50,000r. sterling, are thus yearly brought into the country; but it must not be concealed, that in most of the islands agriculture has suffered, from the attention of the small tenants having been diverted from the land, and, by the in fluence of the landholders, turned almost exclusively to the manufacture of kelp.
The fuci which are chiefly cut on our shores for this manufacture, are Fucus vesiculosus, nodosus, and sena-• tus. In some places, F. loreus and filum are employed, but not to a great extent. By means of a boat and long sharp hooks or bills, F. digitatus is cut in some places; and this species, together with F. saccharinus, bulbosus, and esculentus, form much of the drift-ware employed in making of kelp. Some of these arc no doubt richer in the alkaline salt than others; but of all of them it may be said, that when dry and fit fur burning, they arc capable of yielding about one-fifth of their weight in kelp.
Besides the alkali, kelp affords, as already hinted, a pe culiar simple or hitherto undecomposed substance, named iodine. It was discovered in the year 1812, by a manufac turer of saltpetre at Paris named Courtois, and has since been examined by the most eminent French and English chemists. It is readily procured by pouring concentrated sulphuric acid on the mother water of kelp from which so da has been extracted, or from spent soapers' legs. Heat is speedily produced, and the new substance appears as a violet-coloured gas, perfectly homogeneous and transpa rent. This, on being collected in the usual mode, soon con denses, and assumes the appearance of plumbago. It forms acids with hydrogen, chlorine and tin, call, the hydrionic! chlorionic, and stannionic acids ; and it combines readily with metals. The late Mr Tennant could detect no iodine in sea-water ; so that it appears to be entirely a product of marine plants. French kelp, it is remarked by Sir um phry Davy, yields more iodine than British ; but for this, no reason is assigned. Iodine has a peculiar odour, and is decidedly poisonous. The name is derived from rai3, vio laceous, in allusion to the very striking circumstance of the substance yielding a violet coloured gas on being cxpo•ed to an increase of temperature. The following is the mode of procuring iodine recommended by Dr Wollaston : solve the soluble part of kelp in water ; concentrate the li quid by evaporation, and separate all the crystals that can be obtained; pour the remaining liquid into a clean vessel, and mix with it an excess of sulphuric acid ; boil this li quid for some time ; sulphur is precipitated, and muriatic acid driven off ; decant off the clear liquid, and strain it through wool ; put it into a small flask, and mix it with as much black oxide of manganese as you used before of sul phuric acid ; apply to the top of the flask a glass tube shut at one end ; then upon heating the mixture in the flask, the iodine will sublime into the glass tuhe."*
The species reckoned kelp foci shall now be more par ticularly described ; and as most of them, besides yielding kelp, serve other useful purposes, these shall at the same time be noticed.
Fucus vesiculosus: "The frond is coriaceous, flat, mid ribbed, linear, dichotomous, and quite entire ; the vesicles are spherical, and innate in the membrane of the frond ; the receptacles (containing tubercles and seeds) solitary, ter minal, compressed, turgid, mostly elliptical."—In Scotland this is sometimes called Black tang ; sometimes Kelp ware and when the receptacles are large and swollen, Strawber ry ware. The Norwegians call it Hue tang, because their cows feed on it. It is the Quercus marina or Sea oak of the older writers. F. inflatus of Limexus and Lightfoot, and F. spiralis of English Botany, are to be considered as varieties only of this species. The colour is a pale olive green, which becomes dull and almost black as the plant dries. It grows most plentifully on all our rocky shores, often not mneh below flood-mark. It is readily distinguish ed from F. nodosus by the air vesicles very generally oc curring in parallel pairs, while in F. nodosus they are sin gle ; and from F. serratus, by the edges of the frond being entire, or wanting the serratures which mark that species. It is generally from one to three feet long. It is the spe cies most highly prized for the manufacture of kelp ; being rich in alkaline salts. According to one account, 5 oz. of the ashes of the plant yielded about 24 oz. of alkali ; and Dr Walker states that 1 lb. avoirdupois gave him 3 oz. of kelp. In the north and west of Scotland many hundred tons of this species are for this purpose yearly cut from the rocks, with old reaping hooks. To the Scottish islan ders it is likewise valuable in another way ; it constitutes a considerable part of the winter food of their horses, cattle, and sheep, which seem instinctively to migrate from the hills to the sea-shore at the ebbing of the tide. Lightfoot mentions that during severe snow-storms, stags have been known to descend from the Scottish mountains to the shores, and to foci] chiefly on this species. The same author states, that in some of the islands, the inhabitants cover their cheeses with its ashes, and thus supply the place of salt. Linnaeus says, that the people in Gothland often boil the plant, and mixing it with some coarse flour, feed their pigs with it, and that it has hence received the name of Swine tang. In the Channel islands it affords firing. In Jersey, in particular; it is collected and dried in July, and then housed for winter fuel. It is there also employed in smoke drying pork, beef and fish.t F. nodosua : " The frond is coriaceons, compressed, vein less, sub-dichotomous, branched in a pinuated manner ; the receptacles are distichous, pedunculated, roundish, mostly solitary."—This is very common on the rocky shores of this country, growing about half way between flood-mark and the ebb ; often on the intermediate space between F. vesi culosus and serratus, though it sometimes grows nearest to high-water mark. The fronds are from two to six feet in length, and at short distances swell into large oblong ve sicles or air-bladders ; by which, though not mentioned in the specific character, the plant is more familiarly distin guished, and from which it has derived its title nodosus. Boys amuse themselves by cutting them transversely near the end, and making whistles of them: hence the name sea whistles sometimes bestowed on the plant. ;'he seeds are contained in elliptico-spherical receptacles, which proceed, on short flat peduncles, from the sides of the branches. These receptacles acquire a yellow colour, and give occa-; sion to the name of yellow tang, by which this species is known in Orkney. Like the last species, it is much used by the kelp-makers, and it often gets the name of kelp wrack. Zoophytes are seldom found attached to this fucus ; but dense tufts of the dark-coloured Conferva polymorpha are frequently to be observed on it, and indeed this species of Conferva is scarcely ever to be detected in any other ha bitat.