Wedgwood ware; earthen, stone, metal, and glāss vessels of different kinds; funnels, measures, glass-tubes; spatulas of wood, metal, ivory, and glass; pasteboards, writing paper, unsized paper, clean straws, horns, corks, bladders, linen strips, lutings, cements, paste, glue, portable bellows, brushes, boxes, &c. &c. are all occasionally wanted in a laboratory. See Ure's Dictionary of Chemistry. LAC. A resinous substance, the product of an insect found on several different kinds of trees in the East Indies. These insects pierce the small branches of the trees on which they feed; and the juice that exudes from the wounds is formed by them into a kind of cells for their eggs. Lac is imported into this country adhering to the branches in small transparent grains, or in semi-transparent flat cakes. The first, encrusting the branches, is called stick-lac; the second 'are the grains picked off the branches, and called seed-lac; the third is that which has undergone a simple purification, as we shall presently notice. There is a fourth called lump-lac, made by melting the seed-lac, and forming it into lumps. To purify the lac for use the natives of India put it into long canvas bags, which they heat over a charcoal fire until the resin melts; a portion of the lac then exudes through the bags, which are subsequently twisted, or wrung by means of cross sticks at the ends of the bags, the surface of the latter being scraped at the same time to accelerate the process. The chief consumption of lac in this country is in the_manufacture of sealing-wax and varnishes. It has been a great desideratum among artists to render shell-lac colourless, as, with the exception of its dark brown hue, it possesses all the properties essential to a good spirit varnish in a higher degree than any other known resin. A premium of a gold medal, or thirty guineas, for "a varnish made from shell or seed-lac, equally hard, and as fit for use in the arts," as that at present prepared from other substances, was offered for some years by the Society of Arts. The editor of the Franklin Journal, of Philadelphia, observes, in reference to the foregoing, that "these ends are perfectly attained by the process given by Dr. Hare, which leaves nothing to desire, excepting on the score of economy." Were the oxymuriate of potash to be manufactured in the large way, the two processes, that of making the salt and of bleaching the resin, might be advantageously combined. "Dissolve," (says Dr. Hare,) “ in an iron kettle, one part of pearl-ash in about eight parts of water; add one part of seed or shell-lac, and heat the whole to ebullition; when the lac is dissolved, cool the solution, and impregnate it with chlorine till the lac is all precipitated. The precipitate is white, but its colour is deepened by washing and consolidation; dissolved in alcohol, lac bleached by the process above-mentioned yields a varnish which is as free from colour as any copal varnish." About the same period of time as the publication of the foregoing, the before-mentioned premium of the Society of Arts was claimed by two persons, Mr. George Field, and Mr. Henry Luning. The Society, upon a due examination of both of the processes and products, found them both to answer the intended purpose, and awarded the sum of twenty guineas to each of the candidates. The following is Mr. Field's process: Six ounces of shell-lac, coarsely powdered, are to be dissolved by gentle heat in a pint of spirits of wine; to this is to be added a bleaching liquor, made by dissolving purified carbonate of potash, and then impregnating it with chlorine gas till the silica precipitates, and the solution becomes slightly coloured. Of this bleaching liquor add one or two ounces to the spirituous solution of lac, and stir the whole well together; effervescence takes place, and when this ceases, add more to the bleaching liquor, and thus proceed till the colour of the mixture has become pale. A second bleaching liquor is now to be added, made by diluting muriatic acid with thrice its bulk of water, and dropping into it pulverized red lead, till the last added portions do not become white. Of this acid bleaching liquor, small quantities at a time are to be added to the half bleached lac solution, allowing the effervescence, which takes place on each addition, to cease before a fresh portion is poured in. This is to be continued until the lac, now white, separates from the liquor. The supernatant fluid is now to be poured away, and the lac is to be well washed in repeated waters, and finally wrung as dry as possible in a cloth. The lac obtained in the foregoing process is to be dissolved in a pint of alcohol, more or less, according to the required strength of the varnish; and after standing for some time in a gentle heat, the clear liquor, which is the varnish, is to be poured off from the sediment. Mr. Luning's process is as follows:-Dissolve five ounces of shell-lac in a quart of rectified spirits of wine; boil for a few minutes, with ten ounces of well-burnt and recently heated animal charcoal, when a small quantity of the solution should be drawn off and filtered; if not colourless, a little more charcoal must be added. When all colour is removed, press the liquor through silk, as linen absorbs more varnish, and afterwards filter it through fine blottingpaper. In cases where the wax found combined with the lac is objectionable, filter cold; if the wax be not injurious, filter while hot. This kind of varnish should be used in fa temperature of not less than 60° Fahr.; it dries in a few minutes, and is not afterwards liable to chill or bloom; it is therefore particularly applicable to drawings and prints which have been sized, and may be advantageously used upon oil paintings which have been painted a sufficient time, as it bears out colour with the purest effect. This quality prevents it from obscuring gilding, and renders it a valuable leather varnish to the book-binder, to whose use it has already been applied with happy effect, as it does not yield to the warmth of the hand, and resists damps, which subject bindings to mildew. Its useful applications are very numerous, indeed, to all the purposes of the best hard spirit varnishes: it is to be used under the same conditions, and with the same management. Common seed-lac varnish is usually made by digesting eight ounces of the bright, clear grained lac in a quart of spirits of wine, in a wide-mouthed bottle, putting it in a warm place for two or three days, and occasionally shaking it. When dissolved it may be strained through flannel into another bottle for use. In India, lac is fashioned into rings, beads, and other trinkets. Its colouring matter, which is soluble in water, is employed as a dye. The resinous portion is mixed with about three times its weight of finely powdered sand, to form polishing stones. The lapidaries mix powder of corundum with it in a similar manner. LACQUERING is the application of transparent or coloured varnishes to metals, to prevent their becoming tarnished, or to give them a more agreeable colour. The basis of them is properly the lac described in the preceding article; but other varnishes made by solutions of other resins, and coloured yellow, also obtain the name of lacquer. Strictly speaking, lacquer is a solution of lac in alcohol, to which is added any colouring matter that may be required to produce the desired tint; but the recipes that have been published in various scientific journals contain apparently a great many useless articles. The following is much extolled, in Nicholson's Operative Mechanic, as a lacquer for philosophical instruments : oz. of gum guttæ. 2 oz. of gum sandarac. 2 oz. of gum elemi. 1 oz. of dragon's blood, of the best quality. 1 oz. of seed lac. oz. of terra merita. 20 oz of pure alcohol. Before, however, the reader ventures to meddle with so formidable a list of ingredients as the foregoing, we would recommend him to make trial of the following more simple compound:-Take 8 oz. of spirits of wine, and 1 oz. of annatto, well bruised; mix these in a bottle by themselves: then take 1 oz. of gamboge, and mix it in like manner with the same quantity of spirits. Take seed-lac varnish, (described under the previous article LAC,) what quantity you please, and colour it to your mind with the above mixtures. If it be too yellow, add a little from the annatto bottle; if it be too red, add a little from the gamboge bottle; if the colour be too deep, add a little spirits of wine. In this manner you may colour brass of any desired tint: the articles to be lacquered may be gently heated over a charcoal fire, and then be either dipped into the lacquer, or the lacquer may be evenly spread over them with a brush. LACE. A delicate kind of net-work, composed of silk, flax, or cotton threads, twisted or plaited together. The meshes of this kind of net are of a hexagonal figure, in which thick threads are also woven to form the pattern according to some design; and these threads, which are called gymp, form the ornament of the lace. Buckinghamshire and Bedfordshire have been for many years the counties most celebrated for the manufacture of the pillow or bobbin lace, so called because it is woven by women or children upon a pillow or cushion, by means of bobbins, (which are made of ivory or bone, and each of which contains a small quantity of fine thread,) in such a manner as to make the lace exactly resemble the pattern, which is fixed upon a large round pillow, and pins being stuck into the holes or openings in the pattern, the threads are interwoven by means of the bobbins. At the close of the last century, the manufacturers of Nottingham directed their ingenuity to imitate this species of lace by machinery, in which they have completely succeeded. The Nottingham imitations of lace are of two kinds-point-net and warp-net. The point-net frame is a variety of the stocking frame, which was invented by Mr. John Morris, of Nottingham, in 1764; but it was not at first used to make lace, being intended to make the ancle part of stockings. The machine is an addition to a stocking frame, and operates on the thread in the same way as in stocking weaving for a great part of the process. The Nottingham lace, therefore, is only a modification of the stitch or loop of which stockings are made; all the meshes being formed by a continuance of one thread, which is by the machine formed into loops, a whole course at once, by pressing it down alternately over and under between a number of parallel needles; a second course is then made of similar loops on the same needles, and the loops of the first are drawn through those of the second in such a manner as to form meshes by retaining the first loops; the second are then retained by a third course, and this by a fourth, and so on. The warp-net frame is also a variety of the stocking frame, but the parts are very differently arranged, the movements being produced by treadles, leaving the hands of the workman to manage the machine, which is a piece of mechanism applied in front of the row of needles of the frame. In the warp frame the piece of lace is not formed of one continued thread, as in the point-net frame, but there are as many different threads as there are needles in the frame; these threads are warped, or wound upon a roller or beam the same as a loom; and it is from this circumstance that the machine is called a warp frame. These threads pass through eyes in the ends of small points, called guides, which are opposite the needles; and these guides are fixed on two bars, each of which has half the guides fastened on it; that is, one guide is fast in one bar, and the next in the other, and so on alternately of the whole. Each of the guides presents a thread to its needle, and are all at once moved by the hand to twist the threads two or three times round the needles which are opposite them: the loop is now made in a manner similar to the other frame. The next time, the alternate guides are shifted endways, so as to apply themselves to other needles than those they were opposite before this crosses the thread so as to make a net; but the quantity which is shifted endways is altered every time, by means of the machinery, so as to move a certain number of needles, which number is altered every time to produce the pattern. In 1809, Mr. John Heathcoat invented a machine for weaving the real twisted lace, like that which is made on the pillow. The ground-work of the invention is to extend those threads which form the warp of the lace in parallel lines, and dispose the diagonal threads upon small bobbins, which are detached, and are capable of passing round the extended warp threads, so as to twist with them; by this means, the number of bobbins is reduced to one half. In this machine there are two horizontal beams or rollers, one to contain the thread, and another to receive the lace; also a number of small bobbins to contain the thread. Since Mr. Heathcoat's first invention, the manufacturers of Nottingham, Leicester, Tewkesbury, and many other places, have vied with him and each other in the production of lace-making machinery. In 1824 the different descriptions of machinery for making lace were enumerated under the following heads: the old Loughborough double tier, Heathcoat's; the single tier on Stevenson's principle; improved double tier, Brailey's; single tier on Lever's principle; the old Loughborough improved, with pumping tackle; the pusher principle; the traverse warp, Bevan and Freeman's; traverse warp rotatory, Lindley and Lacey's; the straight bolt, Kendal and Mauley's; the circular bolt, Mauley's; the circular comb, Hervey's; the circular comb improved, Hervey's; and the improved Lever's. The foregoing comprehend the different principles upon which the machinery for making bobbin-net lace have been founded. In 1824 Mr. Longford took out a patent for actuating several of the foregoing machines by rotatory motion, which were previously worked by a beating or lever action of the hands and feet of the operator. Since the last-mentioned period there have been a great many patents taken out for improvements, the description of which alone would occupy a large volume, and require some hundreds of engravings to render them intelligible. We can therefore only refer the reader to such works as are distinguished by subjects of this nature,viz. The London Journal of Arts, The Repertory of Arts, the Register of Patent Inventions, and to the enrolled specifications,-for such further information that he may require. LACTOMETER. An instrument invented by Mr. Dicas, of Liverpool, for the purpose of ascertaining the different qualities of milk from its specific gravity compared with water. On this subject Dr. Ure observes, that it is not possible to infer the quality of milk from the indications merely of a specific gravity instrument, because both cream and water affect the specific gravity of milk alike. "We must first use as a lactometer a graduated glass tube, in which we note the thickness of the stratum of cream afforded, after a proper interval, from a determinate column of new milk; we then apply to the skimmed milk a hydrometric instrument, from which we learn the relative proportions of curd and whey. Thus the combination of the two instruments furnishes a tolerably exact lactometer." J 2 3 LADDER. A portable frame, containing steps for the feet. There are various kinds, most of which are too familiar to the readers of this work to need description; but there is one of a very ingenious description, described under the head of FIRE ESCAPES, invented by Mr. Gregory, which is evidently applicable to a great variety of purposes, wherein common ladders are useless, or of difficult employment. Ladders are very advantageously employed in the raising of weights, by the addition of a pulley-wheel at the top, or suspended over them; passing over this pulley is a rope, to one end of which is attached the article to be raised, (a tea-chest, for instance, out of the hold of a ship;) a man then ascends the ladder to the required height, and steps on to a foot - board, properly contrived for the purpose, which is attached to the other end of the rope just mentioned; the man's weight, then, more than counterpoising the tea-chest, he rapidly descends, while the chest ascends through the same space. this manner the tea is unloaded from the East India Company's dock at Blackwall, and it is very probable there is not a more efficient mode of applying a man's labour for that purpose, and the mechanisin is cheap, convenient, and easily adjustable to the space. The man has only to ascend the steps of the ladder, and he is refreshed in the descent, the frame in which he stands sliding over the inclined plane of the ladder. In Mr. W. Hilton has likewise ingeniously converted the fixed ladder against a trap door, into a crane for lowering heavy or bulky articles, such as a pipe of wine from a warehouse into a cellar, for the communication of which invention to the Society of Arts Mr. Hilton was rewarded by an honorary medal. A very convenient folding ladder is manufactured by Mr. Green, of Goswellstreet, of which the cut in the preceding page is a representation. Fig. 3 shows the ladder as opened out for use; Fig. 2 shows the ladder in section, half open, and the manner in which the rounds are jointed to the side rails; and Fig. 1 exhibits the ladder folded up close, forming exteriorly a round pole, tapered at each end. Mr. Green has likewise contrived an excellent ladder for the purpose of rescuing persons who may have the misfortune to sink under ice. LAKE. A name given to several pigments formed by precipitating colouring matter with some earth or oxide. The principal lakes are carmine, Florence lake, and madder lake; the first of these has been already described under its initial letter. See CARMINE. Florentine lake is prepared from the sediment of the cochineal, which is deposited in the preparation of carmine, and the red liquor also remaining from the same; these are boiled with the requisite quantity of water, and afterwards precipitated with the solution of tin: this precipitate must be frequently edulcorated with water. Exclusive of this, two ounces of fresh cochineal, and one of crystals of tartar, are to be boiled with a sufficient quantity of water, poured off clear, and precipitated with the solution of tin, and the precipitate washed. At the same time two pounds of alum are also to be dissolved in water, precipitated with a lixivium of potash, and the white earth repeatedly washed with boiling water. Finally, both precipitates are to be mixed together in their liquid state, put upon a filter and dried. A cheaper kind of crimson lake is prepared,— Brazil-wood may be employed instead of cochineal, and treated in the foregoing manner. Several modes of preparing fine red lakes from the madder of different countries were communicated to the Society of Arts by Sir H. C Englefield, to whom the Society awarded a gold medal for the same. The following is his process of preparing it from the Dutch crop-madder :-Two ounces troy of the finest quality is to be inclosed in a bag of fine and strong calico, large enough to hold three or four times as much; put it into a large marble or porcelain mortar, and pour on it a pint of clear soft water, cold; press the bag in every direction, and pound and rub it about with a pestle, as much as can be done without tearing it, and when the water is loaded with colour, pour it off. Repeat this process by adding fresh water till all the colour is extracted. Heat all the liquor in an earthen or tinned copper vessel, or what is better, a silver vessel, until it just boils; then pour it into a large earthen vessel, and add to it one ounce troy of alum, dissolved in a pint of boiling soft water, which must be thoroughly mixed. Pour in about an ounce and a half of a saturated solution of sub-carbonate of potash; a precipitation will ensue; let it stand till cold, when the supernatant clear, yellow liquor may be poured off from the red precipitate. A quart of boiling water should again be poured on it and well stirred. When cool, the colour may be separated from it by filtration through paper in the usual way; and boiling water should be poured on it in the filter till it passes through of a light straw colour, and free from an alkaline taste. The colour may now be gently dried, and when quite dry it will be found to weigh half an ounce; just a fourth part of the weight of the madder employed. If less alum be employed, the colour will be somewhat deeper: with less than three-fourths of an ounce, the whole of the colouring matter will not unite with the alumina. One ounce of alum to two ounces of madder is the best proportion. Spanish madder affords a colour of rather a deeper tint than the Dutch madder, but does not appear of so pure a red as the Zealand crop-madder. The lake produced from the foregoing process from Smyrna madder is remarkable for the richness and depth of its tint: the colour may be obtained from the fresh roots of madder, and will prove of equal if not superior quality to the dry. Upon the whole, the author of these processes considers the essential advantage of them to |