We have hitherto spoken only of the extraction of the finest oil, without heat. The application of heat before pressing is however necessary for obtaining the principal supply of oil. The precise order of the several operations, as well as the nature of the machinery employed, differs in different manufactories. In some establishments the oil-cakes, or solid contents of the bags, which remain after the first cold pressing, are taken out of the bags, broken to pieces, and put into mortars to be pounded by pestles worked by machinery. There the paste is again broken down, and the parenchyma of the seed reduced to a fine meal ; thus free egress is allowed to the oil from every vesicle in which it is contained ; but it is now rendered much more clammy by the forcible mixture of the mucilage, and even of the fine parts of the meal. When sufficiently heated it is removed to a chattier, or circular copper-pan ; in which, while it is kept continually stirred by machinery, it is heated to about the temperature of melting bees-wax, either by a charcoal fire, or according to a more recent practice, by steam. It is then, while hot, put into the bags and subjected to a second pressing ; and in some cases the like operations are repeated a third time, by which a further quantity of oil, but of inferior quality, is produced. Sometimes the produce of oil is increased by mixing a little water with the paste ; but this practice is considered to impair the quality of the oiL The oil-cakes which remain after the last process are used as food for cattle, and for various other agricultural purposes ; but of course the cakes vary greatly in richness according to the degree in which they have been divested of oil. There are small mills hi Ilelbuid which have no other employment than extracting oil from the cakes which they purchase from the French and Brabanters, after passing the process of their mills : a clear indication of the superiority of the Dutch practice over that of their neighbours. In some of the Dutch mills, the produce is increased by the application of moderate heat during the grinding process, by enclosing a little furnace in the bed upon which the running-stones roll; but the utmost care la ttecessary to prevent the hest (rum becoming:too considerable, as it causes the ell to dissolve too much of the fermentable substance of the seed, and exposes it to the risk of soon growing very rancid. When the seed is very dry tho process of grinding may be facilitated by the addition of a little water. Tho oil produced by the above process needs little further attention. If left in a cistern, as It is by the Dutch manufacturers, the parenchymatous part, which inevitably /eases away with the oil in some degree in the operation of pressing, will gradually subside, and the oil may be drawn off at various levels, of different degrees of purity ; the bottom being at length removed to a deep and narrow cistern, where it should be left a considerable time or tho dregs to subside.
The annexed cuts represent three stages in the process of obtaining linseed oil by the hydraulic press ; namely, the crushing of the seeds between edge-rollers; the falling of the seeds into bags after grinding; and the piling of the bags in the hydraulic: press.
Mr. Samuelson, in a paper read before the Society of Mechanical Engineers, in 1859, entered fully into the respective merits of the different modes of expressing linseed oil. The latest and best arrange meat he stated to be that of Blundell's double hydrostatic press, with two presses and two pumps, all connected by hydraulic tubing. The larger pump is weighted to 740 lbs. per square inch pressure ; the smaller to 5540 lb. Each press works four bags of seed at once. A pressure of 40 tons is brought to bear on the seed in the first instance; and this is succeeded by one of 300 tons. Each press limitless 36 cwt. of oil cake per day of 11 hours, aud 14 cwt. of oil. It is a curious feature in this new form of hydraulic prose, that it is worked with limpid oil instead of with water. Mr. Samuelson stated that there are now more than one million quarters of linseed pressed for oil anunally in England ; that two-thirds of the whole trade is centred at Hull and that the produce comes out as about 144,000 tons of oil-cake and 56,000 tons of linseed oil.
Linseed oil is used principally as a vehicle for mixing oil-colours for painting, but it is also valuable in several branches of manufacturing industry, and, in a refined or purified state in some medicinal prepara tions. Being a fat or unctuous oil, it is slow in drying, aud as this is a great inconvenience for some purposes in painting, it is sometimes couvertod into drying oil by boiling it with sugar of lend, white vitriol, red lead, or other substances which possess similar properties. The common kinds of drying oil are generally known by the name of boiled oil.
The trade in lard oil in America is so remarkable, in its origin and its nature, as to deserve a few lines of notice. When the British coin miseioners were at the New York Exhibition in 1853, they obtained information, among other things, concerning what in America is called industry. About the year 1830, farmers in the central states,
especially Ohio, unprovided with cheap means of sending their produce to market (for there were no railways in those days), hit upon the plan of converting their corn into living meat, which could " walk itself off to market,"—in other words, fattening hogs. The trade grew year by year; until at length the city of Cincinnati soon became the centre of vast operations. The farmers in the country districts rear hogs, from 100 to 1000 on each farm. Drovers or jobbers come and purchase them, and drive them to Cincinnati, where they sell them to capitalists. These capitalists engage the services of killers, salters, curers, and packers, who keep largo establishments in and near the city. The hogs, when still in the farmers' hands, are allowed to roam about in the woods, and feed themselves on acorns and beech-mast ; but when the Him: for sale is approaching, they are called in, and fattened with an abundance of corn. Their average age at the time of sale is about 15 months, and average weight 200 lbs. When they arrive at the killing establishments, they are driven up an inclined plane to the upper floor or story, where they are penned for a short time. They next descend to the killing-room, where a poleaxe and a knife speedily end their existence. The dead bodies are at once thrown into a huge scalding tank, where the heat of hot water brings the outer skin to a state fit for subsequent operations. They are quickly passed to another room, where all refuse, from within and without the animal, is removed. All this is the work of the killers, who find a useful market for everything—skin, bristles, fat, intestines, blood, hoofs, &c. : nothing is wasted. In 1852 there were ten vast slaughter ing establishments in Cincinnati, at which no less than 400,000 hogs, weighing 80,000,00016s., were slaughtered in twelve months. From the slaughterers, the carcases go to the corers and packers. I Jere the hogs aro cut up into joints with surprising rapidity. Two men, the commissioners say, will cut up 850 hogs in a day of 13 hours, with two other men to carry and lift the carcases, and a fifth to trim the 1700 hams. The hams and shoulders are salted and cured; the middle pieces are salted, to make pickled pork, and packed in barrels For the navy and for foreign trade. The heads, the trimmings, various °the,' parts, and sometimes (in a dull state of the provision market), nearly the whole carcase, are rendered, or digested in steam tanks to get out the fat in the form of lard. In one establishment alone, 3,000,000 lbs. ofiliog'a lard were thus obtained in 1852. The lard is to some extent used as a substitute for butter, in cooking and culinary operations; but it is much more extensively applied to the production of lard-oil and stearine. 1Vhen pressed in a hair-cloth bag, the lard yields from 40 to 70 per cent. of oil ; the rest is a white solid stearinc. The stearin° is used for candles. The lard-oil, after being refiued at a temperature of 200' Fahr., is used for soap-making, and for cheapening finer oils. In 1852, there were more than thirty lard-oil factories in Cincinnati alone. This city is, as wo have said, the greatest centre of the trade; but it is not the only one. In eight of the middle states of the Union, there were, in the season 1852-3, more than 2,000,000 hogs cut up for meat, oil, stearine, &c.—the killing and salting being generally effected during the winter months.
The oil trade of this country is now a very considerable oue. In 1859, the exports iucluded (in round numbers) 20,000 tuns of train oil, blubber, and spermaceti ; 20,000 tuns of olive oil ; 11,000 tuns of various kinds of seed oil; 185,000 cots. of cocoa-nut oil ; and the enormous quantity Of 6S6,000 cwts. of palm oil. Few circumstances, in connection with the recent history of our manufactures, are more important or interesting than the rapid increase in the palm-oil trade. The collecting and shipping of this product are the work of free persons in Africa ; English gold finds its way thither in exchange; and it is possible (though the evidences of the fact are as yet very few) that the slave trade may be lessened by this gradual familiarising of the natives with the commercial dealings of Europe. The imports of whale oil are decaying, partly on account of the scarcity of whales in the North Atlantic, and partly owing to the increase in the use of vegetable oils. Of the seed oils, a large proportion is now Colza, or rape-seed oil, imported for use in lamps. In 1859 our farmers used 95,000 tons of oil cake, in addition to that obtained from oil-mills in this country. Of the 1,270,000 quarters of linseed imported, all but a small proportion was pressed for oil, the seed for sowing being com paratively small in quantity. In the same year, England exported nearly 8,000,000 gallons of seed oil ; this was nearly all linseed oil, and was probably mostly for use in house-painting.