FOOTNOTES:
[B] This paper on "Attraction" is the substance of a lecture which I composed on the basis of notes taken by me when. I had the honour of attending the Prince of Wales at the course given, on the same subject by the late Professor Faraday. The Professor, having seen the resumé I had written, warmly commended the execution, and generously accorded me his sanction to make any use of it, whether for the purpose of a lecture or otherwise, as might seem good to me. It is on the ground of this sanction I feel warranted to print it here.
THE OIL FROM LINSEED.
Various processes have for a long time been in use for the purpose of extracting the oils from different species of nuts and seeds, a few of the more interesting of which are not unworthy of brief notice and description.
In Ceylon, where cocoa-nuts and oil-producing seeds abound, the means employed by the natives in the last century for extracting the oils were of a most primitive character. A few poles were fixed upright in the ground, two horizontal bars attached to them, between which a bag containing the pulp of the seed or nut was placed. A lever was then applied to the horizontal bars, which brought them together, thus creating a pressure which, by squeezing the bag, gradually expressed the oil from the pulpy substance. This rude machine was at that time of day one of the most approved for the purpose.
The system of pestle and mortar was also in use, but as the process was necessarily very slow, this method was seldom resorted to. An improvement on this system was invented by a Mr. Herbert, whose design it had been to construct a powerful and efficient machine which should combine cheapness and simplicity. It consisted of three pieces of wood, viz., an upright piece fixed in the ground, from the lower and upper extremities of which there projected the two other pieces, the top one attached to the joint of a long horizontal lever, and the lower one to the joint of a vertical one. The fixed upright post and the horizontal lever formed the press. The bag of pulp being put between the upright one and the vertical, the pressure was obtained by suspending a negro or a weight from the lever.
In another press of the same or a similar kind, the bags were placed in a horizontal frame, and a loose beam of wood pressed down on it by a lever.
Another form of press had cambs and wedges; also a modification of it by Mr. Hall of Dartford, who applied the pressure by means of a steam-cylinder. The cambs are arranged alternately, so that one is filled while the other is being pressed. This brief notice will suffice to give an idea of such machines as are wrought by lever pressure.
We pass on, therefore, to later inventions and improvements.
First, The Dutch or stamper press, invented in Holland; second, the screw; and, third, the hydraulic:—
(1.) The stamper press is something like a beetling-machine, in which wedges are driven in between the bags, containing, of course in a bruised condition, the seed to be pressed.
(2.) The screw press has an ordinary square-threaded screw, and it acts in the same way as press for making cider or cheese.
(3.) The hydraulic press. Here the pressure is produced by means of a piston driven up by the force of water, the immense power of which is, in great part, due to its almost total incompressibility. This is by far the most perfect form of press. Its power must be familiar to all who remember the lifting of the tubes of the Britannia Bridge, and the launching of the Great Eastern.
An oil-mill is in form something like a flour-mill. The operation begins at the top, where the seed is passed through a flat screw or shaker and then through a pair of rollers, which crush it. These rollers are of unequal diameter, the one being 4 feet, and the other 1 foot; but they are both of the same length, 1 foot 4 inches, and make fifty-six revolutions in a minute. By this arrangement it is found the seed is both better bruised and faster than when, as was formerly the case, the rollers were of the same diameter. A pair of rollers will crush 4-1/2 tons of seed in eleven hours, a quantity enough to keep two sets of hydraulic presses going.
After the seed is crushed in this way, it is passed under a pair of edge stones. These stones weigh about seven tons, are 7 feet 6 inches in diameter and 17 inches broad, and make seventeen revolutions a minute. If of good quality, they will not require to be faced more than once in three years, and they will last from fifteen to twenty. They are fitted with two scrapers, one for raking the seed between the stones, the other for raking it off at the proper period. One pair of stones will grind seed sufficient for two double hydraulic presses, and the operation occupies about twenty-five minutes. The seed is now crushed and ground, but before it is passed on to the press it is transferred to the heating-kettle.
The heating-kettle is composed of two cylindrical castings, one fitting loosely into the other, so that a space is left between them for a free circulation of steam all round both the sides and bottom of the interior vessel. The internal casting is again divided horizontally into two partitions, one above the other therefore, by two plates, between which also there is a space left for the admission and circulation of steam; and a communication is kept up between the upper compartment and the under by means of a stripping valve. Besides this, there is a communication from the internal kettle through the external one, and also a shaft passes between the two horizontal parts to give motion to the stirrer, which revolves thirty-six times a minute. A cover encloses the top, and it is through this the vessel is charged. The upper portion is filled first, where the contents introduced are allowed to remain ten or fifteen minutes, after which the valve is opened and the whole falls into the lower kettle, where it is kept till wanted. The seed is then taken away from the lower kettle by an opening, and bestowed in bags of sufficient size to make a cake of 8 lbs. weight after the oil is pressed out of it. Indeed, the compartments of the heating-kettle are of a size to contain enough to charge one side of a hydraulic press. These, therefore, are so constructed as to render the operation continuous, the upper one being discharged into the under as soon as its contents are withdrawn to the press. The seed is heated to the temperature of 170 degrees Fahr., when it is drawn off and placed in the bags.
In another form of kettle the seed is heated on a hot hearth, and on the top of the hearth is a loose ring, within which a spindle revolves to stir the seed. After the requisite temperature has been reached, the ring is raised and the seed swept into the bags, which are made of horse-hair. There is great loss of heat in this method, however, as the seed is exposed to the atmosphere, which of course cools it.
We now come to the final operation, the mode of expressing the oil. The screw press we do not need to describe, as it consists simply of two plates, brought together by a screw, in the same way as the press used for squeezing apples in the manufacture of cider, and the cheese press. Let us look therefore at the stamper press. It consists of an iron box, open at the top, at each end of which are two plates, capable of containing between them a bag of seed which shall yield a cake weighing 9 lbs. To one of the inner plates of the box is attached a wedge, beside which is inserted another filling up, and then the driving wedge is introduced; and lastly, another block is let in between this wedge and the other plate as soon as the bags have been placed vertically in the press-box. A stamper of wood, worked by cambs on a revolving shaft, is allowed to fall about 1 foot 10 inches, at the rate of fifteen strokes a minute, for about six minutes. This stamper is 16 feet long by 8 inches square, and falls on the head of the wedge, and drives it in to a level at the top of the box. Another stamper is employed to drive down an inverted wedge, so as to release the working one, and enable the attendant to take out the cake. A press of this kind will turn out only about 12 cwts. of cake a day.
We come now to the hydraulic press. This is certainly the most approved invention that has yet been adopted, and it is simply a Bramah press adjusted for the purpose. It has been in use for about thirty years, though it was, of course, at first less skilfully and scientifically constructed than it is now. In one of the earliest of these presses, the box which contains the seed runs on a tramway in order to facilitate its removal from the heating-kettle, so that each time the bags have to be replenished the whole box has to be removed; and this causes no inconsiderable loss both of power and time, for it has, when filled, to be replaced on the ram and lifted bodily upwards in order to bring it flush with the top of the press, which fits the press-box and acts as a point of resistance. In this arrangement there are introduced only one press and one set of small pumps.
The next press we come to is Blundell's, which is admitted to be by far the most efficient in use to-day. Here there are two distinct presses, or a double hydraulic press, fed by two pumps, one 2-1/2 inches and the other 1 inch in diameter, both connected with the separate cylinders by hydraulic tubing. The stroke of these pumps is 5 inches, and they make thirty-six strokes a minute. The larger pump is weighted to 740 lbs. on the square inch, and the smaller to 5540 the square inch. The diameter of the rams is 12 inches, and the stroke 10 inches. Each press is fitted to receive four bags of seed, and it produces 64 lbs. of cake at each operation. After the heated seed has been placed in the bags, the attendant proceeds to fill one press, and then he opens the valve between the large pump and the charged press, which causes the ram to rise till there is a pressure of forty tons, whereupon the safety-valve of the large pump opens, and is kept so by a spring. While this operation is going on, the attendant is occupied with filling the second press; which completed, he opens the communication between the large pump and the second press, taking care first to replace the safety-valve. The ram of this press is then raised to the same height as the other, after which the safety-valve rises a second time. The attendant, as he closes the valve which opens the communication between the large pump and the press, at the same time opens the valve between the small pumps and the presses; and the pressure, amounting to about 300 tons, exerted by the small pump, is allowed to remain on the rams for about seven minutes. From which it appears that, allowing three minutes for emptying and charging the press, the process of expressing the oil takes only three minutes in all; and it is done by this press in this brief time in the most effectual manner. The oil, as it is expressed, passes through the canvas and hair bags to a cistern, known as the spill-tank, which is just large enough to contain the produce of one day's working. The presses are worked by oil instead of water, as it keeps both presses and pumps in better order. Each of them will produce 36 cwts. of cake per day of eleven hours, while the yield of oil is about 14 cwts. The oil is pumped from the spill-tanks to larger ones, capable of holding from 25 to 100 tons, where it remains for some time in order to settle previously to being brought to the market.
I do not intend to enter into the relative merits of the various presses, but content myself with having explained to you the manner in which the oil is produced.
Before concluding, it may be interesting to give you some idea of the vast extent of this manufacture. It appears, according to the official returns, that in the year 1841 we imported 364,000 quarters of seed.