the air; let them stand together, and stir them up several times for the first twenty-four hours; then let them stand a day, and the lime-water will sink below the oil, which must be carefully separated from them. Take this oil, if not sufficiently purified for your purpose, and treat it as directed in Process the Third, diminishing the quantity of pearl-ashes to one ounce, and omitting the lime and chalk.* ROBERT DOSSIE. No. 67. On the Purification of Rapeseed Oil. By C. THENARD.* TO purify oil of rapeseed, mix one hundred parts of the oil with from one and a half to two parts of sulphuric acid, and stir the mixture. The oil will immediately change its colour; it will become turbid and assume a blackish-green tint, and at the end of three quarters of an hour it will be full of flakes. You must then give over stirring it, and add gradually double its weight of water to remove the sulphuric acid, which, if allow ed to remain too long with the oil, would not fail to exercise too strong an action on it, and to char it. The mixture must then be beat for at least half an hour, to bring the molecule of the oil, the acid, and water, into contact with each other; after which it is to be left at rest. When it has rested about eight days, the oil will float on the surface of the water, and the latter will itself float on a black matter, precipitated from the oil by the sul • The dregs remaining after the sundry processes above mentioned will form an excellent manure, as has been since noticed in Dr. Hunter's Georgical Essays. Tilloch, vol. 10, p. 68. From the Journal de Physique, Floreal, an. 9. phuric acid: it is this matter which colours the oil, and prevents it from burning with facility. Three very distinct strata, then, are established, as is here seen: the upper one is oil; the second is aqueous, and contains a little sulphuric acid; and the third is carbonaceous. The oil which forms the upper stratum, after these eight days of rest, is far from being limpid: twenty days, in my opinion, would be necessary for it to purify itself merely by repose; but by filtration it may be immediately obtained perfectly clear and transparent. For this purpose, pounded charcoal, and a piece of linen or cotton cloth, may be employed: the two last substances are preferable to any other. The same cloth will serve several times, only it must be carefully cleaned. By following this process with attention, you may obtain oil which has much less colour, odour, and taste, than that commonly used; which will burn with the greatest facility, and without any residuum; and which is equal to the purest oil sold in the shops, &c. The loss is very inconsiderable. If you are desirous of obtaining it still purer, it may be exposed again to the same treatment; but, in that case, for one hundred parts of oil, one hundredth part of concentrated sulphuric acid will be sufficient. The sulphuric acid will not form in oil which has been once purified a blackish precipitate; on the contrary, it produces a very scanty precipitate, of a grayish-white colour. This precipitate is more difficult to be separated than the former. When the oil has been treated with two hundredth parts of sulphuric acid, if it be suffered to digest for twenty-four hours with the fourth of its weight of chalk or carbonate of lime, or of argil, you will obtain it almost as clear as water. Lime, however, cannot be employed with advantage, as it would occasion too much waste; but, in my opinion, argil would give very advantageous results: it retains, indeed, a pretty large quantity of oil, but, by means of a press, the last portions of the oil may be extracted from the argil almost entirely. No. 68. Plan for an improved Theatre. By Sir GEORGE CAY- (With an engraving.) Brompton, September 25, 1808. SIR-Since the lamentable accident, that has so lately happened to the Covent Garden Theatre, the frequent occurrence of that event to my thoughts has led me to speculate upon the various improvements, that might be made in the construction of theatres. I have taken the liberty of enclosing you the following plan and hints, which I conceive to be worthy your attention, inasmuch as they state undoubted principles, which local convenience may more or less permit to be put in practice, but without an attention to which no theatre can be pronounced well constructed. The science of acoustics is perfectly well understood, and the enclosed rough sketch of the internal plan and elevation of a theatre is modified to the principles of that science, in conjunction with giving the greatest possible convenience of sight to the largest num ber of people the space can contain. It is the property of an elliptical room, to collect all the sound uttered in one of its foci into the opposite focus by reflection; hence, as the ellipsis is a very beautiful curve, and as it is only the parts of a theatre distant from the stage, that require the aid of reflected sound, I have adopted this figure, as the ground plan, plate 10, fig. 1, will * Nicholson, vol. 22, p. 243. show. Here any voice uttered upon the stage at A would be concentrated at the point B, excepting what is absorbed by entering the side boxes. I have drawn the stage semicircular, and on one side arranged the seats concentrical with it. This, I conceive, would be a material benefit to the observers, but it would have this objection, namely, that the seats, if so placed, must rise in steps and have arms to each; hence the necessary allowance of room for the accommodation of the largest persons would be more than necessary for smaller ones; and on no occasion, however pressing, could the advantage be taken of sitting closer. I have also drawn the scenery in a portion of a circle, which would be a most material advantage, both to the hearing and sight, if conveniently practicable: and provided double the height of a scene can be had within the building, it might be managed by suspending the scenes on cords passing over rollers disposed in this form. In constructing the elevation of a theatre, the first consideration is to economise space, hence in the boxes, as at No. 1, fig. 2, after allowing the seats to rise one foot in five for the purpose of clearing the view from the heads of those below, if a line be drawn to the top of the scenery from the eye of the most backward observer, the bottom of the next tier of boxes must just commence at that line, as exhibited by dots. As it is advantageous in the metropolis to make theatres more extensive than the direct voice of an actor can fill with ease, it becomes necessary, to call in the aid of reflected sound, and so to distribute the whole voice as may be deemed most important. I have in the enclosed sketch supposed, that (in a theatre where the extreme part of a pit is one hundred and twenty feet from the centre of the stage) the direct voice is sufficient till within one fourth of the extremity of the building. There fore, the roof is so curved, as to commence its reflection at that point, as may be traced by following the progress of the pulses of sound emitted by the actor at A. One half the roof, as far as C, is allowed to give the sound it receives over this portion of the pit, and the three tiers of boxes. The remaining half of the roof is employed in throwing its sound upon the upper gallery, increasing the density of its reflection as the distance from the stage increases. Although this gallery receives the influence of half the ceiling, yet from the oblique position of it, it will not catch more than half as much sound as the other portion, which is fully required by the distance of the hind part of the gallery, the direct sound being there twentyfive times less dense than in the quarter of the pit next the stage; whereas by the reflection this disproportion will be reduced to about ten times only, and of course it will be as distinctly heard as in the third quarter of the pit. The ratio of sound in the three front boxes compared with that of the first quarter of the pit, is as one sixteenth to one; this, by the reflection of half the roof will be reduced to about one seventh, hence these parts will hear nearly as well as the centre of the pit. In addition to this, the back of each tier of boxes should be covered so as to give a focus of sound either to the front, middle, or last benches, as thought best, this shown at 1, 2, 3. The two latter were altered from the former by dotted coves. The fronts to the boxes should present reflecting curves, to throw their sound within the fourth region of the pit. Fig. 3 is a hind view, showing the proper curve of the roof in this position, where the only object is to keep the diverging rays of sound parallel after reflection, and clear of the sides of the boxes. I think it would not be particularly expensive to have the whole beam and pillar work of the theatre of cast |