Eflimate of Utenfils and Men for the Work of refining Saltpatre.

25

These waters are therefore very different in their nature. The water of the washings is really a mother water. It must be collected in veffels, and treated with potash by the known proceffes. It must be evaporated to 66 degrees (or 1,848 fp. gr.), taking out the muriate of foda as it falls. This folution is to be faturated with 2 or 3 per cent. of potash, then fuffered to fettle, decanted and poured in to cryftallizing veffels, where 20 per cent. of water is to be added to keep the whole of the muriate of foda fufpended.

The waters which are thus obtained by treatment of the mother water, may be mixed with the water of the first crystallization. From thefe the marine falt may be feparated by fimple evaporation; and the nitrate of potafh, which they hold in folution, may be afterwards obtained by cooling.

The small quantity of water made use of to wash and whiten the refined faltpetre, contains nothing but the nitrate of potash: it may therefore be used in the folution of the faltpetre when taken from the tubs.

From this description it follows, that a manufactory for the speedy refining of faltpetre ought to be provided with (1) mallets or rammers for pounding the faltpetre, (2) tubs for washing, (3) a boiler for folution, (4) a cryftallizing veffel of copper or lead, in which the faltpetre is to be obtained by cooling, (5) baskets to drain the crystals, (6) a wooden cafe or hopper for the last washing and draining the faltpetre, (7) fcales and weights for weighing, (8) hydrometers and thermometers to afcertain denfities and temperatures, (9) Fakes to agitate the liquor in the cryftallizing veffel, (10) skimmers to take out the crystals and convey them to the baskets, (11) fyphons or hand-pumps to empty the boilers. The number and dimenfions of these feveral articles must vary according to the quantity of faltpetre intended to be refined.

If it be proposed to refine ten thousand weight of crude faltpetre per day, the requifite men and utenfils may be determined as follows:

Part of the ground near the magazine may be difpofed for conveniently breaking and pounding the faltpetre.

This ground fhould be paved with large flat stones very uniformly, or with thick pieces of wood. Mallets fimilar to those used in pulverizing gypsum may be applied to this use. Two men are fufficient to weigh and pound the faltpetre, and ftow it in the magazine. As the three washings require two days, and each tub can hold only five or fix hundred pounds of faltpetre, it would require twenty days to refine ten thousand weight (with one tub). These tubs are two feet and a half in height, and the fame in diameter. They must be very well made, in order that the water of the washing may not leak out. They are to be placed folidly on a plane flightly inclined, of fuch a material as fhall not imbibe the water which may be fpilt during the operation, but tranfmit it to a refervoir placed at the extremity of the row of tubs,

Twenty of these tubs must be difpofed in two parallel lines; the planes on which they are fet may incline towards each other, and form, by their union, the gutter or cavity for tranfmitting into the common refervoir fuch waters as may efcape. These tubs are perforated at the distance of two inches from the bottom. The aperture is clofed by a fpigot Four men may be appropriated to the washing of the faltpetre. It is a part of their duty to convey the faltpetre from the magazine to the tubs, and from the tubs to the boiler. VOL. II.-APRIL 1798.

E

I

It is fcarcely neceffary to obferve that the tubs muft ftand fufficiently apart, and be so difpofed that the work may be eafy and convenient.

A boiler of a conical form, five feet wide and four deep, will ferve for three operations per day, and confequently to refine fifteen thoufand weight. A fingle man is fufficient to attend the boiler.

The veffel for cryftallization is of lead or copper, and must be as near the boiler as poffible. Its depth is fifteen inches, its length ten feet, and its width eight. It must be placed on a very folid fupport, fo that its bottom may every where reft upon it. It is convenient to raife this fupport of masonry about twelve inches above the ground. By this means the borders of the cryftallizing veffel will be 27 inches above the floor, which will render the operation cafy and convenient.

It has appeared to us of advantage to give to the bottom of the cryftallizing veffel, an inclination of four inches from the fides to the middle, merely in the longitudinal direction. The folutions may be emptied for feveral fucceffive times from the boilers into the cryftallizing veffel, after having taken out the cryftals depofited from each folution.

Four men appear neceflary for the operation of the cryftallizing veffel. Their bufinefs is to agitate the fluid continually with the rakes. They collect without intermission on the borders of the veffel the cryftals which fall down, and convey them with a skimmer to the baskets prepared for their reception and draining. These fame workmen put the faltpetre into the wooden veffel for the last washing and drainage, and carry the refined faltpetre into the magazine.

For want of a large cooler for crystallization a shallow boiler may be used, or the fame veffels which ferve for cryftallization in the present works for refining this falt.

To prepare the faltpetre for the manufacture of gunpowder, it may be dried, after refining, by two proceffes, (1) by expofing it to the open air, or the fun, for feveral hours, upon tables; or (2) by expofing it in a fhallow metallic veffel for two hours, to the heat of 40 or 50 degrees (about 135 Fahrenheit). In either cafe it must be agitated and stirred. with fcarcely any interruption, in order to dry it speedily and equally.

Confiderable experience has fhewn us, that the procefs here defcribed is the moft fimple and economical. But, to prevent others from trying fuch methods as might seem promifing, though we have thought fit to reject them, it will be proper to offer the following remarks: 1. Trial has been made to diffolve the faltpetre, cryftallize it, and then wash it to feparate the fea-falt.

This procefs appears most advantageous at firft fight, because it faves the pounding; but it has great inconveniences. 1. Crude faltpetre diffolved in 50 per cent. of water, and poured into the vessel for crystallization, does not deposit the same quantity of faltpetre as it would do if washed before the folution. This difference depends on the muriate of foda in the crude faltpetre, which facilitates the folution of the nitrate of potash, and confequently the water of the cryftallizing veffels must retain in folution more nitrate of potafh, when the crude faltpetre is diffolved, than when it has been previously washed with

cold

old water, and deprived of the marine falt which it contains. 2. The washing of faltpetre, when it is performed after the folution and cryftallization, requires 40 or 50 per cent. of water instead of 35.

3. Trial has been made to diffolve faltpetre in 20 or 25 per cent. of boiling water; to take out the fea-falt in proportion as it falls by the ebullition of the liquor; to dilute the fluid with 30 per cent. of additional water, and then to convey it into the vessel of crystallization. It was expected that the washings with cold water might be avoided, or confiderably diminished by thefe means; but, not to mention that ebullition maintained for four or five hours to feparate the fea-falt fuppofes very great confumption of time, fuel, and faltpetre, the washings are ftill indifpenfable to remove the colouring principle, and to carry off the last portions of muriate of soda.

4. It may be thought that the quantity of water in the washings might probably be diminished; but there is reafon to fear that when the faltpetre is loaded with fea-falt, it cannot be perfectly refined by the ufe of a lefs quantity of water than we have prefcribed.

5. The operator may alfo be tempted to diminish the proportion of water employed in the folution; but we are convinced by numerous experiments, that this proportion is the most fuitable. If it be increased, the faltpetre remains diffolved in the fluid; if it be diminished, it fettles or falls down in a folid mafs. Obfervation has proved, that the degree of faturation beft adapted to this work, is between the 66th and 68th degrees of the hydrometer (fp. gr. 1.848, and 1.896.)

6. It may likewise be thought more fimple and economical to treat the solution of crude faltpetre with potash. But it is to be feared in this cafe that part of the alkali might be employed in decompofing the muriate of foda, to convert it into muriate of potash; and it must be observed, that this last falt is not at all proper to decompose the earthy nitrates, notwithstanding the affertions of skilful chemifts to that effect.

It appears, therefore, more convenient to defer the treatment of the mother waters, and not to use potash till after the fea-falt has been separated by evaporation.

This process, therefore, unites a number of advantages.

1. It confumes much less fuel: for, inftead of two long folutions and ebullitions, nothing more is required than to give the water a boiling heat in order to diffolve the faltpetre.

2. It requires lefs time. Three days are fufficient to purify the faltpetre to the degree fuitable for making gunpowder.

3. It difpofes the faltpetre to dry more readily. As the cryftals are no larger than fmall needles, a few hours' expofure to the air are fufficient for its complete deficcation. This advantage is inestimable, particularly in a season wherein several months would be required to drain the large loaves of nitre; and in which, confequently, the fabrication of gunpowder would be either retarded or fufpended, and the drying-houfes encumbered with quantities of humid nitre.

4. It requires lefs space. A boiler five feet in diameter and four in depth, a veel for cryftallization of a few feet dimenfions, and thirty tubs, are perfectly fufficient to refine fifteen thousand weight daily.

5. It occafions lefs lofs. Very accurate experiments have shown that the folutions ufed in the ancient procefs occafioned a lofs of faltpetre, by miere evaporation, which amounted to 7 per cent. of the original quantity. By this new procefs, the water which holds the faltE 2 petre

petre in folution is never heated to boiling, the falt does not remain in the boiler, and theevaporation is almost nothing.

VI.

On the Light emitted by Superfaturated Borate of Soda, or Common Borax. By Mr. F. AccUM.

SIR,

THAT

To Mr. NICHOLSON.

HAT two flints and several other filiceous ftones, ftruck against each other, appearlu. minous on the fide ftruck upon; and that phosphate of lime, tremolite, fugar, gum elemi, black jack, and various refins become luminous, and emit phosphoric sparks in the dark, when scratched with a sharp inftrument, or struck against one another, are well known to every tyro of natural philofophy; but that fuperfaturated borate of foda poffeffes this pro perty in the highest degree, has not perhaps been hitherto remarked. Two pieces of this falt, of confiderable magnitude, struck against another, or a swift blow with any fharp in ftrument upon it, produces fuch a flash of white light, as none of the before-mentioned fubftances are capable of giving. It deferves therefore a place under the clafs of those kinds of phofphoric fubftances which give a perceptible light by attrition or percuffion, without having been exposed either to the folar or artificial light; for which reason I take the liberty of laying this before you, begging you will give these lines a place in your much-admired Journal of Natural Philofophy.

I remain, Sir, your very humble fervant,

Hay-Market, No. 17.

VII.

FREDERICK ACCUM:

New Conftruction of the Air Pump. By Sir GEORGE, S. MACKENZIE, Bart:

FIG. I. plate 1, reprefents a section of the barrel. C is a cup for oil to moisten the collar

of leathers L, in which the piston rod R works. is a plug-P P P P is the piston, which is folid, except a fpace for a collar of leathers A, through which the wire W attached to the valve V, paffes into a perforation in the piston rod. N is a fmall nut to prevent the valve from rifing too high. This method of lifting the valve was invented many years ago by Dr. Rutherford of Edinburgh. X is a perforation in the side of the bottom of the barrel,. into which is inferted a piece of metal, as in fig. 5, with a filk valve tied over it, opening downwards into the pipe K. E is the pipe leading to the receiver.

The conftruction of the bottom of the barrel is feen in fig. 2 and 3. Fig. 4 is the valve and wire. Fig. 5 fhews the conftruction of the pipe E leading to the receiver, which is better than bent copper tubes, as thefe are apt to crack. Fig. 6 is caft folid and bored,