or POTAMO, was a philosopher of Alexandria. He kept a middle course between the scepticism of the Pyrrhonians and the presumption of the dogmatists; but attached himself to none of the schools of philosophy of his time. He was the first projector of the Eclectic sect; for though the mode of philosophising had been pretty common before, he was the first that attempted to institute a new sect on this principle. Diogenes Laertius relates, that not long before he wrote his Lives of the Philosophers, an Eclectic sect, ἐκλεκτικοὶ τῶν δόγματων, had been introduced by Potamo of Alexandria, who selected tenets from every former sect. He then proceeds to quote a few particulars of his system from his Eclectic institutes, respecting the principles of reasoning, and certain general topics of philosophical inquiry; from which nothing further can be learned, than that Potamo endeavoured to reconcile the precepts of Plato with those of other masters. As nothing remains concerning this philosopher besides the brief account just referred to in Laertius, an obscure passage in Suidas, and another still more obscure in Porphyry; it is probable that his attempt to institute a school upon the Eclectic plan proved unsuccessful. The time when Potamo flourished is uncertain. Suidas places him under Augustus; but it is more probable, from the account of Laertius, that he began his undertaking about the close of the second century.
**Potash**, the lixivious ashes of certain vegetables, used in making glass, soap, &c. See Glass, Soap, &c. For an account of the properties and combinations of potash, see Chemistry. Potash was till lately considered as a simple substance; but it appears from the unexpected discoveries of Mr Davy in galvanism to be a compound of a peculiar metallic substance and oxygen. Soda is also a compound of a similar nature. For an account of Mr Davy's discoveries, see Zinc. Here we treat only of the manufacture of potash.
The method of making potash is directed by Dr Shaw as follows. Burn a quantity of billet-wood to gray ashes; and taking several pounds of these ashes, boil them in water, so as to make a very strong lixivium, or ley. Let this ley be strained through a coarse linen cloth, to keep out any black parts of the half-burnt wood that might happen to remain in the ashes; then evaporate this strained ley in an iron pan over a quick fire almost to dryness; then taking out the matter remaining at the bottom, and putting it into an iron crucible, let it in a strong fire till the matter is melted, and then immediately pour it out upon an iron plate, where it soon cools, and appears in the form of a solid lump of potash*. Much after this manner is potash made in the large way, for the service of the soap-boiler, glass-maker, fuller, &c., but according to the difference of the wood, or combustible matter employed, with the manner of turning it, and conducting the process, different kinds of potash are prepared. There are certain saline plants that yield this potash to great advantage, as particularly the plant kali; there are others that afford it in less plenty, and of an inferior quality, as bean-stalks, &c.; but in general, all vegetable subjects afford it of one kind or other, and may most of them be made to yield it tolerably perfect after the manner of the process already laid down, even the lop-pings, roots, and refuse parts of ordinary trees, vine clippings, &c. The fixed salts of all vegetables, excepting the kali and marine plants, when reduced to absolute purity, or entirely separated from the other principles, appear to be one and the same thing: whence it should seem, says Dr Shaw, that by a suitable management good saleable potash might be made in all places where vegetable matters abound. For if by examining Russia (A) potash, for example, we find that its superiority or excellence depends upon its being clear of earth, or upon its containing a large proportion of oil, or refined salt, these advantages may, by properly regulating the operation, be given to English potashes, so as perhaps to render the latter as good as the former: but where the potash of any remarkable saline vegetable is to be imitated, that of the kali, for example, the doctor recommends a prudent sprinkling of the subject with salt, or sea-water, in the burning; and by these ways, properly diversified, any principle that is naturally wanting might be artificially introduced so as to perfect the art of potash.
Above half a century ago, Mr Stephens, encouraged by the Society of Arts, &c., and by a parliamentary grant of £3000, established a manufacture of potash in phen's manufactory in North America, which produced such as was so perfectly good as to answer in bleaching and other uses the purposes of pearl-ash; and which at the same time afforded a very large produce. But the very great heat which his process required, occasioned the destruction of a very extensive apparatus; and other circumstances concurred to disappoint the hopes and check the spirit of the proprietors. The manufacture was, however, afterwards undertaken and prosecuted by others. Mr Stephens's apparatus was as follows: Fig. 1. A is the bed of the kiln, which flies off about four feet by two from the grate, more or less according to the size; C is the ash-hole, 2½ or 3 feet deep. Fig. 2. B represents quadrangular bars of iron, with their opposite angles placed upwards and downwards, not above an inch asunder. Fig. 3. A, B, and C, are three steepers five feet deep, and of any width from four to eight feet square, of the best white pine or cypress plank, with square joints and strong oak frames, placed each over a receiver, with a cock to let off the ley, and a vent just beneath the surface of the grating. E represents three receivers, standing each under, and projecting out, from its steeper. They must be made of the best stuff, carefully put together, and laid in tough clay well rammed within the ground, their tops being level with the surface: they need not be so large as the steepers by six, eight, or twelve inches. Fig. 4. F represents a false bottom or lattice of boards, eight inches deep and five square, with a hole in the under edge of every partition for the ley to pass into the steeper. Fig. 5. A is the vessel over the furnace in which the ley and ashes are mixed; B is a hole or funnel a few inches from the back of the furnace, with an iron socket to let the pipe through the hinder part of the arch, to reach down within two inches of the floor of the furnace. C is a cast-
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(A) According to Sir Peter Warren, the best woods for making Russian potash are, oak, ash, poplar, hickory, elm, hazel, and beech. They must be cut in November, December, January, and February, split and stacked to dry. After 12 months, in warm open weather, it must be burnt on a brick hearth by a slow fire in a kiln, or close place; the ashes must be sifted through two sieves, one finer than the other, and then put up in brick troughs or wooden backs, covered with rain or river water, and must remain well marthed and incorporated five months. Brick furnaces shaped like bakers' ovens must be heated with a strong fire of oak or ash, burning night and day; the prepared ashes must be gradually thrown on the fire, when they will run into metal like lead: the fire must not go out till the furnace is nigh filled with potashes. The ashes must then be broken to be taken out, but the larger the pieces the better; they must be preserved from the air in tight casks, the large pieces by themselves, and the dust by itself. east-iron cauldron for boiling the ley to dryness when pearl-ashes is made. D is a vessel whence the liquor is let into the cauldron as it evaporates. The mortar for building the furnace should be made of loam; the arch should be 18 inches thick, and the floor should be laid with tiles on a layer of sand an inch thick, with neat joints.
Mr. Stephens's process, both with and without the kiln, was as follows. Cut timber, felled at any season, into lengths of about eight feet: lay from three to ten of them lengthwise in a heap upon dry ground, and fill the vacancies between with smaller wood: the sooner it is burnt after felling, the better. Set fire to it by laying embers on the bottom logs at each end; and for burning the brush and lappings, with other smaller woods, lay them lengthwise on the ground, top to top, lapping over a little, with the butt ends outwards, and as close as a faggot; laying the larger woods on top till the heap is full four feet high; the length of the brush set against each other making the breadth of the heap. As to the choice of the timber, old hollow trees, if not dead, are best: pine, cypress, and cedar, are to be totally rejected.
As soon as the pile is burnt down, rake such ashes as lie round the outside a little in towards the middle; add no fresh fuel, nor throw on any brands. Let the ashes lie without stirring till you can just bear your hand in them; then carry them to a house, or under a shed, on a plank floor raised a little from the earth and well jointed; there wet them till brought nearly to the consistence of mortar in the first mixture of lime and sand, and ram them in a heap, in which they must lie full 20 days, or some months if you please; observing to be more sparing of water in winter, and ramming them closer, and sometimes wetting the top that it may never grow quite dry.
Wood may also be burnt in a kiln, as fig. 1. and 2.; and then it must be cut into such lengths as may be most convenient for carriage, and best suit the size of the kiln. The mouth of the ash-hole must be close stopped by daubing the joints of the lid with loam, or throwing a bank of sand or earth against it: keep the bed of the kiln filled with wood up to the surface, but not above it, and let it burn incessantly till the ashes rise within six or eight inches of the grate. Draw them out whilst red-hot, and in that state sprinkle them with ley, from four to five carats weight; weigh a small phial which holds about four ounces very exactly; then fill it with water and weigh that also: divide the weight of water into equal parts till you come to $\frac{1}{4}$ of the whole, which is called a carat, $\frac{1}{4}$ two carats, &c., until you have a weight equal to $\frac{1}{4}$ of the whole water, which is called 32 carats: all which small weights, together with one equal to the phial filled with water, are to be kept for weighing the ley in the said phial till they are made damp; then ram them as before in a heap, but separate from the ashes made as above. N.B. By kiln-burning a stronger ley may be more certainly procured than by the other way, where rain may chance to fall on the ashes before they can be removed.
The ashes thus prepared are to be put into vats or steepers, fig. 3., with a false latticed bottom, as fig. 4.; first putting coarse wheat or rye straw about a foot thick on the lattice or grating; on which put ashes to within four or five inches of the top, ramming them all the way up, especially at the sides, with a small light rammer, as tight as you can, without bursting the vat. Form on the top of the steeper a hollow basin in the ashes four or five inches deep, leaving the ashes four or five inches thick on the sides, by raising a small bank round the sides, so that the liquor may not overflow the edges of the ashes at top; keep this basin constantly filled with soft water in the steeper A, until the ashes will imbibe no more, which will be in 24 hours or more, according as it is rammed; then turn the cock, and let off what shall be soaked through into the receiver or lower chamber of the stepper, and no more; for if the several runnings are not kept separate, the ley will not be brought to its due strength. Follow that stepper with fresh water on the same ashes for several other runnings, which will each come off in a few days, till the liquor has neither smell nor taste; then heave out the ashes, and charge the stepper afresh.
Upon drawing off the first running from the stepper A, fig. 3., fill the stepper B with ashes as before, and put into its hollow at the top the ley to first run off, and the smaller or half leys also, till full, and draw off as directed for the stepper A; if this weighs 18 carats or more, pump it into the cistern F as fit for use; if it be short of that, pass it off as half ley to the stepper C, and through fresh ashes till strong enough. With kiln-ashes only, from water passing through the first stepper, it will be strong enough for the cistern, if the ashes are well prepared. If your water be hard, let it stand two or three days exposed to the air and sun in a shallow back, and it will be soft. When you use kiln-ashes with others, lay them at bottom.
The ley must be conveyed from the cistern F, as it is wanted to the vessel A fig. 5.; where with every gallon of proof ley mix three ounces of fine, light, wood ashes; and to the ley that is one-fourth over-proof put six ounces of ashes; and if two-fifths over-proof 12 ounces, increasing or decreasing according to the strength of the ley.
For evaporating the ley and melting the salt, heat a furnace till you bring it very near a white heat, of which the side-doors being red-hot is a mark. This will take 48 hours or more if the furnace be quite cold; when thorough hot, a little fuel keeps it so. Then, through the cock of the vessel A, pass the mixture by the funnel B into the furnace, not so as to reach much beyond the middle of the floor, before it changes from dark to bright red, letting the heat prevail towards front or back as you see necessary. When the mass begins to gather about the flues or in heaps, run in no more till the furnace is cleared by driving the fire backwards. You must have two funnels, one soon choking; in an hour or less will issue out a red-hot stream of melted salt, which is potash, to be broken to pieces as soon as cold, and packed in tight close casks, being in no respect inferior to the best foreign ash whatever.
The best potash is made from barilla, and comes from Spain. The plants from which it is procured are found in great plenty about Cartagena, where they are indigenous, and may be collected in a swamp called Almogar cask of that place; the Suyoner barilla is the best. They are found, besides all along that coast, on the borders of the Mediterranean for 60 leagues in length, and 8 in breadth. About 150,000 quintals of it are annually exported from Spain. It produces a revenue of 25,500l. H h 2 a-year; Potash.
Dr Percival's account of making potash.
In the 70th volume of the Philosophical Transactions we have an account of a method of procuring this salt from the putrid water which runs from dunghills. The process is very easy, consisting only in simple evaporation of the fluid, and calcining the impure salt till most of the foulness is burnt out. From 24 wine-pipes full of this muck-water were obtained 9 cwt. 1 q. 12 lb. of saleable potash, valued at 42s. per cwt.: the expense of manufacturing them being only valued at 1q. 9s.
The potash thus made is of a grayish white appearance; deliquesces a little in moist air; but if kept in a dry room, near the fire, acquires a powdery surface. It is hard and of a spongy texture when broken, with many small crystals in its substance. The colour of its internal parts is dusky and variegated. To the taste it is acrid, saline, and sulphureous. It emits no smell of volatile alkali, either in a solid form, dissolved, or when added to lime-water; neither does it communicate the sapphire-colour to a solution of blue vitriol. Silver is quickly tinged black by it; a proof that it contains much phosphorus. Ten grains of this potash required 11 drops of the weak spirit of vitriol to separate it. The like quantity of salt of tartar required 24 drops: a strong effervescence occurred in both mixtures; and a sulphureous vapour exhaled from the former. A tea spoonful of the syrup of violets diluted with an ounce of water was changed into a bright green colour by five grains of the salt of tartar; but ten grains of this potash were necessary to produce the same hue in a similar mixture. Half an ounce of the salt dissolved entirely in half a pint of hot water; but when the liquor was cold, a large purple sediment subsided to the bottom; and it was found that this sediment amounted to about two-thirds of the whole quantity of ashes used.
Dr Percival, the author of this paper, concludes with observing, that this potash is a true fixed vegetable alkali, produced by putrefaction; that the quantity of alkali contained in it may be estimated at one-third of its weight, whereas the white Mucovoy ashes are said to yield only one-eighth part; that no quicklime appears to be contained in this potash, for a solution of it poured from its sediment remained clear though long exposed to the air: that it would be worth trying, whether the large purple sediment, which subsides when this potash is lixiviated, might not be applied to the manufacture of Prussian blue, or used in the manner recommended by Macquer for dyeing wool and silks; and that this manufacture will furnish the farmer with top-dressing for his garden and land, of great fertilizing powers. See Phil. Trans., vol. lxx. p. 345.
These are the processes most essentially different from one another which have appeared concerning the manufacture of this useful salt. Some indeed have attempted to compose it on the supposition that alkali consisted of an earth combined in a peculiar manner with a certain acid. But the little success of all these attempts show that they have been built on a false principle. The only method of producing alkaline salts originally is from the ashes of vegetables; and the vegetable substances which yield the largest quantity of them are tartar and marine plants. From the former the purest and strongest vegetable alkali is obtained, and from the latter the mineral alkali. From other vegetables, as fern, broom, bean-stalks, &c., an alkaline salt is produced, but so impure, and in such small quantity, that no manufacture of it can be established in this country with any reasonable expectation of profit.
Dr Watson (the present bishop of Landaff) suggests, that the investigation of a method of extracting its alkaline part from rock-salt would be a most serviceable discovery. We have inexhaustible mines of rock-salt in this country, which (he observes) the proprietors can afford at ten shillings a ton. A ton of rock-salt contains about half a ton of mineral alkali, which is for most purposes far preferable to potash. To those who have leisure to attempt such a discovery, he gives the following hint: whether the alkaline part of rock-salt may not be obtained by calcining it in conjunction with charcoal in open fires? His reason for this conjecture is founded upon the following experiment: upon burning sea-wreck to a black coal and stopping the process at that point, he has obtained great plenty of common salt, but no mineral alkali from the black ashes, though we are certain, that when the black ashes are thoroughly calcined, or reduced to white ashes, mineral alkali may be obtained from them. This makes it probable, that the common salt contained in the black ashes of sea-wreck is decomposed, and changed into a mineral alkali, during the burning of the black ashes. There are reasons to suppose, that the cinder of pit-coal would answer the purpose better than charcoal. Chem. Eff., vol. i. p. 136, &c.
The potashes of different countries vary much in quality; and the experiments of Dr Home, in his Treatise on Bleaching, seem to set forth their different properties in the clearest point of view. The different kinds tried by him were,
1. Blue pearl-ashes.—These appear to be a pure alkaline salt, mixed with a small quantity of vitriolated tartar and earth. Half a pound of this, filtered and evaporated, yielded 5½ ounces of pure salt.—Here, however, we must observe, that though the quantity was so far diminished by this operation, yet we are not to imagine that the whole of this diminution was owing to impurities; for all salts are destroyed in some measure by solution in water and evaporation.
2. White pearl-ashes are nearly of the same quality with the former; half a pound of them giving five ounces and seven drams of pure salt, with some vitriolated tartar and earth.
3. Rustia or Mucovoy ashes have very much the appearance of flaked lime, and are, like it, friable between the fingers. They adhere to the tongue; and their alkaline taste soon goes away, leaving in the mouth a strong taste of lime. Some small bits of charcoal are observable in their composition, and they never turn moist in the air. Half a pound of the salt lixiviated with water, and evaporated, gave only 10 drams 15 grains of very caustic salt. These consist therefore of a small quantity of alkaline salt united with a large quantity of lime.
4. Caustic-ashes are of the colour of iron-stone, and extremely hard, with many shining particles of charcoal in in them. They have a saline taste, with a considerable degree of pungency; feel gritty in the mouth when broke in pieces by the teeth, and will dissolve in water. To extract the pure salt, half a pound of the ashes were boiled in a pint of water; then that water poured off, and half a pint put on the ashes again; and so on, till the ashes tasted no more salt. This boiling took 24 hours, and the last water that came off had a strong taste of sulphur, and was blackish. A piece of silver put in the decoction was in a few minutes turned almost black; but though the decoction was evaporated considerably, it did not turn silver black more speedily than before. The whole, when totally evaporated, yielded only 10 drams of a brown salt having a strong caustic alkaline taste. Some Cathub-ashes powdered, and often washed in water, so that the salts were all carried off, were infused in water. After standing some time, there was a weak lime-water, with something of a saline taste, but no pellicle. Some of this residuum was put into a reverberatory furnace for two hours; after which it afforded good lime-water. Cathub-ashes then appear to contain an earth half vitrified, some lime, alkaline salts, and a quantity of sulphur.
5. Murcifl ashes are of a paler colour than the former, with some small pieces of charcoal in their composition. They have a strong saline taste; and so great pungency, that they cannot be held long in the mouth. Half a pound dissolved in water, filtered and evaporated, yielded 11 drams one scruple and two grains of alkaline residuum. The decoction blackened silver, but not so strongly as the former; and by evaporation it quickly lost that quality.
Our author next proceeds to consider the probability of manufacturing these ashes in this country. On which subject he has the following observations.—"The blue and white pearl-ashes we have discovered to be pure alkaline salts, without any considerable mixture of heterogeneous bodies. Their purity shows the lixiviate to have been strained through some close substance, such as linen or flannel. The blue ashes show by their colour that they have sustained the most fire. But both of them are so much alike, that the one may be substituted for the other; and therefore we shall consider them in one view.
"Every one knows that alkaline salts, such as these, are got from all plants except the alkaliferous, and from all trees except the most resinous, which afford them in very small quantity. These plants or trees, when found, are pulled or felled in the spring, dried, and burnt to ashes. By the affusion of warm water the salts are dissolved, and, by straining, separated from the earth along with the water. This saline liquor, which is called a lixiviate, is evaporated over a fire; and what remains is an alkaline salt of the same kind with the pearl-ashes.
"I was informed by a skilful bleacher in Ireland, that he practised a more expeditious way of extracting the salts. He bought the ashes of different vegetables from the commonalty for 9s. a bushel. From these a very strong ley was made, into which dry straw was dipped until it sucked up all the ley. This straw was afterwards dried and burnt, and gave him salts which he showed me, almost as good and pure as the pearl-ashes. This method I have several times tried; but could never burn the straw to white ashes, the fats diminishing the inflammability of the straw. It is a very expeditious method if it can be practised. But I can see no occasion for bringing the ley into a solid form, as the salts must again be dissolved in water before they can be used. The strength of the ley can easily be determined by the hydrostatic balance.
"Though I make no question, that the quantity of salt, in plants of the same species, will vary in different soils and climates; yet it would be of advantage to have the proportion ascertained in general. Some trials of this kind I have made.
"Two pounds of fern which had been pulled August 16th were dried, and burnt to white ashes. These weighed 7 dr. and tasted very salt. When lixiviated, strained, and evaporated, they gave me 49 gr. of salt, about the eighth part of the ashes. If the fern had been pulled in April, it would have afforded more salt. Why then should we not prepare salts from this vegetable? There is more of it growing on our hills than would serve all our bleachfields. The Irish make great use of it.
"From 11 oz. of tobacco-ashes I had 1 oz. of salt. Two ounces of peat-ashes afforded half a drachm of salt. Nettles, I am informed, afford much salt. Furze and broom, natives of this country, are very fit for this purpose.
"But the kelp as it grows in such plenty along our shores, and contains more salt than any other vegetable I know, would be the most proper, were it not for a mixture of some substance that renders it unfit for bleaching, at least of fine cloths, after they have obtained a tolerable degree of whiteness. It is observed by bleachers, that in these circumstances, it leaves a great yellowness in the linen. As these ashes are much used in Ireland, and as it is not uncommon to bleach coarse cloths with them in Scotland, a disquisition into their nature, and some attempts to purify them, may not be improper. There are no ashes sold so cheap as these; for the best gives but 2l. the 2000 weight (B). They may, therefore, allow of more labour to be expended on them, and come cheaper at long-run than the foreign salts.
"I dried some tea-ware, and burnt it, though I found that last operation very difficult. When I had kept it fused in the fire for two hours, it weighed 3½ oz. I poured on the ashes an English pint and a half of cold water, that I might have as little of the sulphur as possible. This ley, after it had stood for some hours, was poured off clear, and had but a slight tendency to a green colour. I made a second infusion with milk-warm water, and poured it off from the sediment. This had a darker colour than the former; was kept separated from it, and evaporated by itself. There was a third infusion made;
(B) "Since this treatise was written, however, the price of kelp has been advanced to 7l. or upwards the 2000 weight; so that those who would now attempt any thing of this kind, must also manufacture the kelp themselves." Potash made; but having no salt taste, it was thrown away. The second infusion seemed to contain more sulphur than the first; and a piece of white linen kept in it half an hour, while it was boiling, was tinged yellow, and could not be washed white again. The earthy part remaining, weighed, when well dried, 1 oz. 2 dr. The saline decoction evaporated by degrees, and set at different times in a cellar to crystallize, afforded me 5 dr. 46 gr. The liquor, when entirely evaporated, left 4½ dr. of a yellow salt, which appeared to be a strong alkaline. The salts which crystallized seemed to be mostly sea-salt, with a considerable quantity of sulphur, and some alkaline salt. There appeared no signs of the bitter in these salts, as their solution did not turn turbid with the oil of tartar. Nor is any of the bitter to be expected in kelp ashes, although it probably is to be found in the recent vegetable; because the alkaline salts formed by the fire must have changed it into a neutral. The ley made warm with water, being evaporated, left 4 dr. of a black bitter salt, which, from its quantity of sulphur, appeared unfit for bleaching. These ashes, then, seem to be a composition of somewhat less than the fourth of sulphur, the same quantity of sea-salt, about a fourth of alkaline salt, and somewhat more than a fourth of earth. The alkaline salt contained in kelp ashes amounts to one penny a pound. This cheapness makes it worth our pains to bestow some labour on them.
"If the bad effects in bleaching with kelp-ashes arise from the sea-salt, as some of the most knowing bleachers think, they can be freed from it in an easy manner. Let a lixivium of kelp-ashes be made with cold water, for that does not extract so much of the sulphur; it must stand but for a short time, for these salts dissolve easily; decant it, and evaporate the ley. As the boiling continues, the sea-salt will crystallize. When that is all separated, the remaining ley will contain alkaline salt with some sulphur. This operation every master of a bleachfield may learn and oversee, without taking up much of his time. A similar process is carried on by common servants in the alum-works, who have by practice learned it from others.
"I had some hopes that the sulphur might be carried off by long roasting, such as these salts undergo before they are fused in order to be turned into glass; because I had observed, that the longer time they were kept in the fire, the freer were they from this sulphurous part.
"I ordered a quantity of kelp ashes to be kept in the furnace of a glasshouse, where the heat was just below the vitrifying point, for 24 hours. During this time they had lost almost four-fifths of their weight. They were now much freer from their sulphur, and were of a light colour; but much of the alkaline salt had been driven off with the oils. If a ley is much impregnated with this sulphurous matter, it appears to be carried off in a great measure by long boiling.
"We come now to explain the method of manufacturing the white Muscovy ashes. We have shown, by undoubted experiments, that the greatest part of these ashes consists of lime; and yet we have several acts of parliament which forbid the use of that material under severe penalties. The parliament were in the right to discharge its use, upon the disadvantageous reports which were made to them. We shall immediately see how dangerous a material it is when used improperly, or without the mixture of alkaline salts, which render it safe, and more soluble in water. But I will venture to say, that experiment will not support the prejudice entertained with regard to it, if carried any further.
"Since bleaching, then, cannot be carried on without it (for those ashes which contain it are quite necessary in that operation), and since we import them from foreign countries, let these prejudices against it cease, and let us only consider how we may render our own lime as safe as the foreign. If we can do that, the wisdom of the legislature will be as ready to abrogate these acts as they were to make them.
"By my experiments on the white Muscovy ashes, I got about the eighth part of alkaline salts from them. This made me expect, that, by mixing in the same proportion quicklime and alkaline salts, I should be able to produce Muscovy ashes.
"To an ounce of quicklime and a dram of white pearl-ashes, I added about a gill of water, and boiled them together till the water was all evaporated. The taste of this substance was little different from lime. To recover the salts again from the lime, I dissolved it in water, strained off the liquor, and evaporated it. Instead of the dram of salts, I had but two grains of a substance which was more earthy than saline.
"To 3 drams of quicklime, and as much potashes, I added a muttonkin of water, and kept it boiling for two hours till it was evaporated. I dissolved it again in water, which being filtered and evaporated, gave me 1½ dram of a caustic salt, that liquefied in the air when it had been but four minutes from the fire. It appears, then, that the alkaline salts are destroyed by lime, and that a great part of them can never be again recovered. From the remaining lime, after the salts were extracted, I got strong lime-water, but without a pellicle. This shows, that a quantity of alkaline salts, equal to the lime, boiled with it for two hours, are not able to fix all the soluble part of the lime.
"From these experiments we may draw some corollaries with regard to the present subject. 1st, That evaporating the water from the lime and salts by boiling, is a most unfrugal way of preparing these white ashes. 2ndly, That these ashes ought to be kept close shut up in casks; for if exposed to the open air, though in a room, the alternate moisture and drought must fix their most useful parts. This I have found to be fact: for the salts that I made became less pungent by keeping; and I have observed, that the surface of the Muscovy ashes lost all pungency by being exposed to the air, while their internal parts still retained it. 3rdly, That all boiling is prejudicial to these Muscovy ashes, as it fixes, and that quickly, their most subtile and probably their most serviceable parts.
"Let us now proceed to another method of making these white ashes. I imagined, that if the salts were dissolved in water, and the quicklime flaked with that, the mass would soon dry without the assistance of fire. In this way I added equal parts of both; but the composition was so strong, that it blistered my tongue if it but touched it. When the fourth part was alkaline salt, it blistered my tongue when kept to it a few seconds. I could taste the salts plainly in the composition, when they made but the thirty-second part of the whole.
"I thought, when composed with the eighteenth..." part of salt, it had, when fresh made, just the taste and look of the Muscovy ashes; nor could any person have distinguished them. This I once imagined was the proportion; but when I found that the saline pungency soon turned weaker by keeping, and that this composition would not afford the same quantity of salts that the Muscovy ashes did, I saw that a much greater quantity of salts was necessary. The proportion appears to be one of salts to four of lime, prepared in this last way. Three drams of ashes prepared in this way, and kept for a fortnight, gave me but 15 grains of salt; which is but the half of what the Muscovy would have afforded. I find, if the quicklime is first quenched, it does not fix the salts so much; and therefore is better and cheaper. One dram of potashes dissolved in a little water, and added to three drams of quenched lime, gave me 44 grains of a very caustic salt. I prefer this method as the best.
"The manufacturers of this salt probably pour the lixivium upon the lime, as they can know by its specific gravity what quantity of salts is in the water, and so save themselves the expense of procuring the salts in a dry form.
"The manufacture of the Marcott and Cashub ashes remains yet to be explained. We have discovered that both of them contained sulphur, earth, alkaline salts, and lime; and differ in nothing but in the Cashub's having more sulphur than the Marcott ashes. We shall therefore consider them together.
"Whether these two species of ashes are of any use in bleaching, may be, and has already been, disputed. I find they contain no other principles, the sulphurous part excepted, than the former ashes combined together. Why then should we expect any other effects from the same ingredients in the Marcott and Cashub ashes, than what we have from either of the pearl and Muscovy ashes mixed together? The sulphurous principle in the former must have very bad effects; as I find by experiment, that it leaves a yellowness on cloth that is very hard to be washed out. It is owing to this sulphurous principle that linen, after it has been washed with soap, and is pretty well advanced in whiteness, is apt to be discoloured by leys which is brought to boil; for, by boiling, the sulphurous part is extracted from the ashes, and the ley becomes of a deep brown colour. Daily practice, then, shows the disadvantage of this sulphurous principle. Besides, as sulphur unites itself quickly and firmly with alkaline salts, it must weaken or altogether destroy a great quantity of these in the Marcott and Cashub ashes, and so render them of no effect in bleaching. These two reasons seem to me sufficient to exclude them from the bleachfield; especially as, by increasing the other materials, we can attain perhaps more speedily the same ends.
"However, as custom has introduced them into general practice, we shall consider how they are to be manufactured. Dr Mitchell has, in a very ingenious and useful paper, contained in the Philosophical Transactions for the year 1748, delivered an account transmitted to him by Linnaeus of the method of making potashes in Sweden. This account was contained in an academical dissertation of one Lundmark upon this subject at Aboe in Sweden. The substance of this account is, 'That birch or alder is burnt by a slow fire to ashes, and made into a paste with water. This paste is plated over a row of green pine or fir logs. Above that is laid transversely another row of the same; and that likewise is plastered over. In this way they continue building and plastering till the pile be of a considerable height. This pile is set on fire; and whenever the ashes begin to run, it is overturned, and the melted ashes are beat with flexible sticks, so that the ashes incrust the logs of wood, and become as hard as stone.' This, in the Doctor's opinion, is the method of making the potashes that come from Sweden, Russia, and Danzig; and that there is no other difference between the ashes made in those different countries, but that the Russian, containing more salt, must be made into a paste with a strong ley.
"There would appear, by my experiments, a greater difference than this between the Swedish ashes, if that is the true process, and those I have examined. I had discovered the greatest part of the Muscovy ashes to be lime. I suspected it might enter into the composition of the Marcott and Cashub; and have accordingly discovered it there. Without the same grounds, none would ever have searched for it. Whence then comes this lime? It must either enter into its composition, or arise from the materials managed according as the process directs.
"I have tried the birch ashes made into a paste with water. I have tried common charcoal made into a paste with a third part of potashes, and kept them in a strong reverberatory heat for some hours, and yet no such caustic substance appeared. I have kept earth and salts of kelp-ashes fused together for 24 hours in the furnace of a glass-house, where the heat was just below the degree of vitrification; and yet no remarkable causticity appeared afterwards in the concreted mass. But supposing that there did, will ever this account for the generation of lime? These chemists do not assert that it is a calcareous causticity. The earth of vegetables kept in fusion with their salts, is so far from turning into a quicklime, that the mass takes the opposite course, and becomes glaas. Bodies that, by the laws of nature, are vitreifiable, can never, so far as we know, become calcareous. In one or other of these two substances all bodies terminate that are changeable by fire; and vegetables are of the former kind. Here it may be asked, Why then, since they endure such a fire, are they not vitrified? the objection would be just, did they contain nothing else but what was found in vegetables. But if we once allow that lime is one of the materials, the difficulty is easily solved: for lime, we know, in proportion as it is mixed, hinders the vitrification of all bodies. In effect, the earthy part in these ashes is almost vitrified; and I think that I have carried the vitrification yet farther in that part; but I never was able, with the utmost heat of a reverberatory furnace, continued for six hours, to produce anything like a thorough vitrification in these ashes. The heat of the fire used in the process would seem to be very great; and must, if it were not very difficult, reduce them to glaas. The invitrifiable nature of these salts, so far from being an objection, becomes a strong proof of my opinion.
"These salts have a remarkable pungency. This we have already seen is the natural effect of quicklime on salts.
"These salts are found to be the fittest for making soap, and to incorporate soonest and best with oils." Salts, we know, of themselves do not readily unite with oil; but when once mixed with quicklime, they have a greater tendency to union.
"Again, I find that these ashes are more easily fluxed than charcoal made into a paste with the third part salt; which is much more than the ashes contain. Now, it is observed that quicklime increases the fluxing power of alkaline salts; for the common caustic made of quicklime and alkaline salts is sooner fused than the latter alone.
"From these reasons, and the experiments that discover lime in these ashes, I am led to think, that it is not generated by the process, but mixed with the ashes when they are made into a paste. The following experiment is a convincing proof of what I have been endeavouring to make out.
"I boiled some peat straw in a strong ley of pearl-ashes burnt into a black coal, and made it into a paste with water. Another quantity of straw was boiled in a ley made of one part of quicklime and four parts of pearl salts, the ley being poured off turbid from the lime. This straw was likewise burnt when dry, and made into a paste. These two substances were put into separate crucibles, and fluxed in a reverberatory furnace. The latter appeared to resemble the Marcott and Cathub ashes more than the former, which seemed to want their pungency."
Though the only method of preparing the alkaline salt originally is by the combustion of vegetables, yet there are some neutral salts from which if we were possible to expel the acid, we should have it in our power to procure the finest pearl-ashes in vast quantity. These are vitriolated tartar, nitre, &c. But there are objections to all those. The vitriolated tartar, or any other salt in which the vitriolic acid enters, cannot be decomposed without converting the acid into sulphur by charcoal dust; in which case it is as difficult to get free of the sulphur as of the acid. With respect to nitre, though its acid may be expelled by fire, yet it is too high-priced, and too much used in other manufactures, to be thought of for this purpose.