in painting and sculpture, the situation of a figure with regard to the eye, and of the several principal members thereof with regard to one another, whereby its action is expressed. A considerable part of the art of a painter, consists in adjusting the postures; in giving the most agreeable ones to his figures, in accommodating them to the characters of the respective figures and the part each has in the action, and in conducting and in pursuing them throughout.
Postures are either natural or artificial.
Natural postures are such as nature seems to have had a view to in the mechanism of the body, or rather such as the ordinary actions and occasions of life lead us to exhibit while young, and while the joints, muscles, ligaments, &c., are flexible.
Artificial postures, are those which some extraordinary views, or studies, occasion us to learn; as those of dancing, fencing, &c. Such also are those of our balance and posture-masters.
A painter would be strangely puzzled with the figure of Clark (a late famous posture-master in London) in a history-piece. This man, we are told in the Phil. Trans. had such an absolute command of his muscles, &c., that he could disjoint almost his whole body; so that he imposed on the great surgeon Mullens, who looked upon him as in such a miserable condition, he would not undertake his care. Though a well-made man, he would appear with all the deformities imaginable; hunch-backed, pot-bellied, sharp-breasted, &c. He disjointed his arms, shoulders, legs, and thighs; and rendered himself such an object of pity, that he has frequently extorted money, in quality of a cripple, from the same company in which he had the minute before been in quality of a comrade. He would make his hips stand a considerable way out from his loins, and so high as to invade the place of his back. Yet his face was the most changeable part about him, and showed more postures than all the rest. Of himself he could exhibit all the uncouth odd faces of a Quaker's meeting.
POT-ASH, the lixivious ashes of certain vegetables, used in making of glass, soap, &c. See Glass, Soap, &c.
The method of making pot-ash is directed by Dr Shaw, as follows. Burn a quantity of billet-wood to grey 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, set 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 pot-ash. Much after this manner is pot-ash made in the large way of business, 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 pot-ash are prepared. There are certain saline-plants that yield this pot-ash 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 loppings, 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: when it should seem, says Dr Shaw, that by a suitable management, good saleable pot-ash might be made in all places where vegetable matters abound. For if by examining Russia pot-ash, for example, we find that its superior excellence depends upon its being clear of earth, or upon its containing a large proportion of oil, or fixed salt, these advantages may, by properly regulating the operation, be given to English pot-ashes, so as perhaps to render the latter as good as the former: but where the pot-ash 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 pot-ash.
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 dung-hills. The process is very simple, 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 pot-ash, valued at 42 s. per Cwt.; the expense of manufacturing them being only valued at 4l. 9s.
The pot-ash thus made is of a greyish 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 acid, saline, and sulphurous. 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 phlogiston. Ten grains of this pot-ash 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 Potash 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 pot-ash 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.
These are the two 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 expectations of profit. The pot-ashes of different countries also 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. See Chemistry, p. 21.
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. Rustic or Molyneux 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 mouldy in the air. Half a pound of the salt leached 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 ironstone, and extremely hard, with many shining particles of charcoal 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 taste 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 Caustic 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. Caustic ashes then appear to contain an earth half vitrified, some lime, alkaline salts, and a quantity of sulphur.
5. Marcelli 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 refrinous, 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 infusion 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 salts 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 hydrometrical 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 salted 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 oz. of peat-ashes afforded half a drachm of salt. Nettles, I am informed, afford much salt. Furz 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 disqualification 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 (a). 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 sea-ware, and burnt it, though I found that last operation very difficult. When I had kept them fused in the fire for two hours, they 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; 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 bittern in these salts, as their solution did not turn turbid with the oil of tartar. Nor was any of the bittern 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 below 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 lixiviate 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 glass-house, 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
(A) Since this treatise was written, however, the price of kelp has been advanced to 2l. or upwards the 2000 weight; so that those who would now attempt any thing of this kind, must also manufacture the kelp themselves. gate 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 quick-lime and alkaline salts, I should be able to produce Muscovy ashes.
"To an ounce of quick-lime and a drachm 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 drachm of salts, I had but 2 gr. of a substance which was more earthy than saline.
"To 3 dr. of quicklime, and as much potashes, I added a mottledkin 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 1/3 dr. 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 quick-lime 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 salt to four of lime, prepared in this last way. Three drachms 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 quick-lime is first quenched, it does not fix the salts so much; and therefore is better and cheaper. One drachm of potashes dissolved in a little water, and added to 3 drachms 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 lixivie 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 contain 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 ley which is brought to boil; for, by boiling, the sulphurous part is extracted from these 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 end.
"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 Dr Linnæus, 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 plastered 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 Dantzig: and that there is no other difference betwixt 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, betwixt 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 Marcolf and Calthub; 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 potash, 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 inviolate 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 soaps and beets 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 pease-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 pearl salte, 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 Marcolf and Calthub 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 it is 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, but especially sea-salt, on account of the inexhaustible quantities of it to be met with in the waters of the ocean. Unhappily, however, there are some objections to every one of these. 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; and if we attempt it by frequent solutions in water, we destroy the phlogiston of the sulphur, and have only vitriolated tartar again instead of alkali. See Chemistry, n° 225, 324.
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. A potash manufacture from sea-salt has indeed been lately erected in England. The principle on which this was established is, that the acid of sea-salt may be extracted by means of lime; and accordingly we find that the saline efflorescence, which frequently appears on walls, consists chiefly of the marine alkali deprived of its acid. But this, tho' delivered on the credit of a very eminent chemist, we can affirm from our own observation to be a mistake. Of the many cases in which we have examined this efflorescence, only one was found to be alkaline; the others uniformly appeared to be true Glauber's salt composed of the vitriolic acid and fusible alkali. Neither did this appear to be formed by any decomposition of salt originally in the plaster, but to be a real generation of both acid and alkali where none of them existed before. See Efflorescence, in the Appendix.