a black liquor used in writing, generally made of an infusion of galls, copperas, and gum-arabic. The properties which this liquor ought to have, are,

1. To flow freely from the pen, and sink a little into the paper, that the writing be not easily discharged. 2. A very deep black colour, which should be as deep at first as at any time afterwards. 3. Durability, so that the writing may not be subject to decay by age. 4. Ink should be destitute of any corrosive quality, that it may not destroy the paper, or go through it in such a manner as to render the writing illegible. No kind of ink, however, hath yet appeared which is possessed of all these qualities. The ink used by the ancients was possessed of the second, third, and fourth qualities above-mentioned, but wanted the first. Dr Lewis hath discovered its composition from some pages in ancient authors. "Pliny and Vitruvius (says he) expressly mention the preparation of, or what we now call lamp-black, and the composition of writing-ink from lamp-black and gum. Dioscorides is more particular, setting down the proportions of the two ingredients, viz. three ounces of the root to one of the gum. It seems the mixture was formed into cakes or rolls; which being dried in the sun, were occasionally tempered with water, as the cakes of Indian ink are among us for painting."

In Mr Delaval's Treatise on Colours, p. 37, he acquaints us, that with an infusion of galls and iron filings, he had not only made an exceedingly black and durable ink, but by its means, without the addition of any acid, dyed silk and woollen cloth of a good and lasting black. This kind of ink, however, though the colour is far superior to that of any other, hath the inconvenience of being very easily discharged, either by the smallest quantity of any acid, or even by simple water; because it doth not penetrate the paper. Ink in such a manner as is necessary to preserve it from the instantaneous action of the acid or of the water. During the action of the infusion of galls upon the iron in making this kind of ink, a very considerable effervescence takes place, and a quantity of air is discharged, the nature of which hath not yet been examined.

The materials usually employed for the making of ink are, common green vitriol, or copperas and galls; but almost all of them are deficient in durability, which is a property of such importance, that Dr Lewis hath thought the subject of ink-making not unworthy of his attention. From experiments made by that author, he infers, that the decay of inks is chiefly owing to a deficiency of galls; that the galls are the most perishable ingredient, the quantity of these, which gives the greatest blackness at first (which is about equal parts with the vitriol), being insufficient to maintain the colour: that, for a durable ink, the quantity of galls cannot be much less than three times that of the vitriol; that it cannot be much greater without lessening the blackness of the ink: that by diminishing the quantity of water, the ink is rendered blacker and more durable; that distilled water, rain-water, and hard spring-water, have the same effects; that white-wine produces a deeper black colour than water; that the colour produced by vinegar is deeper than that by wine; that proof-spirit extracts only a reddish brown tinge; that the last-mentioned tincture sinks into, and spreads upon, the paper; and hence the impropriety of adding spirit of wine to ink, as is frequently directed, to prevent mouldiness or freezing: that other astringents, as oak-bark, bistort, floe-bark, &c. are not so effectual as galls, nor give so good a black, the colour produced by most of these, excepting oak-bark, being greenish: that the juice of floes do not produce a black colour with martial vitriol; but that, nevertheless, the writing made with it becomes black, and is found to be more durable than common ink: that inks made with saturated solutions of iron in nitrous, marine, or acetous acids, in tartar, or in lemon-juice, were much inferior to the ink made with martial vitriol; that the colour of ink is deprived by adding quicklime, which is done with an intention of destroying any superabundant acid which may be supposed to be the cause of the loss of the colour of ink: that the best method of preventing the effects of this superabundant acid is probably by adding pieces of iron to engage it; and that this conjecture is confirmed by an instance the author had heard, of the great durability of the colour of an ink in which pieces of iron had been long immersed: and lastly, that a decoction of logwood used instead of water, sensibly improves both the beauty and deepness of the black, without disposing it to fade. The same author observes, that the addition of gum-arabic is not only useful, by keeping the colouring matter suspended in the fluid, but also by preventing the ink from spreading, by which means a greater quantity of it is collected on each stroke of the pen. Sugar, which is sometimes added to ink, is found to be much less effectual than gums, and to have the inconvenience of preventing the drying of the ink. The colour of ink is found to be greatly injured by keeping the ink in vessels made of copper or of lead, and probably of any other metal, excepting iron, which the vitriolic acid can dissolve.

The foregoing experiments point out for the best proportions of the ingredients for ink, One part of green vitriol, one part of powdered logwood, and three parts of powdered galls. The best menstruum appears to be vinegar or white-wine, though for common use water is sufficient. If the ink be required to be of a full colour, a quart, or at most three pints, of liquor, may be allowed to three ounces of galls, and to one ounce of each of the other two ingredients. Half an ounce of gum may be added to each pint of the liquor. The ingredients may be all put together at once in a convenient vessel, and well shaken four or five times each day. In 10 or 12 days the ink will be fit for use, though it will improve by remaining longer on the ingredients. Or it may be made more expeditiously, by adding the gum and vitriol to a decoction of galls and logwood in the menstruum. To the ink, after it has been separated from the feculencies, some coarse powder of galls, from which the fine dust has been sifted, together with one or two pieces of iron, may be added, by which its durability will be secured.

In some attempts made by the Doctor to endow writing ink with the great durability of that of the ancients, as well as the properties which it has at present, he first thought of using animal glues, and then of oily matters. "I mixed both lamp-black (says he) and ivory-black with solution of gum arabic, made of such consistence as just to flow sufficiently from the pen. The liquors wrote of a fine black colour; but when dry, part of the colour could be rubbed off, especially in moist weather, and a pencil dipped in water washed it away entirely.

"I tried solutions of the animal-glues with the same event. If glaigs or fish-glue being the most difficultly dissoluble of these kinds of bodies, I made a decoction of it in water, of such strength that the liquor concreted into a jelly before it was quite cold: with this jelly, kept fluid by sufficient heat, I mixed some ivory-black: characters drawn with this mixture on paper bore rubbing much better than the others, but were discharged without much difficulty by a wet pencil.

"It was now suspected, that the colour could not be sufficiently fixed on paper without an oily cement. As oils themselves are made miscible with watery fluids by the intervention of gum, I mixed some of the softer painters varnish, after mentioned, with about half its weight of a thick mucilage of gum arabic, working them well together in a mortar till they united into a smooth uniform mass: this was beaten with lamp-black, and some water added by little and little, the rubbing being continued till the mixture was diluted to a due consistence for writing. It wrote freely, and of a full brownish-black colour: the characters could not be discharged by rubbing, but water washed them out, though not near so readily as any of the foregoing. Instead of the painters varnish or boiled oil, I mixed raw linseed oil in the same manner with mucilage and lamp-black; and on diluting the mixture with water, obtained an ink not greatly different from the other.

"Though these oily mixtures answered better than those with simple gums or glues, it was apprehended that their being dischargeable by water would render them unfit for the purposes intended. The only way..." of obviating this imperfection appeared to be, by using a paper which should admit the black liquid to sink a little into its substance. Accordingly I took some of the more sinking kinds of paper, and common paper made damp as for printing; and had the satisfaction to find, that neither the oily nor the simple gummy mixtures spread upon them so much as might have been expected, and that the characters were as fixed as could be desired, for they could not be washed out without rubbing off part of the substance of the paper itself.

"All these inks must be now and then stirred or shaken during the time of use, to mix up the black powder, which settles by degrees to the bottom: those with oil must be well shaken also, though not used, once a day, or at least once in three or four days, to keep the oil united with the water and gum; for if once the oil separates, which it is apt to do by standing at rest for some days, it can no longer be mixed with the thin fluid by any agitation. But though this imperfect union of the ingredients renders their inks less fit for general use than those commonly employed, I apprehend there are many occasions in which these kinds of inconveniences will not be thought to counterbalance the advantage of having writings which we may be assured will be as lasting as the paper they are written upon. And indeed the inconvenience may be in a great measure obviated by using cotton in the inkstand, which, imbuing the fluid, prevents the separation of the black powder diffused through it.

"All the inks, however, made on the principle we are now speaking of, can be discharged by washing, unless the paper admits them to sink into its substance. The ancients were not insensible of this imperfection; and sometimes endeavoured to obviate it, according to Pliny, by using vinegar instead of water, for tempering the mixture of lamp-black and gum. I tried vinegar, and found it to be of some advantage, not as giving any improvement to the cement, but by promoting the sinking of the matter into the paper. As this washing out of the ink may be prevented by using a kind of paper easy enough to be procured, it is scarcely to be considered as an imperfection; and indeed, on other kinds of paper, it is an imperfection only so far as it may give occasion to fraud, for none of these inks are in danger of being otherwise discharged than by design. The vitriolic inks themselves, and those of printed books and copperplates, are all dischargeable; nor can it be expected of the ink-maker to render writings secure from frauds.

"But a further improvement may yet be made, namely, that of uniting the ancient and modern inks together; or using the common vitriolic ink instead of water, for tempering the ancient mixture of gum and lamp-black. By this method it should seem that the writings would have all the durability of those of former times, with all the advantage that results from the vitriolic ink fixing itself in the paper. Even where the common vitriolic mixture is depended on for the ink, it may in many cases be improved by a small addition of the ancient composition, or of the common Indian ink which answers the same purpose: when the vitriolic ink is dilute, and flows too pale from the pen, that the fine strokes, on first writing, are scarcely visible, the addition of a little Indian ink is the readiest means of giving it the due blackness. By this admixture it may be presumed also that the vitriolic ink will be made more durable, the Indian ink in some measure covering it, and defending it from the action of the air. In all cases, where Indian ink or other similar compositions are employed, cotton should be used in the inkstand, as already mentioned, to prevent the settling of the black powder."

Since the invention of printing much less attention than formerly has been paid to the making of ink, so that now the art seems to be in a great measure lost. This will appear from a comparison of some ancient manuscripts with the writings of modern times. It being of the utmost importance, however, that public records, wills, and other valuable papers, which cannot admit of being printed, should be written with ink of a durable quality, this inattention seems to have been very culpable, and a restoration of the method of making writing ink a very valuable acquisition. "The necessity (says Mr Aftle) of paying greater attention to this matter may readily be seen, by comparing the rolls of records that have been written from the 15th century to the end of the 17th, with the writings we have remaining of various writings from the 5th to the 12th centuries. Notwithstanding the superior antiquity of the latter, they are in excellent preservation; but we frequently find the former, though of more modern date, so much defaced that they are scarcely legible."

Our author agrees with Dr Lewis in the opinion that the ancient inks were composed of root or ivory black instead of the galls, copperas, and gums, which form the composition of ours. Besides their black inks, however, the ancients used various other colours, as red, gold and silver, purple, &c. Green ink was frequently used in Latin manuscripts, especially in the latter ages; and it was frequently employed in signatures by the guardians of the Greek emperors till their wards were of age. Blue or yellow ink was seldom used except in manuscripts; but (says Mr Aftle) "the yellow has not been much in use, as far as we can learn, these 600 years." Some kinds of characters, particularly the metallic, were burnished. Wax was used by the Latins and Greeks as a varnish, but especially by the former, and particularly in the 9th century. It continued a long time in vogue.

A treatise upon inks was published by Peter Caniparius professor of medicine at Venice; of which an edition was printed at London in 1660. It is divided into five parts. The first treats of inks made from pyrites, stones, and metals; the second of such as are made from metals and calcis; the third from roots and vitriols; the fourth of the different kinds of inks used by the librarii or book-writers, by printers, and engravers; likewise of staining or writing upon marble, stucco, or faïence, and of cautical modes of writing; also of liquids for painting or colouring leather and linen or woollen cloths; restoring inks that had been decayed by time; together with many methods of effacing writing, restoring decayed paper, and different modes of secret writing. The fifth treats of writing inks made in different countries from gums, woods, the juices of plants, &c. as well as of different kinds of varnishes. The sixth treats of the different methods of extracting vitriol, and the chemical uses of it. Weckerus de Secretis, a treatise printed at Basle in 1612, contains a number of curious particulars concerning ink. He gives also receipts for making gold and silver inks, composed both with these metals and without them; directions for making inks for secret writing, and for defacing them; though in this last part there are many particulars bordering too much on the marvellous.

In the Philosophical Transactions for 1787, Dr Blagden gives some account of a method of restoring decayed inks so as to render them legible. His experiments originated from a conversation with Mr Aitle already quoted, on the question whether the inks made eight or ten centuries ago, and which are found to have preserved their colour very well, were made of the same materials now employed or not? In order to decide the question, Mr Aitle furnished the Doctor with several manuscripts on parchment and vellum from the 9th to the 15th centuries inclusively. Some of these were still very black; others of different shades, from a deep yellowish brown to a very pale yellow, in some parts so faint that it could scarcely be seen. This was tried with simple and phlogisticated alkalies, the mineral acids, and infusion of galls. From these experiments it appeared that the ink anciently employed was of the same nature as at present: the letters turned of a reddish or yellowish brown with alkalies became pale, and were at length obliterated by the dilute mineral acids. The drop of acid liquor, which had been put upon a letter, changed to a deep blue or green on the addition of phlogisticated alkali; with an infusion of galls, in some cases the letters acquired a deep tinge, in others a slight one. Hence (says the Doctor) it is evident, that one of the ingredients was iron, which there is no reason to doubt was joined with the vitriolic acid; and the colour of the more perfect MSS., which in some was a deep black, and in others a purplish black, together with the restitution of that colour in those which had lost it by the infusion of galls, sufficiently proved that another of the ingredients was astringent matter, which from history appears to have been that of galls. No trace of a black pigment of any sort was discovered; the drop of acid, which had completely extracted a letter, appearing of an uniform pale and ferruginous colour, without an atom of black powder, or other extraneous matter floating in it."

As this account differs very materially from the former extracted from Mr Aitle's writings, so the reason given for the continuance of the colour differs no less. This, according to Dr Blagden, "seems to depend very much on a better preparation of the material upon which the writing was made, namely the parchment or vellum; the blackest letters being generally those which had sunk into it the deepest. Some degree of effervescence was commonly to be perceived when acids were in contact with the surface of these old vellums. I was led, however, to suspect, that the ancient inks contained rather a less proportion of iron than the more modern; for, in general, the tinge of colour produced by the phlogisticated alkali in the acid laid upon them, seemed less deep; which, however, might depend in part upon the length of time they had been kept: and perhaps more gum was used in them, or they were washed over with some kind of varnish, tho' not such as gave any glofs."

Among the specimens with which our author was favoured by Mr Aitle, there was one which differed very materially from the rest. It was said to be a manuscript of the 15th century: the letters were of a full engrossing hand, angular without any fine strokes, broad, and very black. None of the chemical solvents above mentioned seemed to produce any effect. Most of them seemed rather to make the letters blacker, probably by cleaning the surface; and the acids, after having been rubbed strongly upon the letters, did not strike any deeper tinge with the phlogisticated alkali. Nothing could obliterate these but what took off part of the vellum; when small rolls of a dirty matter were to be perceived. "It is therefore unquestionable (says the Doctor) that no iron was used in this ink; and, from its refilance to the chemical solvents, as well as a certain clotted appearance in the letters when examined closely, and in some places a slight degree of glofs, I have little doubt that they were formed of a footy or carbonaceous powder and oil, probably something like our present printer's ink; and am not without suspicion that they were actually printed."

On examining this MS. more fully, our author was convinced that it was really a part of a very ancient printed book. In considering the methods of restoring the legibility of decayed writings, our author observes, that perhaps one of the best may be to join phlogisticated alkali with the calx of iron which remains; because the precipitate formed by these two substances greatly exceeds that of the iron alone. On this subject Dr Blagden disagrees with Mr Bergmann; but to bring the matter to a test, the following experiments were made.

1. The phlogisticated alkali was rubbed in different quantities upon the bare writing. This, in general, produced little effect; though, in a few instances, it gave a bluish tinge to the letters, and increased their intensity; "probably (says the Doctor) where something of an acid nature had contributed to the diminution of their colour." 2. By adding, besides the alkali, a dilute mineral acid to the writing, our author found his expectations fully answered; the letters then changing quickly to a very deep and beautiful blue. It is but of little consequence whether the acid or phlogisticated alkali be first added; though upon farther consideration the Doctor inclined to begin with the alkali. The reason is, that when the alkali is first put on, the colour seems to spread less, and thus not hurt the legibility of the writing so much as would otherwise be done. His method is to spread the alkali thin over the writing with a feather, then to touch it as gently as possible upon or nearly over the letters with the diluted acid by means of a feather or bit of stick cut to a blunt point. The moment that the acid liquor is applied, the letters turn to a fine blue, beyond comparison stronger than the original trace of the letter; and by applying a bit of blotting-paper to suck up the superfluous liquid, we may in a great measure avoid the staining of the parchment; for it is this superfluous liquor which, absorbing part of the colouring matter from the letters, becomes a dye to whatever it touches. Care ought, however, to be taken. not to allow the blotting paper to come in contact with the letters, because the colouring matter may easily be rubbed off while soft and wet. Any one of the three mineral acids will answer the purpose effectually: Dr Plagden commonly uses the marine. But whichever of the three is used, it ought to be diluted so far as not to be in danger of corroding the parchment; after which the degree of strength seems not to be a matter of great nicety.

Another method of restoring the legibility of old writings is by wetting them with an infusion of galls in white wine: but this is subject to the same inconvenience with the former, and is besides less efficacious. The Doctor is of opinion that the acid of the galls by itself would be better for the purpose than the infusion of the whole substance of them; and he thinks also that a preferable kind of phlogisticated alkali might be prepared either by purifying the common kind from iron as much as possible, or by making use of the volatile alkali instead of the fixed. Mr Astle mentions a method of restoring the legibility of decayed writings; but says that it ought not to be hazarded lest a suspicion of deceit should arise.

In the Monthly Review of this volume of the Transactions, we find a method proposed of preventing ink from decaying, which seems very likely to answer the purpose. It consists in washing over the paper to be written upon with the colouring matter of Prussian blue, which will not deprave it in colour, or any other respect. By writing upon it with common ink afterwards, a ground of Prussian blue is formed under every stroke; and this remains strong after the black has been decayed by the weather, or destroyed by acids. Thus the ink will bear a larger proportion of vitriol at first, and will have the advantage of looking blacker when first written.

Indian Ink, a valuable black for water-colours, brought from China and other parts of the East Indies, sometimes in large rolls, but more commonly in small quadrangular cakes, and generally marked with Chinese characters. Dr Lewis, from experiments made on this substance, hath shown that it is composed of fine lamp-black and animal-glue: and accordingly, for the preparation of it, he desires us to mix the lamp-black with as much melted glue as is sufficient to give it a tenacity proper for being made into cakes; and these when dry, he tells us, answered as well as those imported from the East Indies, both with regard to the colour and the freedom of working. Ivory-black, and other charcoal blacks, levigated to a great degree of fineness, answered as well as the lamp black; but in the slate in which ivory-black is commonly sold, it proved much too gritty, and separated too hastily from the water.

Printing Ink, is totally different from Indian ink, or that made use of in writing. It is an oily composition, of the consistence of an ointment: the method of preparing it was long kept a secret by those whose employment it was to make it, and who were interested in concealing it; and even yet is but imperfectly known. The properties of good printing ink are, to work clean and easily, without daubing the types, or tearing the paper; to have a fine black colour; to wash easily off the types; to dry soon; and to preserve its colour, without turning brown. This last, which is a most necessary property, is effectually obtained by setting fire to the oil with which the printing ink is made for a few moments, and then extinguishing it by covering the vessel (A). It is made to wash easily off the types, by using soap as an ingredient; and its working clean depends on its having a proper degree of strength, which is given by a certain addition of rosin. A good deal, however, depends on the proportion of the ingredients to each other; for if too much soap is added, the ink will work very foul, and daub the types to a great degree. The same thing will happen from using too much black, at the same time that both the soap and black hinder the ink from drying; while too much oil and rosin tear the paper, and hinder it from washing off.—The following receipt has been found to make printing ink of a tolerable good quality. "Take a Scots pint of linseed oil, and set it over a pretty brisk fire in an iron or copper vessel capable of holding three or four times as much. When it boils strongly, and emits a thick smoke, kindle it with a piece of paper, and immediately take the vessel off the fire. Let the oil burn for about a minute; then extinguish it by covering the vessel; after it has grown pretty cool, add two pounds of black rosin, and one pound of hard soap cut into thin slices. If the oil is very hot when the soap is added, almost the whole mixture will run over the vessel. The mixture is then to be set again over the fire; and when the ingredients are thoroughly melted, a pound of lamp-black, previously put through a lawn sieve, is to be stirred into it. The whole ought then to be ground on a marble stone, or in a mill like the levigating mill described under the article Chemistry, n° 599."

Though the above receipt is greatly superior to any that hath been hitherto published, all of which are capitally deficient in not mentioning the necessary ingredients of rosin and soap; yet it must be acknowledged, that ink made in this manner is inferior in point of colour, and is likewise more apt to daub the types and make an indistinct impression, than such as is prepared by some of those who make the manufacture of this commodity their employment; so that either a variation in the proportion of the ingredients, a nicety in the mixture, or some additional ingredient, seems necessary to bring it to the requisite perfection.

Ink for the Rolling Press, is made of linseed oil burnt in the same manner as that for common printing-ink, and then mixed with Flanfort-black, and finely ground. There are no certain proportions which can be determined in this kind of ink; every workman adding oil or black to his ink as he thinks proper, in order to make it suit his own taste.—Some, however, mix a portion of common boiled oil, which has never been burnt: but this must necessarily be a bad practice, as such oil is apt to go through the paper; a fault very common in prints, especially if the paper

(A) This is mentioned by Dr Lewis in his Philosophical Commerce of Arts; but he seems not to have been acquainted with the method of giving it the other necessary properties. Ink is not very thick. No soap is added; because the ink is not cleared off from the copperplates with alkaline ley as in common printing, but with a brush dipped in oil.

Ink is also an appellation given to any coloured liquor used in writing, whether red, yellow, green, &c. Many different kinds of these inks may be prepared by the directions given under the article Colour-Making, which it would be superfluous here to repeat.

Sympathetic Ink, a liquor with which a person may write, and yet nothing appear on the paper after it is dry, till some other means are used, such as holding the paper to the fire, rubbing it over with some other liquor, &c.

These kinds of ink may be divided into seven classes, and that with respect to the means used to make them visible; viz. 1. Such as become visible by passing another liquor over them, or by exposing them to the vapour of that liquor. 2. Those that do not appear so long as they are kept close, but soon become visible on being exposed to the air. 3. Such as appear by trewing or sifting some very fine powder of any colour over them. 4. Those which become visible by being exposed to the fire. 5. Such as become visible by heat, but disappear again by cold or the moisture of the air. 6. Those which become visible by being wetted with water. 7. Such as appear of various colours, red, yellow, blue, &c.

I. The first class contains four kinds of ink, viz. solutions of lead, bismuth, gold, and green vitriol. The first two become visible in the same manner, viz. by the contact of sulphurous liquids or fumes. For the first, a solution of common sugar-of-lead in water will answer as well as more troublesome preparations. If you write with this solution with a clean pen, the writing when dry will be totally invisible; but if it be wetted with a solution of hepar fulphuris, or of orpiment, dissolved by means of quick-lime; or if it be exposed to the strong vapours of these solutions, but especially to the vapour of volatile tincture of sulphur; the writing will appear of a brown colour, more or less deep according to the strength of the sulphurous fume. By the same means, what is wrote with the solution of bismuth in spirit of nitre will appear of a deep black.

The sympathetic ink prepared from gold depends on the property by which that metal precipitates from its solvent on the addition of a solution of tin. If you write with a solution of gold in aqua regia, and let the paper dry gently in the shade, nothing will appear for the first seven or eight hours. Dip a pencil or a small fine sponge in the solution of tin, and drawing it lightly over the invisible characters, they will immediately appear, of a purple colour.

Characters wrote with a solution of green vitriol carefully depurated, will likewise be invisible when the paper is dry; but if wetted with an infusion of galls, they will immediately appear as if wrote with common ink. If, instead of this infusion, a solution of the phlogisticated alkali, impregnated with the colouring matter Prussian blue is made up of, the writing will appear of a very deep blue.

II. To the second class belong the solutions of all those metals which are apt to attract phlogiston from the air, such as lead, bismuth, silver, &c. The sympathetic ink of gold already mentioned belongs also to this class; for if the characters wrote with it are long exposed to the air, they become by degrees of a deep violet colour, nearly approaching to black. In like manner, characters wrote with a solution of silver in aquafortis are invisible when newly dried, but being exposed to the sun, appear of a grey colony-like flate. To this class also belong solutions of lead in vinegar; copper in aquafortis; tin in aqua regia; emery, and some kinds of pyrites, in spirit of salt; mercury in aquafortis; or iron, in vinegar. Each of these has a particular colour when exposed to the air; but they have the disagreeable property of corroding the paper, so that after some time the characters appear like holes cut out of the paper.

III. The third class of sympathetic inks contains such liquids as have some kind of glutinous viscoity, and at the same time are long drying; by which means, though the eye cannot discern the characters wrote with them upon paper, the powders strewed upon them immediately adhere, and thus make the writing become visible. Of this kind are urine, milk, the juices of some vegetables, weak solutions of the deliquescent salts, &c.

IV. This class, comprehending all those that become visible by being exposed to the fire, is very extensive, as it contains all those colourless liquids in which the matter dissolved is capable of being reduced, or of reducing the paper, into a sort of charcoal by a small heat. A very easily procured ink of this kind is oil of vitriol diluted with as much water as will prevent it from corroding the paper. Letters wrote with this fluid are perfectly invisible when dry, but instantly appear as black as if wrote with the finest ink on being held near the fire. Juice of lemons or onions, a solution of sal-ammoniac, green vitriol, &c. will answer the same purpose, though not so easily, or with so little heat.

V. The fifth class comprehends only solutions of regulus of cobalt in spirit of salt; for the properties of which, see Chemistry, no 822.

VI. This class comprehends such inks as become visible when characters wrote with them are wetted with water. They are made of all such substances as deposit a copious sediment when mixed with water, dissolving only imperfectly in that fluid. Of this kind are dried alum, sugar of lead, vitriol, &c. We have therefore only to write with a strong solution of these salts upon paper, and the characters will be invisible when dry; but when we apply water, the small portion of dried salt cannot again be dissolved in the water. Hence the insoluble part becomes visible on the paper, and shows the characters wrote in white, grey, brown, or any other colour which the precipitate assumes.

VII. Characters may be made to appear of a fine crimson, purple, or yellow, by writing on paper with solution of tin in aqua regia, and then passing over it a pencil dipped in a decoction of cochineal, Brazil-wood, logwood, yellow wood, &c.—For an account of the nature of all these sympathetic inks; however, and the principles on which they are made, see the articles Chemistry and Colour-Making, passim.

Ink-Stones, a kind of small round stones of a white, red, red, grey, yellow, or black, colour, containing a quantity of native martial vitriol, whence they derive the property of making ink, and from thence their name. They are almost entirely soluble in water, and besides their other ingredients, contain also a portion of copper and zinc.

IRIS-STONE. See Moon-Stone.