I N K
and filver and copper, tho' both foluble in aqua fortis, yet produce no change of colour by thefe precipitations made by mixing the folutions of them on paper ; and this example of fuch an effefl in the folutions of gold and tin, is an excep- tion to the general rules in the folutions of metals, and their effects on one another.
The purple colour of thefe letters may be again effaced by rubbing fome fimple aqua-regia over the paper, and may be made to appear again by rubbing over that the folution of tin. Kunkel, Caffius, and Orfchal, with fome other writers, made the firft fteps toward this difcovery, by their attempt to give cryftal, by means of gold, the colour of the oriental rubies. There are befide thefe mineral preparations, fome vegetable ones, which give the fame phenomena, but thefe are the more certain.
Of the fecond-kind of fympatbetic Inks, or thofe which ap- pear on being expofed only to the air, is the golden Ink made by adding to a folution of gold in aqua regia, fo much wa- ter that the liquor (hall not ftain a white paper ; letters written with this will not appear till the paper has been ex- pofed fome hours to the open air, and they will then begin to acquire a colour by degrees till they at length become of a deep violet colour tending to black. If, inftead of expofing the paper to the air, it be kept in a box clofe (hut up, or clofely folded in other paper, it will re- main invilible two or three months ; but at the end of that time it will begin to appear, and will by degrees become of a deep violet colour. So long as the gold remains united to its diffolvent, it is yellow ; but the acid that diffolves it be- ing of a volatile nature, the greater part of it evaporates, and leaves no more than is juft necefi'ary to colour the calx of gold which remains upon the paper.
The fecond of thefe is the filver Ink, made by a folution of filver in aqua-fortis, weakned by diftilled water till it will not itain the paper. Letters written with this will be invi- fible for three or four months, if fhut up in a box ; but if it be expofed to the fun it becomes legible in about an hour, becaufe by this means the evaporation of the acid is accele- rated. The letters written with this Ink are of a flate co- lour ; and that from the fulphureous nature of the aqua fortis, every thing that is fulphureous blackening filver. This blackifh colour, however, is not permanent ; for the ful- phureous part finally evaporating, the letters are left to their natural appearance, and are of a true filver colour, if the filver that was ufed was fine, and the place open. In this clafs there may alfo be placed feveral metallic diflblu- tions ; as that of lead in vinegar, and of copper in aqua fortis, which give at length a brownifh colour upon the pa- per ; as alfo the folution of tin in aqua regia, of mercury in aqua fortis, of iron in vinegar, of emery and feveral of the pyritre in fpirit of fait. But all thefe, tho' they give letters which are after fome time legible on being expofed to the air, are alfo made to appear inftantly on the paper, on holding them to the fire. Each of thefe folutions gives its own particular colour ; but they have all this difadvantage, that in time they eat away the paper, and the letters are feen in the lhape of h many holes.
Of the third clafs of Ink, or that which appears on rubbing over the paper with a brown or black powder, are almoft all the glutinous expreffed juices of plants, which are themfelves of no remarkable colours, the milk of animals, or any other thick and vifcous fluids. To ufe thefe, the letters muft be written on a white paper, and when dry there is to be thrown over them the fine powder of any coloured earth, or other fuch fubftance ; and the writing will afterwards appear co- loured, becaufe its vifcous quality remains fuflicienfly in it for the entangling and retaining this fine powder, tho* it falls eafily off from every other part of the paper. Mem. Acad. Scienc. Par. 1737.
Of the fourth clafs, or thofe Inks which become vifible on holding them to the fire, there arc a vaft number, and in- deed all infufions, the matter of which is readily burnt to a fort of charcoal by a little fire, will anfwer this purpofe. The niceft of this kind is the fal armoniac Ink, made by dif- folving a fcruplc of fal armoniac in two ounces of fair water. Letters written with this folution are inviflble on the paper till it is held before the fire, or has an iron a little heated palled over it. The rationale of this is, that the inflam- mable part of the fal armoniac is burnt to a charcoal by a heat which is not fufHcient to fcorch the paper ; and this is the cafe with all the reft of this claf*. The letters written with this folution are, however, of no great duration ; for the fait being apt to moiften in the air, the letters foon fpread, and run together in a confufed manner. The fifth clafs of fympatbetic Ink, contains only one yet known kind.
This, tho' in itfelf invifible, becomes of a bluifh green when held to the fire, and this colour difappears again as the pa- per cools, and is to be produced again on holding it again to the fire ; and this for a long time, and a repeated feries of trying ; it may alfo, according to the different manner of treating it, be made to appear blue, green, yellow, red, and of fome other colours.
A certain German chemift fhewed the Academy at Paris a 3
INK-
fait of a rofe-colour, which became blue on holding it to the fire, and at the fame time ibewed the ore from which he procured the fait, which he calied an ore of marcahte, a name given by many to the bifmuth ore; He added, that this was the mineral from which the line blue fmalt of Snee- berg was prepared, and that no other ore but this afforded it, and that he made the tincture from this mineral with aqua fortis, which he fixed with fea-fa!t. This was the fubitance of what the German declared, and from which Mr. Heliot took the hint for his difcovery of this remarkable Ink. The fait was after many experiments at length found to be produced from an arfenic ore ; and it was found, that all the cobalts and ores of bifmuth afford a tincture capable of thefe changes by fire. The method of preparing it is this ; pour upon two ounces of arfenic ore grofsly powdered, a mixture of five ounces of aquafortis, and live ounces ot common water; alter the firifc ebullition is over, place the veflel in a gentle fand-heat, and let it frand there till no more air-bubbles leem to alcend ; after this increale the hrc, fo as to make the liquor boil for about a quarter of an hour j after this the liquor will become of a reddiih colour, and when cold it is to be decanted clear off from the fediment into a phial ; and after ftanding in that fome time, is to be again decanted off from what is precipi- tated there ; and fo on for three or four times, till it is quite clear ; for it muff not be filtered left the acid mould take fomething from the paper that might fpoU the effects. When the liquor is clear, there muff be added to it two ounces of white fea-falt ; this mixture is to be evaporated over a gentle fand heat, till there remains only a dry faline mafs. When the liquor is grown hot, it changes from its orange colour to a fair red ; and when the aqueous humidity is evaporated, it becomes of a beautiful emerald colour, and from this, as it dries up, it changes by degrees to a dirty green like that of verdigreafe in the cake. As it becomes nearly dry, it muft be ftirred about with a glafs-rod or peftle, to keep it from uniting into a mafs ; and it moft not be kept over the fire till perfectly dry, becaufe by that means the colour is often loft, and the fait from green becomes of a dufky yellow ; but if it be taken from the fire while it is green, it gradually becomes reddiih as it cools, and finally is of a beautiful rofe- colour.
This is the fait the German produced before the Academy of Paris. This fait is to be put into another vefiel with a quantity of fimple diftilled water, and this is to be fet alfo in a warm digeftion, till there remains nothing but a white powder precipitated to the botom, which is the ftony part of the ore ufed in the preparation, the fait having been all ta- ken up by the water. If the firft quantity of water does not take up all the fait, but leaves the precipitate coloured, it muft be poured off and more put on, till all the fait is diffolved. Eight or nine ounces of water is the ufual pro- portion to the fait made from an ounce of the tincture ; and if the ore that was ufed has been a good one, this quantity of liquor will be of the colour of the lilac flowers. If the ore has contained copper, which is the cafe in a great many of the bifmuth ores, the liquor will ftill produce its effects as a fympatbetic Ink ; bnt it will be of a bad colour ; and if look'd on in the bottle it will appear of different colours ac- cording as it is held with regard to the light, like the infu- fion of the nephritic wood. This liquor does not retain its virtue fo long upon the paper as that which has been made from a purer ore, and which in the bottle appears the fame in all lights.
The manner of ufing this fympatbetic Ink, is this : Write with it on a fine and fmooth paper, or draw with a black lead pencil the figure of a plant or tree on the paper, then trace over the fame lines with this liquor ; let it dry in the open air, and then rub off the black lines with bread, and the paper will appear altogether fair, tho' the lines made by the Ink are in reality funk deep into it. On holding this pa- per to the fire the lines will all appear, and the figure of the plant or the letters will be painted in a beautiful bluifh green, which will continue fo long as the paper is warm; but when it is cold again, they will wholly difappear. The lines therefore difappear much fooner in winter than in fum- mer ; and in very hot weather it is often neceffary to lay the paper on a marble, or other very cold body, in order to pro- duce tkis effect. At any time if the paper be fcorched in the experiment, the colour of the lines will not difappear again, even if ice be laid upon them. If the writing be expofed for three or four days to a humid air, the lines will appear of a fine pale red. If the impregnation of the ore of bifmuth, inftead of fea-fa!t, have alum added to it, and the whole procefs be continued as before defcribed ; and if letters be written with the red liquor as it is taken out of the veuel, the letters will not appear even on holding it to the fire ; but if the paper be wetted over with a clear folu- tion of marine fait, and then left to dfy, and afterwards held to the fire, the letters will appear blue. The fame alfo will be the effect, if the writing be expofed to the vapour of hot fpirit of fait. When this preparation ia thus made with alum, inftead of common fait, the liquor never becomes green, but continues red, and never changes colour in the
drying
