1 N K. 



Again, take a book four or fivp inches thick, and on the 

 flrli leaf write any thing with the lull liquor ; turn to the other 

 riid of the book, and nib there with a rag, dipt in the fiift 



liquor, on that 



the writing, and leave alfo the rag there," clapping a paper 

 over it ; tlien nimbly /hutting the book, ftrike four or five 

 fmart ftrokcs thereon with your hand, and turning the other 

 fide uppermoft, clap it into a prefs, or lay it under a good 

 weight for a quarter of an hour, or even half that time ; 

 then will the writing done with the invifible ink be found le- 

 gible there. 



The above operation may be varied, by writing the invifi- 

 ble charafters with the folution of bifmuth in nitrous acid, 

 and expofing them tn the \'apours of liv^-r of fulphur, or moif- 

 tcning them with a folution of liver of fulpliur. 



Diffolve white or green vitriol in water, and writing with 



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 tlie gold remains united to its dilfolvent. it is yel- 



part, as near as you can guofs, oppofite to low ; but the acid that diflblves it bemg of a volatile nature, 

 " ' ■ ■ ■ the greater part of it evaporates, and leaves no more than is 



jufl neccfTary to colour the calx of gold which remains upon 

 the paper. 



The fecond of thefe is the filver ink, m.nde by a folution 

 of iilvcr in aquafortis, weakened by diftilled water, till it 

 will not llain the paper. Letters written with this will be 

 iovifible for three or four months, if fiiut up in a box ; but 

 if they be expofed to the fun they become legible in about 

 an hour, becaufe by this means the evaporation of the acid 

 is accelerated. The letters written with this ink are of a 

 (late colour ; and that fron^ the fulphureoiis nature of the 

 aquafortis, every thing that is fulpliureous blackening fil- 



the folution, nothing will appear. Boil galls in water, and ver. This blackilTi colour, however, is not permanent ; for 



dip a linen rag in the decodHon, and with it rub the place be- 

 fore writ, and it will appear black and legible. Rub it over 

 again with fpirit of vitriol, or its oil, and the writing will dif- 

 appear again ; rub it over again with oil of tartar per deli- 

 quium, the letters will appear again, but of a yellow co- 

 lour. If the blacknefs of ordinary ink be dellroyed by a 

 fufficient quantity of nitrous acid, the writing made by it 

 will remain invifible till it be moillened with liquid fixed 

 alkali. 



The golden fympathet'ic tni. — This is made by difiblving 

 in aqua regia as much gold as that menftruum can take up, 

 and then adding to the hquor five or fix times as much water; 



in another 

 giai 



uing 

 ■efii^l 



there mull be fomc tin dilTolved 



the fulpliureous part finally evaporating, the letters are left 

 to their natural appearance, and are of a fine true filver co- 

 lour, if the filver that wasufed was fine, and the place open. 

 In this clafs there may alfo be placed fcveral metaUic dif. 

 folutions ; as thit of lead in vinegar, and of copper in aqua- 

 fortis, which give at length a brownifli colour upon the 

 paper ; as alfo the folution of tin in aqua regia, of mercury 

 in aquafortis, of iron in vinegar, of emery and fevcral of 

 the pyrita in fpirit of fait. But all thefe, though they 

 give letters which are after fome time legible, on being ex» 

 pofcd to the air, are alfo made to appear inltantly on the 

 paper, holding tliem to the fire. Each of thefe folutions 

 gives its own particular colour ; but they have all this difad- 



d when that menftruum has alfo taken up as much of vantage, that in time they eat away the paper, and the letters 



are fecn in the lliape of fo many holes. 



Of the third clafs of ink, or that which appears on rub- 

 bing over the paper with a brown or black powder, are alnioll 



all the gh; 

 fel ^- 



effedi 



the metal as it can, there is to be added to it an eqtial 



tily of common water. The letters mud be written on white 



paper with the folution of gold, and the writing being dried 



in the fliade, the letters will not appear, at lead not for feven 



or eight hours afterwards; dip a pencil in tht- fohition of tin, 



and rub that over the folution of gold with which the letters 



were written, and they will appear of a beautiful purple. 



It might be fuppofed, that any other metals which were fo- 



luble in the fame acid menftruum would equally produce this 



effeft ; but experiment (hews, that this is not the cafe ; and 



filver and copper, though both foluble in aquafortis, yet 



produce no change of colour by thefe precipitations made by 



mixing the folutions of them on paper; and this example of Mem"! Acad. Science, Par. 17^7 



of plants, which are thi 



of no remarkable colour, the mtlk of animals, or any 

 other thick and vifcoiis 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 ap- 

 pear coloured, becaufe its vifcous quality remains fufiiciently 

 in it for the entangling and retaining this fine powder, 

 though it falls eafily off from every other part of the paper. 



luch an effedl in the folutions of gold and tin, is an excep- 

 pon to the geneial rules in the folutions of metals, and their 

 effedls 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, Caflius and Orfchal, with fome other writers, 

 made the firfl ftcps towards this difcovery, by their attempt 

 to give cryftal, by means of gold, the colour of the oriental 

 rubies. There are, bcfide thefe mineral preparatioss, fome 

 vegetable ones, which give the fame phenomena, but thefe 

 arc the more certain. 



Among the methods which Ovid teaches young women to 

 deceive their guardians, when they write to their lovers, he 

 mentions that of writing with new milk, and of making the 

 writirig legible by coal-duft, or foot. 



" Tuta quoque eft, fallitque oculos, e lafle rccenti 

 Litera : carbonis pulvere tange : leges.'' 



De Arte Amaudi, 1. iii. v. 629, 

 Aufuiiius propofes the fame means to Paulinus, and he after- 

 wards teaches other methods of fecret writing. Aufon. 

 Epift. xxiii. v. 21, &c. Eneas, in Poliorccticis, cap. ^i, 

 , . . , , , , . , ""^ Gellius, lib. xvii. cap. cj, mention the like. Plinv, 



Ot the fecond kind of fympathetic inks, or tliofe which lib. x.-<vi. cap. 8. mentions the milky fap of certain plants 

 appear on bi-ing expofed only to the air, is the golden ink for a (itnilar pnrpofe. 



made by adding to a folution of gold in aqua regia, fo much Of tiie fourth clafs, or thofe inks which become vifible 

 .'^i!?^" '., ?i • '^"'n'' '. ""' '^^.'n ^/''^''^ P^'P'-*'" • •'-■"'^''s ?" holding them to die fire, there are a vaft number ; and 



indeed all infulions, the matter of which is readily burnt 



written with this will not appear till the paper has b 



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 ex- 

 pofing the paper to the air, it be kept.in a box clofe Ihut up, 

 or cli»fcly folded in other paper, it will remain invifible two 



to a fort of charcoal by a little fire, will anfwer tliis pur- 

 pofe. The nicell of this kind is the fal ammoniac ink, 

 made by diffolving a fcruple of fal ammoni.ic in two ounces 

 of fair water. Letters written with this folution arc in- 

 vifible on the paper, till it ie held before the fu-e, or has an 



