I N K. 



I iron a little heated paflcd over it. The rationale of this 

 ! is, that the iiiSammable part of the fal ammoni.io is burnt 

 i to charcoal by a hc-at wliich is net fuflicicnt to Icoroh the 

 paper ; and this is tlie cafe with all the rcll of this clafs. 

 The letters written uitli this folution are, however, of no 

 great duration ; for the fait being apt to moiften in the 

 air, the Liters foon fpread, and run together in a confufed 

 manner. 



Dr. Lewis fays, that all the falts which he has tried: 



produce this efFecl in a greater or lefs degree ; nitre, alum, 



I tartar, very weakly ; fea-fiilt more ftrongly ; fixed alka- 



I line falts ftill niflre fo ; but fal ammoniac the ftrongeft of 



! all. Metallic folutions, made in acids, and diluted fo as not 



' to corrode the paper, aft in the fame manner. The juice of 



lemons and milk have alfo been ufed as fympathetic inks, 



the writing with whicli -appears upon the application of , heat 



' fnfficient to decompofe the oily and mucilaginous parts of 



thefe iiquoFS. 



Ti'.e fifth clafs of fympathetic ink contains only one yet 

 known kind. 



This, though in itfelf invifible, becomes of a blueifh green 

 when held to the fire ; and this colour difappears again as the 

 paper cools, and is to be produced again on holding it again 

 to the fire : and this for a long time, and a repeated fe- 

 ries of trials. It may alfo, according to the different 

 manner of treating it, be made to appear blue, green, ycllo'v, 

 red, and fome other colours. This property of the tinging ' 

 matter of bifmuth ore was publilhed at full length by a Ger- 

 man lady, in 1705'. 



A certain German cliemift fliewed the academy at Paris 

 fait of rofe-water, which became blue on holding it to the 

 tire, and at the fame time {hewed the ore from which he pro- 

 cured the fait, which he called an ore of marcafite, a name 

 given by many to the bifmuth ore. He added, that this was 

 the mineral from which the fine blue fmalt of Sneeberg was 

 prepared, and that no other ore but this afforded it, and that 

 lie made the tinfture from this mineral with aquafortis, 

 which he fixed with fea-falt. 



This was the fubftance of what the German declared, 

 and from what Mr. Hellot, in 1736, 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 five ounces of common water. 

 After the firft ehulhtion is over, place the veffel in a 

 gentle fand-heat, and let it ftand there till no more air- 

 bubbles feem to afcend ; after this increafe the fire, fo as 

 to make the liquor boil for about a quarter of an hour ; 

 after this the liquor will become of a reddilh colour, and 

 when cold it is to be decanted clear off from this fediment 

 into a phial; and after ftanding in that fome time, it is 

 to be again decanted off from what is precipitated there ; 

 and fo on for three or four times till it is quite clear ; for it 

 muft not be filtered, left the acid fhould take fomeihing 

 from the paper that might fpoil the effedts. When the 

 liquor is clear, there muft 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 wlion 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 verdigris 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 muft not be kept over 

 the fire till perfeftly dry, hecaufe by that means the colour 

 is often loft, and the fait from green becomes of a dufliy 

 yellow ; but if it be taken from the fire while it is green, 

 it gradua ly becomes reddifti as it cools, and finally is of a 

 rofe-colour. The manner of uling this fympathetic ink 

 is this : write with it on a fine and fniooth paper, or draw 

 with a black-lead pencil the figure of a plant or tree on the 

 paper ; then trace over tlie fame hues with this liquor ; let 

 it dry in the open air, and then rub off the black fines 

 with bread, and the paper will appear altogether fair, 

 though the lines made by the ink are in reality funk deep 

 into it. On holding this paper to the fire the lines will all 

 appear, and the figure of the plant or the letters will be 

 painted in a beautiful blueifh green, which will continue 

 fo long as the paper is warm ; but when it is cold again, 

 they will wholly difappear. The lines, therefore, dif- 

 appear much fooner in winter than in fummer ; and in very- 

 hot weather it is often neceffary to lay the paper on a 

 marble, or other very cold body, in order to produce this 

 effeft. At any time, if the paper be fcorched in the ex- 

 periment, the colour of the lines will not difappear again, 

 even if ice be laid upon them. If the writing be ex- 

 pofed for three or four days to a humid air, the hnes will 

 appear of a fine pale red. If the impregnation of the ore 

 of bifmuth, inftead of fea-falt, have alum added to it, 

 and the whole proccfs be continued as before defcribed ; 

 and if letters be written with the red liquor as it is takea 

 out of the veffel, the letters will not appear even on hold- 

 ing it to the fire ; but if the paper be wetted over with a 

 clear folution of marine fait, and then left to ilry, and 

 afterwards held to the fire, the letters will appear blue. 

 The fame alfo will be the effed, if the writing be cxpofed 

 to the vapour of hot fpirit of fait. When this preparation 

 is thus made with alum, inftead of common fait, the 

 liquor never becomes green, but continues red, and never 

 changes colour in the drying, or afterwards. The green 

 colour feems to be wholly the effeft of the fea-falt ; for not 

 only this aluminate impregnation, but others in which other 

 falts had been employed, were always found to be of a dif- 

 ferent colour. 



Glauber's fait, ufed inftead of common fea-falt, left 

 the mafs red in the fame manner as alum did. Nitre 

 added, inftead of fea-falt, gave the precipitate or dried 

 fait a beautiful purple colour, which became white on the 

 inllant that water was poured upon it, and a rofe-coloured 

 tindture was drawn from it, which gave fines or letters 

 on paper, which continued invifible as long as they were 

 cold, but aflumed a beautiful red oa holding the paper 

 to the fire ; which colour they retained no longer than 

 wl'.ile the paper centinued warm, difappearing afterwards 

 in the iVme manner with the green colours made by the 

 fea-falt ; and if a fimple folution of fea-falt be rubbed 

 over the paper, and fuffered to dry, and the paper be af- 

 terwards heated, the lines appear blue. Borax has the 

 fame effect in this preparation with the nitre. All thefe 

 experiments were made with the neutral falts ; but in or- 

 der to try what would be the effefts of alkalies in tlte 

 mixture, Mr. Hellot added to three ounces of the im- 

 pregnation of the ore in aquafortis, pure fait of tartar, 

 till the ebullition ceafed ; but the confequence of this 

 was no great precipitation, but merely the fubfiding of a 

 fmall white fediment ; this mixture being evaporated 

 nearly to a drynefs, the remaining mafs, fo long as it wras 

 warm, appeared of a beautiful purple ; but this becam- 

 paler as it dried, and turned while in an iuftant on pour- 



inj 



