INK. ' f 63 
are apt to attrafl oxygen from the air, fuch as lead, bif- 
muth, filver, &c. The fympathetic ink of gold, already- 
mentioned, belongs alfo to this clafs; for, if the charac¬ 
ters written with it are long expofed to the air, they be¬ 
come by degrees of a deep violet-colour, nearly approach¬ 
ing to black. In like manner, characters written with a 
lolution of nitrat of filver are invifible when newly dried, 
but, being expofed to the fun, appear of a grey colour like 
flate. To this clafs alfo belong folutions of fuga-r of lead, 
nitrats of copper and of mercury, acetat of iron, and 
muriat of tin. Each of thefe has a particular colour 
■when expofed to the air; but they corrode the paper. 
3. Such as appear by firewing or ffling fame very fine pow¬ 
der of any colour over them. —This third clafs of fympathetic 
inks contains fuch liquids as have fomekind of glutinous 
vifeofity, and at the fame time are long in drying; by 
which means, though the eye cannot difeern the charac¬ 
ters written with them upon paper, the powders ftrewed 
upon them immediately adhere, and thus make the writ¬ 
ing become vifible. Of this kind are urine, milk, the 
juices of fome vegetables, weak folutions of the deliquef- 
cent falts, See. This kind of fympathetic ink is an old 
invention. Among the methods by which Ovid teaches 
young women to deceive their guardians, when they write 
to their lovers, he mentions that of writing with ne\y 
milk, and of making the writing legible by coal-duft or 
foot: 
‘Tuta quoque efi,fallitque oculos, e laEle recenti 
* Litera : carbonis pulvere tange, leges. 
It is obvious, that any other colourlefs and glutinous 
juice, w'hich will hold fall the black powder ftrewed over 
it, will anfwer the purpofe as well as milk; and therefore 
Pliny recommends the milky juice of certain plants to be 
tiled. 
4. Thofe that become vifible by being expofed to the fire. — This 
clafs, comprehending all thofe that become vifible by be¬ 
ing expofed to the fire, is very extenfive, as it contains 
all thofe colourlefs liquids in which the matter diflolved 
is capable of being reduced, or of reducing the paper, 
into a fort of charcoal by a fmall heat. Sulphuric acid 
diluted with as much water aswill prevent it from cor¬ 
roding the paper makes a good ink of this kind. Letters 
written with this fluid are invifible when dry, but inftantly 
on being held near the fire appear as black as if written 
with the ftneft ink. Juice of lemons or onions, a folution 
of fal-ammoniac, green vitriol, See. anfwer the fame pur¬ 
pofe. 
5. Such as become vifible by the heat , but difappear again by 
cold or the moifiure of the air. —This fifth clafs comprehends 
only a folution of muriat of cobalt; for the properties of 
which, fee the article Chemistry, vol. iv. p. 271. where 
we have given directions for making a green fympathetic 
ink. M. Heliot obferves, that, if nitre or borax be added 
to the nitrous folution, the characters written with it be¬ 
come rofe-coloured when heated; and, if-fea-falt is after¬ 
wards palled over them, they become blue; that, with al¬ 
kali fufficient to faturate the acjd, they change purple and 
red with heat. 
A blue fympathetic ink may be made from cobalt in 
the following manner: Take of an earthy ore of cobalt r 
as free from iron as poflible, one ounce. Bruife iv, but 
not to too fine a powder. Then put it into a cylindrical 
glafs, with fixteen ounces of diftilled vinegar, and fet the 
mixture in hot land for the fpace of fix days, ftirring it 
frequently ; or elfe boil it direCtly till there remain but 
four ounces. Filter and evaporate it to one-half. Ifyour 
folution be of a rofe-colour, you may be certain that 
your cobalt is of the right fort. A red-brown colour is 
a fign of the folution containing iron; in which cafe the 
procefs fails. To two ounces of the folution thus re¬ 
duced, add two drams of common fait. Set the whole 
in a warm place to diflolve, and the ink is made. 
For making a fympathetic ink of this clafs, the fol¬ 
lowing procefs, by M. Meyer, may be worthy of the rea- 
Vol. XL No. 734. 
der’s notice. It was entered upon in confequence of a 
receipt for rofe-coloured fympathetic ink fbown to him 
by a traveller. In that receipt cobalt was the principal 
ingredient, and therefore the firft object w r as to procure 
cobalt; but M. Meyer, being unwilling'to facrifice purs 
pieces of cobalt of any conliderable fize, made choice of 
one which was vilibly mixed with bifmuth, iron, and 
quartz. He endeavoured to feparate the bifmuth as much 
as poflible, and alfo the arfenic, if it (hould contain any, 
by bringing it flowly to A red heat; and he fucceeded 
pretty well, as the bifmuth flowed from it in abundance; 
and the arfenic, the quantity of which was final!, was vo~ 
latiiifed ; many globules of bifmuth ftill adhered to it. 
By bringing it repeatedly to a red heat, and then quench¬ 
ing it in water, it was reduced to fuch a ftate as to be 
eafily pulveriled. Having poured nitrous acid upon the 
powder, he obtained by digeftion a beautiful rofe-red fo¬ 
lution ; the filiceous earth was feparated in the form of a 
white flime, and by diluting it with water, there was de- 
pofited a white powder, which was oxyd of bifmuth 
The folution being.filtred, he added to it a folution of 
potafli, and obtained a precipitate inclining more to a 
yellow than to a red colour. He again poured over it a 
little of the nitrous acid, by which-a part of the oxyd 
was re-diflolved of a red colour; the remaining part, 
which had a dark brown colour, was oxyd of iron. From 
the folution, by the addition of potafti, a precipi¬ 
tate was formed, which was now reddifh. Having by 
this procefs obtained it pure, that he might now prepare 
from it the wiftied-for red ink, he diflolved the waftied 
pure oxyd of cobalt in different acids. That diflolved in 
the nitrous acid with a mixture of nitre, gave a green ink 
like the common ; that diflolved in the l'ulphurous acid, 
without the addition of falts, gave a reddilh ink, which 
remained after it was expofed to heat, and would not 
again difiippear, even when a folution of nitre was ap¬ 
plied; and that diflolved in the muriatic acid, gave a 
green ink, darker and more beautiful than the common. 
By dilfolving it, however, in the acetous acid, and ad¬ 
ding a little nitre, he obtained what he had in view; for 
it gave, on the application of heat, an ink of a red co¬ 
lour, like that of the rofa cent folia, which again difap- 
geared when the paper became cold, but could ftill be 
renewed by heat carefully applied. 
Thus it appears, that, of all the kinds of fympathetic 
ink, the molt curious is that made with cobalt; fince the 
characters or figures traced out with this ink may be 
made to difappear and to re-appear at pleafure. This 
property is peculiar to ink obtained from cobalt; for all 
the other kinds are at firft invifible, until fome fubftance 
has been applied to make them appear; but, when once 
they have appeared, they remain That made with cobalt 
may be made to appear and to difappear any number of 
times, at pleafure, by repeating the application of heat. 
With this kind of ink fome very ingenious and amuflng 
tricks, fuch as the following, may be performed. 
To make a drawing which fliall alternately reprefent 
fummer and winter.-—We fhall fuppofe that you have pro¬ 
vided the green ink of which the eompofition is explained 
under the article Ch-emistry, vol. iv. p. 271. Draw a 
landfcape, and delineate the ground and the trunks of the 
trees with the ufual colours employed for that purpofe; 
but the grafs and leaves of the trees with the liquor 
above-mentioned. By thefe means you will have a draw¬ 
ing, which at the commom temperature of the .atmof- 
phere will reprefent a winter-piece; but, if it be expofed 
to a'proper degree of heat, not too ftrong, you will fee 
the ground become covered with verdure and the trees 
with leaves, fd as to prefent a view in fummer. Small 
fire-fereens painted in this manner were formerly made 
at Paris. Thole to whom they were prefented, if unac¬ 
quainted with the artifice, were aftonifbed to find, when 
they made ufe of them, that the views they exhibited were 
totally changed ; and, as a fereen is often moved nearer 
to or farther from the fire without any apparent defign, the 
S changes 
