The weekly herald. [volume] (Cleveland, Tenn.) 1876-1888, 1877-05-03
SCIENTIFIC

Postal Card Hk.-With the numerous
useful and useless little inventions for
which country Is noted, II Is sur
prising that no one has yet placed In
the market any kind Of invisible ink
for postal cards. Although we do not
believe that such ink would prove very
useful, 1t would probably meet with
considerable sale, partially from the
novelty Of the thing. Many kinds of
magic ink have been known for years,
but most Of them possess some disad-
vantage. A writer In the Deutsche In.
dustris ZeituRg, in discussing the subject,
claims that postal card ink should
possess the following properties: l.
The writing must, Of curse be invisi-
BILE at first. 2. IL must be Of such a
nature al 10 be rendered visible quickly
and easily by means in the possession
Of every one at all times. 3. There
should not be several kinds Of this ink
In market, n OF which Is developed
by light, another by heat, a third by
common salt solution, etc., so that, on
receiving a blank card, the receiver
would be at a loss to know how to de-
velop 1t. This writer then goes on to
recommend the use Of a solution of
nitrate or chloride of cobalt, mixed with
a little sugar or gum 10 make it flow
easily from the pen. Such writing Is
made visible by moderate heat, even a
burning match being sufficient. DR..
Bottger, In a note TO this article, recon-
NERDS the use of ferrOcyamde Of potas-
sum, or yellow prussiate Of potash,
which is readily developed by the sui
pilates of copper and iron (blue OF
green vitriol), the former yielding
brown, the latter blue, letters.
DR.. Bottger is an exceedingly ingen-
iOus chemist, and most of his sugges-
tons are very practical, but we beg TO
differ with him this time. Very few
persons, except chemists, may be sup
posed to have solutions of either blue
or green vitriol always at hand; and
even a chemist, unless notified TO this
effect beforehand, would not think TO
try the effect of these solutions until he
had tried several other reagents. This
could be remedied by writing on the
back of the card with common black
ink an ambiguous or n~onsensical SEN
tence containing the name of the de-
velOper to be employed. For chemists,
a convenient ink would be a solution of
some lead salt, which is developed by
sulphuretted hydrogen.
For ordinary use the most convenient
ink is an iron salt; the common tine
ture of iron of the drug store will
answer, if diluted. This writing is
scarcely perceptible when dry, hut
comes out a beautiful black on pouring
over it some ordinary green or black
tea. The tannin in the tea unites with
the iron to form a black ink precisely
like that used for ordinary correspon-
dence.
Another ink, less convenient for the
writer, but more convenient for the re-
ceiver, is to write or the card with thin
starch paste. When perfectly dry, the
card is flowed with a solution of iodine
in very strong alcohol. This imparts
a reddish color to the card, but does not
develop the writing, owing to the
absence of water. The receiver has
only to dip the card in water, when
the writing will appear in blue charac-
tiers. We offer this as a suggestion to
manufacturers of magic inks. OF
course, the operation of flowing with
tincture of iodine might be left for the
person who receives it, provided he
were furnished with a key to the de-
veloper required. In this case dilute
alcohol, or a solution of iodide of potas-
stun, could be employed as a solvent
for the iodine.
Another method. more curious than
useful, consists in writing on the card
with a solution Of paraffiin in benzol.
When the solvent has evaporated the
parafflin is invisible, but becomes visible
on being busted with lampblack or
powdered graphite, or smoking over a
candle flame.