What is "sympathetic ink"

Ink \Ink\, n. [OE. enke, inke, OF. enque, F. encre, L. encaustum the purple red ink with which the Roman emperors signed their edicts, Gr. ?, fr. ? burnt in, encaustic, fr. ? to burn in. See Encaustic, Caustic.]

1. A fluid, or a viscous material or preparation of various kinds (commonly black or colored), used in writing or printing.

   Make there a prick with ink.
   --Chaucer.

   Deformed monsters, foul and black as ink.
   --Spenser.

2. A pigment. See India ink, under India. Note: Ordinarily, black ink is made from nutgalls and a solution of some salt of iron, and consists essentially of a tannate or gallate of iron; sometimes indigo sulphate, or other coloring matter, is added. Other black inks contain potassium chromate, and extract of logwood, salts of vanadium, etc. Blue ink is usually a solution of Prussian blue. Red ink was formerly made from carmine (cochineal), Brazil wood, etc., but potassium eosin is now used. Also red, blue, violet, and yellow inks are largely made from aniline dyes. Indelible ink is usually a weak solution of silver nitrate, but carbon in the form of lampblack or India ink, salts of molybdenum, vanadium, etc., are also used. Sympathetic inks may be made of milk, salts of cobalt, etc. See Sympathetic ink (below). Copying ink, a peculiar ink used for writings of which copies by impression are to be taken. Ink bag (Zo["o]l.), an ink sac. Ink berry. (Bot.)

   1. A shrub of the Holly family ( Ilex glabra), found in sandy grounds along the coast from New England to Florida, and producing a small black berry.

   2. The West Indian indigo berry. See Indigo.

      Ink plant (Bot.), a New Zealand shrub ( Coriaria thymifolia), the berries of which yield a juice which forms an ink.

      Ink powder, a powder from which ink is made by solution.

      Ink sac (Zo["o]l.), an organ, found in most cephalopods, containing an inky fluid which can be ejected from a duct opening at the base of the siphon. The fluid serves to cloud the water, and enable these animals to escape from their enemies. See Illust. of Dibranchiata.

      Printer's ink, or Printing ink. See under Printing.

      Sympathetic ink, a writing fluid of such a nature that what is written remains invisible till the action of a reagent on the characters makes it visible.

Sympathetic \Sym`pa*thet"ic\, a. [See Sympathy, and cf. Pathetic.]

1. Inclined to sympathy; sympathizing.

   Far wiser he, whose sympathetic mind Exults in all the good of all mankind.
   --Goldsmith.

2. Produced by, or expressive of, sympathy.

   Ope the sacred source of sympathetic tears.
   --Gray.

3. (Physiol.)

   1. Produced by sympathy; -- applied particularly to symptoms or affections. See Sympathy.

   2. Of or relating to the sympathetic nervous system or some of its branches; produced by stimulation on the sympathetic nervious system or some part of it; as, the sympathetic saliva, a modified form of saliva, produced from some of the salivary glands by stimulation of a sympathetic nerve fiber.

      Sympathetic ink. (Chem.) See under Ink.

      Sympathetic nerve (Anat.), any nerve of the sympathetic system; especially, the axial chain of ganglions and nerves belonging to the sympathetic system.

      Sympathetic powder (Alchemy), a kind of powder long supposed to be able to cure a wound if applied to the weapon that inflicted it, or even to a portion of the bloody clothes.
      --Dunglison.

      Sympathetic sounds (Physics), sounds produced from solid bodies by means of vibrations which have been communicated to them from some other sounding body, by means of the air or an intervening solid.

      Sympathetic system (Anat.), a system of nerves and nerve ganglions connected with the alimentary canal, the vascular system, and the glandular organs of most vertebrates, and controlling more or less their actions. The axial part of the system and its principal ganglions and nerves are situated in the body cavity and form a chain of ganglions on each side of the vertebral column connected with numerous other ganglions and nerve plexuses.

n. (context dated English) invisible ink