Using a small amount of moisture—usually obtained by secretly licking a fingertip—to make two cards cling together as if they were one, has served several purposes. Perhaps the earliest instance is adhering two cards secretly to assist in a Double Lift, after which the double is tossed into the air. When the double card is placed onto the deck, the adhesion is broken and just the top card is removed, effecting a change or switch. This idea, “Die Verwandlung einer Karte”, was described by Carl Willmann in Die Zauberwelt, Vol. 4 No. 4, Apr. 1898, p. 54. Willmann attributes the trick to Martignoni.
Almost twenty years later, Louis F. Christianer built significantly on the idea of sticking two cards together with moisture. The pair of adhered cards is used to conceal a selection and to locate it in a shuffled deck by using the pair as an impromptu thick card. The joined pair remains together when boomeranged into the air with a third card, and all three are caught in the deck, after which the selection is displayed as having appeared or been caught between the two scaled cards. Christianer published this trick as “The Divining Cards” in his booklet Modern Magical Effects, 1917, p. 16.
See also: Saliva Card Vanish.