Spirit gum is a cosmetic adhesive originally made from pine sap. Today however, the term is often used to loosely refer to any light adhesive.
In order for spirit gum to achieve its maximum adhesion, both surfaces to be glued should be brushed with spirit gum and be allowed to dry briefly unattached. Then both surfaces should be tapped with a clean finger briskly to bring up it's tackiness. The surfaces should then be pushed firmly into place.
Numerous formulations of spirit gum exist, varying in viscosity, sheen, and tackiness. More recently, synthetic or rendered spirit gums have come on the market. These may help to alleviate the fumes, allergic reactions, or improve the effectiveness.
True spirit gum is irritating to the eye, and in general, use around the eye should be avoided altogether.