What is "oil of birch"

Birch \Birch\ (b[~e]rch), n.; pl. Birches (-[e^]z). [OE. birche, birk, AS. birce, beorc; akin to Icel. bj["o]rk, Sw. bj["o]rk, Dan. birk, D. berk, OHG. piricha, MHG. birche, birke, G. birke, Russ. bereza, Pol. brzoza, Serv. breza, Skr. bh[=u]rja. [root]254. Cf. 1st Birk.]

1. A tree of several species, constituting the genus Betula; as, the white or common birch ( Betula alba) (also called silver birch and lady birch); the dwarf birch ( Betula glandulosa); the paper or canoe birch ( Betula papyracea); the yellow birch ( Betula lutea); the black or cherry birch ( Betula lenta).

2. The wood or timber of the birch.

3. A birch twig or birch twigs, used for flogging.

   Note: The twigs of the common European birch (B. alba), being tough and slender, were formerly much used for rods in schools. They were also made into brooms.

   The threatening twigs of birch.
   --Shak.

4. A birch-bark canoe. Birch of Jamaica, a species ( Bursera gummifera) of turpentine tree. Birch partridge. (Zo["o]l.) See Ruffed grouse. Birch wine, wine made of the spring sap of the birch. Oil of birch.

   1. An oil obtained from the bark of the common European birch ( Betula alba), and used in the preparation of genuine (and sometimes of the imitation) Russia leather, to which it gives its peculiar odor.

   2. An oil prepared from the black birch ( Betula lenta), said to be identical with the oil of wintergreen, for which it is largely sold.