154 ENGLISH BOTANY. 



tlie liark is romovrd ; lionoc iliai, bark wliicli presents tlio appearance of not liavin«jf 

 been easily detaclieil, fetches a far lower price than that which seems to have been 

 removed with facility. The richest bark is always obtained in the warmest spring, 

 as it then contains most sap : a few days only of cold weather previous to felling 

 and stripping causes a very perceptible reduction in the proportion of tannin and 

 sap. The bark of coppice trees about twelve years old contains more tannin than 

 that of younger trees, and the latter more than that of old trees. According 

 to Dr. Stenhouse, the tannin of oak-bark docs not afford i^jTOgallic acid when 

 subjected to the destructive distillation, like the tannin of gall-nuts ; from which 

 circumstance it may be concluded that the tannin of the bark is not identical with 

 that of galls. An analysis of oak-bark by M. Geiger, afforded 5 to 6 per cent, of 

 tannin. Sir H. Davy estimated the entire bark of middle-sized oak cut in the spring 

 to contain G per cent. In tbis, and all other astringent barks, the tannin is con- 

 tained solely in the inner white layers next to the alburnum ; the middle, coloured 

 portion contains most of the extractive matter ; and the epidermis, or exterior, con- 

 tains little extractive matter and no tannin. According to common estimation, fi-om 

 three and a half to four pounds of oak- bark are required for the production of one 

 pound of leather. The quality of leather made by means of oak-bark is considei-ed to 

 be superior to that of the leather made with either of the numerous tanning materials 

 which are now so extensively employed in the place of bark. The process of tanning 

 with bark, however, requires the longest time. The present price of English oak- 

 bark is from 5Z. to 87. a ton. The price of foreign oak-bark, duty paid, per ton, is as 

 follows : — 



Dutch, from 61. 10s. to Gl. 10.s\ ; Flemish, from 57. 10s. to 71. 10s. ; and German, 

 from 47. to 57. 



Oak-bark, on account of its tanidn, has been used as an astringent medicine since 

 the days of the Greek physicians. It is a very powerful astringent, and its decoction 

 is an excellent gai^gle for relaxed sore throats ; as well as a good lotion for ulcers, &c. 

 It is not so much used in medicine now as formerly, quinine and astringents of foreign 

 origin having in a great measure taken its place. The acorn-cups of a species of oak, 

 the Quercns JEcfiloi^s, which grows in the Levant, are most valuable articles of export 

 from the Morea, and from Smyrna and are known by the name of Valonia ; above 

 7,500 tons of these cups being imported into this country from thence every year. 

 They are sold almost wholly to tanners and dyers. It is said that the leather pro- 

 duced by means of valonia is harder and less permeable to water than that made 

 with oak-bark, and so heavy as to constitute this the cheapest of all tanning 

 materials, catechu or terra japonica only excepted. The leather produced by a mixture 

 of valonia and oak-bark is of very excellent quality. 



The tannin of valonia appears to be different from that of nut-galls, as it affords 

 no pyrogallic acid on destnactive distillation. Dr. Stenhouse found only a trace of 

 gallic acid in this tanning material. An infusion of valonia speedily affords the 

 deposit of " bloom." 



The various parts of the oak-tree arc subject to the attacks of different species of 

 insects belonging to the genus Cynips. They are commonly called gall-flies, and 

 produce various excrescences upon the leaves, stem, &c. Kn-by and Spence's work 

 on Entomology tells us that the insect that produces the gall-nut is the Cynips Scn'p- 

 torum. They attack chiefly a species of ofik veiy common in Asia Minor (jQuerais 

 ivfectoria), in many parts of -v^liich the galls are collected by the poorer inhabitants 

 and exported from Smyi'na, Aleppo, and other ports in the Levant, as well as from 

 the East Indies. Ollivier says that the insect li\es on this species of Quercus only. 



