154 ENGLISH BOTANY. 
the bark is removed; hence that bark which presents the appearance of not having 
been easily detached, 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 does not afford pyrogallic 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 6 per cent. In this, 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, from 
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 considered 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 51. to 8l. a ton. The price of foreign oak-bark, duty paid, per ton, is as 
follows :— 
Dutch, from 57. 10s. to 67. 10s. ; Flemish, from 51. 10s. to 71. 10s.; and German, 
from 4/. to 51. 
Oak-bark, on account of its tannin, 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 gargle 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 Quercus Agilops, 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 destructive 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 are 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. Kirby and Spence’s work 
on Entomology tells us that the insect that produces the gall-nut is the Cynips Scrip- 
torum. They attack chiefly a species of oak very common in Asia Minor (Quercus 
infectoria), in many parts of which the galls are collected by the poorer inhabitants 
and exported from Smyrna, Aleppo, and other ports in the Levant, as well as from 
the East Indies, Ollivier says that the insect lives on this species of Quercus only, 
