They are, therefore, oxygenated volatile oils?
MRS. B.
Not exactly; for the process does not appear to consist so much in the oxygenation of the oil, as in the combustion of a portion of its hydrogen, and a small portion of its carbon. For when resins are artificially made by the combination of volatile oils with oxygen, the vessel in which the process is performed is bedewed with water, and the air included within is loaded with carbonic acid.
EMILY.
This process must be, in some respects, similar to that for preparing drying oils?
MRS. B.
Yes; and it is by this operation that both of them acquire a greater degree of consistence. Pitch, tar, and turpentine, are the most common resins; they exude from the pine and fir trees. Copal, mastic, and frankincense, are also of this class of vegetable substances.
EMILY.
Is it of these resins that the mastic and copal varnishes, so much used in painting, are made?
MRS. B.