OILS 295 
various parts of many plants, and agree in forming an ink- 
like product when combined with an iron salt. Though 
chemically more or less diverse they mostly resemble indican 
and hematoxylin in being glucosides, and are believed to 
be usually waste products of the plant producing them. A 
property of tannins which renders them especially valuable 
to the dyer is that they are readily absorbed in solution by 
cotton, linen, and silk, and will then precipitate various dyes 
within the fiber, thus serving as a mordant. But the chief 
property which gives industrial importance to plants rich 
in tannins is the power which these substances have of so 
combining with animal skins as to render them permanently 
pliable and resistent of decay. Hence it is that a hide soaked, 
under proper conditions, in an extract of tan-bark becomes 
leather. At the same time, the staining powers of the tannin © 
and associated substances may be taken advantage of to 
impart a strong color to the product. 
79. Oils, whether fixed or volatile, are very generally pres- 
ent throughout the vegetable kingdom; and, as we have 
already seen, they are often of much economic importance 
as food or flavoring, and in medicine. They are of scarcely 
less value in the industrial arts, immense quantities of dif- 
ferent vegetable oils being consumed in the manufacture of 
paints, printing-ink, varnishes, soaps, and perfumery, and 
as lubricants and illuminants. 
As vehicles for pigments fixed oils are selected which not 
only will hold the particles of coloring matter in perfect sus- 
pension, and so make it easy to spread them evenly over a 
surface, but which also will harden promptly when thus 
spread into a film exposed to the air. Oils which harden in 
this way are called drying oils although the change which takes 
place depends not upon the evaporation of a volatile solvent, 
as in the drying of certain varnishes, but upon the absorption 
of oxygen which changes the oil into a varnish-like substance. 
Linseed-oil, which is obtained by pressure from the seeds of 
flax (Fig. 217), is the one most widely used by painters. Its 
“drying” qualities are much improved by boiling. For use 
in printing-ink the oil is boiled until it is very thick. Other 
drying oils which are somewhat superior to linseed-oil are 
