OILS 295 



various parts of many i)lants, and agree in forming an ink- 

 like product when combined with an iron salt. Though 

 chemicahy more or less diverse they mostly resemble indican 

 and hsematoxylin 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 thej^ 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 

 alread}^ 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 j)igments 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 

 "drjdng" 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 



