A 1929 article from The Melliand, Part IV of a series on the physics and chemistry of dyeing, reporting experiments on how silk and wool take up acids, acid dyestuffs and o-nitraniline. It gives tables of equilibrium absorption (limit values in gram equivalents per 100 g fibre) for many acids and dye acids on degummed silk, compared with wool, silkworm gut, acetylated wool and casein. The authors conclude that wool fixes 3-4 times more acid than silk, that dyeing involves both salt formation and solution (Henry's law), and discuss solution versus adsorption and the partly crystalline structure of silk. Includes two graphs and eleven data tables.