On this fact depends the process of mordanting wool with potassium bichromate, alum, alumina sulphate, ferrous sulphate, copper sulphate, etc. The exact nature of the action which occurs is not properly understood, but there is reason for thinking that the wool fibre has the capacity of assimilating both the acid and the basic constituents of the salt employed. Excessive treatment with many metallic salts tends to make the (p. 013) wool harsh to the feel, partly owing to the scales being opened out and partly owing to the feel naturally imparted by the absorbed metallic salt. The normal salts of the alkaline metals, such as sodium chloride, potassium sulphate, sodium sulphate, etc., have no action whatever on the wool fibre. Wool has a strong affinity for many colouring matters. For some of the natural colours, turmeric, saffron, anotta, etc., and for the neutral and basic coal-tar colours it has a direct affinity, and will combine with them from their aqueous solutions. Wool is of a very permeable character, so that it is readily penetrated by dye liquors; in the case of wool fabrics much depends, however, upon the amount of felting to which the fabric has been subjected. If wool be boiled in water for a considerable time it will be observed that it loses much of its beautiful lustre, feels harsher to the touch, and also becomes felted and matted together. This has to be carefully guarded against in all dyeing operations, where the handling or moving of the yarns is apt to produce this unfortunate effect. After prolonged boiling the fibre shows signs of slight decomposition, from the traces of sulphuretted hydrogen and ammonia gases which it evolves. When wool is dried at 212 deg. F. it assumes a husky, harsh feel, and its strength is perceptibly impaired. According to Dr. Bowman, the wool fibre really undergoes a slight chemical change at this temperature, which becomes more obvious at 230 deg. F., while at about 260 deg. F. the fibre begins to disintegrate. According to the researches of Persoz, however, temperatures ranging from 260 deg. F. to 380 deg. F. can be employed without any harm to the wool, if it has previously been soaked in a 10 per cent. solution of glycerine. When wool is heated to 212 deg. F. (100 deg. Cent.) it becomes (p. 014) quite pliant and plastic and may be moulded into almost any shape, which it still retains when cold. This fact is of much interest in the processes of finishing various goods, of embossing velvet where designs are stamped on the woven fabric while hot, and in the crabbing and steaming of woollen goods, making hats, etc. CHAPTER II. (p. 015) PROCESSES PREPARATORY TO DYEING, SCOURING AND BLEACHING OF WOOL.