The fatty matters which the fibre contains may belong to the components of the fibre itself and be natural matters, but in the case of wool yarns and cloths they are mostly dressing oils, from which the dyer cannot be too anxious to free the wool before dyeing. Some practical methods of preparatory treatment of the fibres before dyeing may therefore be described here with advantage. Cotton is boiled off at actual boiling heat for two hours, with 8 per cent. of its weight of carbonate of soda and a little soft soap, which treatment is sufficient for dark colours. For light colours it is necessary that the cotton be bleached. Wool is scoured with soda and soap in the proportion of 10 lb. soda and 2 lb. Marseilles soap for 100 lb. wool. Silk is scoured by boiling for one and a half hours in a boiling bath with 30 per cent. of its weight of soap. For light colours a second boiling should be given, with 15 per cent. The careful cleaning of wool previous to dyeing is of exceptional importance. Raw wool is cleaned with carbonate of soda and ammonia. For 50 lb. wool to be cleaned 6 lb. carbonate of soda and 1-1/2 lb. (p. 079) ammonia are added to a bath of 150 gallons water. The wool is laid down in it for twenty minutes at 35 deg. C., taken up, squeezed, treated for fifteen minutes in another bath, with 5 lb. carbonate of soda and then rinsed. The first bath must be renewed as often as possible, because it contains all the impurities. In the case of woollen yarn 30 lb. require two tubs of 40 gallons capacity. The first tub is to contain 35 gallons water and 2 lb. ammonia at 10 deg. Be. After working the skeins for three minutes in it they are left to stand for fifteen minutes, then wrung out, and the operation is repeated in the second tub. Finally, the yarn is rinsed several times in soft water. Woollen piece goods are treated in a large wooden tub at 40 deg. C. with 4 lb. carbonate of soda and 2 lb. carbonate of ammonia for 80 lb. material. The pieces are moved about for twenty minutes, laid down in the bath overnight, again turned for ten minutes and hydro-extracted. They may also be handled for forty minutes in a bath of 2 oz. ammonia for 100 lb. wool at 60 deg. C., and then for twenty minutes in clear water at 60 deg. C. After wetting or preparatory treatment, it will be best to proceed immediately to dyeing; if the fibres be left in a heap for too long a time, there is danger that they may become heated, or at least that the moisture may be irregularly distributed by the occurrence of partial drying, causing an uneven fixation of the colour in the first stages of dyeing. The first two conditions of successful dyeing are, therefore, a suitable wetting out and scouring. The dyer, however, must not be less careful to see that the dye-bath is what it ought to be.