DYE MATERIALS .

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difference may be thus shortly stated. M. Persoz holds that the colour isfixed to the surface only of the fibre ; Mr. Walter Crum thinks the fibre tobe perfectly permeated by the colour. No other portion of our subject is sodifficult as the one upon which we are now engaged, and none so little re-munerative, for were our knowledge precise and complete upon this point thegain would not be great.

The practical view of the question leads to the following considerations :—A mordant is a substance which can exert an affinity for the fibrous materialto which it is applied, at the same time possessing an attraction for colouringmatters. Mordants are not very numerous, and the class may be subdividedinto mordants proper and mordants of a dubious nature. The first divisioncontains those metallic salts whose oxides appear to effect an intimatechemical union with the fibre and colouring matter. The second divisionincludes a number of substances, which, possessing some affinity for colouringmatter, do not appear to combine with the cloth in a chemical, but ratherto adhere to it in a mechanical manner. Under the first division may bementioned the so-called mineral mordants, salts of iron, zinc, alumina, tin,and copper; to the second division belong some vegetable and animalmatters, as galls, oils, albumen, caseine. With wool, silk, hair, skin, horn,and bone there exists naturally an affinity for colouring matters, but notto an extent sufficient to yield general good results. There are only a fewcolours that can combine with either vegetable or animal tissues without theaid of a mordant, and of these the majority are much improved both inbrilliancy and fastness by the presence of a mordant. The colouringmatters of madder, logwood, fustic, and many other dyes are by themselvescapable of imparting a feeble colour to unmordanted calico, but with theassistance of mordants they communicate fast and brilliant hues. Althoughseveral colouring matters can impart their shade to woollen fabrics withouta mordant the hue is poor and feeble, lacking lustre, and not possessed°f any great stability. The colouring matters of indigo, safflower, andannatto impart colour to cotton fabrics without the agency of a mordant,which does not much improve the depth and colour; and for woollen tissuesmay be added picric acid, archil, and many of the aniline colours. Silkbehaves very similarly to wool, and with the exception of indigo—a dye suigeneris —all the colours are loose and unstable. There are certain conditionsabsolutely necessary to the constitution of a true mordant. The first issolubility. Unless the metallic oxide to be used as a mordant can thoroughlypermeate the fibres, the colour will not resist washing, at least with soap andhot water; and such complete permeation cannot take place except themordant be in the condition of a clear solution. The property of becomingmsoluble is the second essential of a mordant; it is apparent that if a mordantbe carried into the pores of the fibre simply on account of its solubility, thatthe same reason would cause its return, unless means were taken to fix it per-manently upon the spot to which it has penetrated. This insolubility isobtained in various ways. The metallic oxide is combined with a volatilea cid, such as acetic acid, which, after volatilisation, leaves the oxide in-solubly fixed in the fibre ; this method is most generally employed. Or theoxide may be combined with a non-volatile acid, the acid being removed bychemical means, such as an alkaline bath, exposure to vapours of ammonia,

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