42. It has been proposed to divide the colouring par ticles of vegetables into extractive and resinous ; but this division is attended with no advantage, and indeed only serves to convey very imperfect and erroneous ideas of their properties, as some of the most important dyes arc not at all affected by the usual solvents which act upon these two classes of substances. The truth is, that the colouring particles possess chemical properties which are peculiar to themselves, and distinguish them from all other substances; though these properties are different in different colouring matters. They unite with acids, alkalies, metallic oxides, and some of the earthy bodies, particularly alumine. They are said to precipi tate oxides and alumine from the acids which hold them in contbination ; but this is doubtful, or at least the de composition is very partial. They more frequently form triple compounds with the salts, and in this state of com bination unite with the stuffs to which they are present F 2 ed. t a mor inti• manner than they would do with out t o inn 11t1 t 1 nl tin •t 43 col !W• p t tI it s le I nit. however, unite inehs i matt Is with tI L•se ch, and there k • yulrrOith 1 I lit cull t fits :Mill(' 10 the it respective TI e 11n 1s1 t (1111111011 soil cut, are 55 ate', acids, pull .Ikalit s; alcohol I, se loom use d. unless when we wish to .1 1 on t Ir (morning matter or small bodies.
est. sr istances, 5% di I is Iii0St CNICIISirely employ ed, t Ali on account of its abundance, and the great Ilhlch it exerts over almost all colouring math r•, aik arc VIII ployed to promote the solution of the colour' g pril elide contained in indigo, the flow •rs t-f h st kc.; and the acids are used in some cases to tassoi5c co.eurs, and in others lo precipi tate the colouring priniples from their solutions in alkal 44 Before CI doming principles are exposed to the action e a sokent, tier Ilion to be divided as minutely. as possi dc, by ni chailieal means. This is obtained by tri.nritfm,; thin in a inortar, or by means of a mullet-, and ..fterwards passing them, in a reduced state, through a fine sieve, that no gross particles may escape notice. The m t. od, by which the division of colouring substan ces is produced, tare according to the consistence, stn uctore, and volatility ol the matters upon which the opera tion is performed. In sonic cases, the substance con taining the colouring principle is reduced to chips or scales, by means of a sharp instrument; in others, it is perfectly sufficient to bruise it under uifiers, in the same manner as tan is ground. In whatever way pul verization is performed, the operation is attended with the diffusion of a subtile powder through the workshop, which is both injurious to respiration, and the cause of a considerable loss to the manufacturer, particularly if the article he expensive. These disadvantages may be
guarded against, by pet forming the process of ti nitration in cos et cd places, or moistening the matters acted upon with water. Auer the colouring principle is sufficiently comminuted, it tray be dissolved, without difficulty, in some of the solvents formerly mentioned.
43. With the exception or some colouring matters of a resinous or starchy nature, water may be regarded as a Will cruel solvent of that class of bodies ; while it pos sesses the important property of forming a very imper fect union with them, and readily parts with them again to the stuff. Warm water extracts the colouring. prin ciple molt• copiously than cold water; hut di•erent sub stances require different degrees of temperature for that purpose. Sonic colouring principles can only be dis solved by long protracted maceration; others require a very gentle heat, and suffer in their brilliancy by being exposed to too elevated a temperature; and others still yield but little of their colour, uniess they be raised to a considerable degree of heat. A knowledge of these cir cumstances is ahuolutely necessary to the dyer, in order that 1.e may connuct his operations with certainty and success.
46. A great deal has been ascribed to the qualities of waters used in ely e Mg, nothing 14( Mg more common than to refer the brilliancy of sonic colours, and the poverty of others, entirely to this cause. Without adopting all that has been pnblished on this subject, it must be allowed that waters col tribine essentially to the qua lities of dye s; and it may be added, that different colours, and even the same colour in different states, require that waters of very different natures should be employed.
Rapid and miming waters are getiCrally the purest, and standing waters most impregnated 55hi, earthy and saline maturs. In some cases, Wive% er, the. latter mar lw used with great udvantige in dyeing, because the putrid animal and vegetable matters suspended in then), con tribute to form ammonia and sulphureted hydrogen, which precipitate the earthy and metallic principles.
47. Waters holding calcareous salts in solution, are particularly prejudicial to the dyeing cotton ol a red co lour. The Time which is precipitated during the pro cess attaches itself to the stuff, and obscures the colour to such a degree that it is almost impossible to revive it. But if the object be to obtain a dark colour, these salts are rather beneficial than hurtful, as they tend to in c•ease the hotly of the red colour and its 1-n°(111k:100ns; hence they may be used with advantat:,e when it is intend ed to convert scarlet into crimson.