DYEING. 



^sixteenth of potash, and as much of 

 powdered chalk, are also added. In this 

 mixture, the sulphuric acid combines with 

 the lead, and is precipitated ; and the 

 alumen, or base of the alum, combines 

 with the acetous acid, as it parts from the 

 lead, and forms acetite of alumen ; the 

 chalk and potash serve to saturate the ex- 

 cess of acid. 



The final effect of alumining, in what- 

 ever way performed, and whatever che- 

 mical changes may have taken place in 

 it, consists in the combination of alumen 

 with the stuff'; this union is probably im- 

 perfect, and the acids but partially sepa- 

 rated at first, but becomes complete when 

 the stuff is afterwards impregnated with 

 the colouring substance. 



The attraction of alumen for animal 

 substances may be shewn by direct ex- 

 periment ; for if a solution of alum is mix- 

 ed with a solution of glue, on adding an 

 alkali, the glue is precipitated in combi- 

 nation with the alumen. 



The attraction of alumen for most co- 

 louring substances may also be proved 

 by experiment. If a solution of a co- 

 louring substance be mixed with a solu- 

 tion of alum, and an alkali be added, 

 which decomposes the alum, the colour- 

 ing matter will be precipitated combined 

 with the alum, and the liquor will re- 

 main clear. The matter precipitated is 

 called a lake. In this experiment too 

 much alkali must not be added, because 

 alkalies are capable of dissolving most 

 lakes. 



No direct experiment has yet shewn 

 that alumen attracts any vegetable sub- 

 stances, except colouring matters: its at- 

 traction to them seems much weaker than 

 that which it has for animal substances; 

 hence the acetite of alumen is a better 

 mordant than alum for linen and cotton, 

 as has been observed ; and upon this de- 

 pend the different means employed to 

 increase the fixidityof the colouring par- 

 ticles in dyeing these substances. 



Lime is the only earth, besides alum, 

 which is employed in dyeing: the affinity 

 of lime for cloth is sufficiently strong; it 

 is, however, found to answer the purpose 

 of a mordant less perfectly than alumen, 

 on account of the colour, which is not so 

 good. It is employed either in the state 

 of lime-water, or of that of sulphate of 

 lime dissolved in water. 



Metallic oxides have so great an at- 

 traction for many colouring substances, 

 that they quit the acids in which they 

 were dissolved, and are precipitated in 



combination with them : they hav also 

 the property of uniting with animal sub- 

 stances; it is therefore natural, that they 

 should serve as a bond of union between 

 the colouring particles and animal sub- 

 stances ; but, besides the attraction of the 

 oxides for colouring substances, and for 

 animal matter, their solutions in acids 

 possess qualities which render them more 

 or less fit to act as mordants : thus, those 

 oxides which easily part with their acids, 

 such as that of tin, are capable of com- 

 bining with animal substances, without 

 the aid of colouring particles: it is suffi- 

 cient to impregnate wool or silk with a 

 solution of tin, although they be after- 

 wards carefully washed, which is not the 

 case with other metallic solutions. 



Some metallic substances afford in 

 combination only a white and colourless 

 basis ; and some, by the admixture of 

 their own colour, modify that which is 

 proper to the colouring partides; but in 

 many metallic oxides, the colour varies 

 according to the proportion of the oxy- 

 gen they contain, and the quantity of this 

 is easily liable to change. Upon these 

 circumstances their properties in dyeing 

 principally depend. 



The attraction of metallic oxides for 

 substances of vegetable origin is much 

 weaker than for animal substances, and 

 we are even ignorant whether they are 

 capable of contracting a real union with 

 them or not: metallic solutions are there- 

 fore ill-fitted to serve as mordants for co- 

 lours in linen or cotton, except iron, the 

 oxide of which unites firmly with vege- 

 table substances, as is shewn by iron 

 moulds, which are owing to a real com- 

 bination of this oxide. When the co- 

 louring particles have precipitated a me- 

 tallic oxide from its menstruum, the su- 

 pernatent liquor contains the disengaged 

 acid, which is commonly capable of dis- 

 solving a portion of the compound of 

 colouring substance and oxide, so that 

 the liquor remains coloured : but some- 

 times the whole'ofthe colouring matter 

 is precipitated, when the proportions 

 have been accurately adjusted: this pre- 

 cipitation is facilitated, and rendered 

 more complete, by the presence of the 

 stuff, which assists, by the tendency it has 

 to unite with the compound of oxide and 

 colouring matter. 



Uncombined metallic oxides have also 

 a very evident action on many colouring- 

 substances when boiled with them, and 

 modify their colour; the oxide of tin, in 

 particular, increases the brightness and 

 fixidity of several. 



