The discovery of this mode of dyeing scarlet may be considered asthe most re markable mra in the art of dyeing. The .ancients applied the name scarlet to a co lour obtained from kermes, which was much inferior in beauty, to the colour procured from cochineal.
Dufay, Hellot, Macquer, and Berthol let, were suscessively charged by the French government with the care of im proving the art of dyeing. Dufay was the first who entertained just, though imper fect ideas of the nature of colouring sub stances, and the power by which they adhere ; he examined certain processes with great sagacity, and established the surest methods that could at that time be employed, for determining the goodness of a colour. Hellot published a metho dical description of the processes used in dyeing wool, which even now is the best treatise we have on the subject. Macquer has given an exact description of the processes employed in dyeing silk ; he has made us acquainted with the combinations of the colouring princi ple of Prussian blue ; he has endeavour ed to make an application of it to the art of dyeing, and has given us a process for communicating the most brilliant co lours to silk by means of cochineal. He intended to publish a general treatise on the art of dyeing, of which he gave the prospectus in 1782; but his death, which took place in 1784, prevented the execu tion of any part of the work.
Berthollet succeeded Macquer ; his treatise on dyeing is one of the best epi. tomes on the subject ; and chemistry and the arts of dyeing, and of bleach ing, have been much indebted to his la bours.
In his theory of dyeing, he refers all the combinations produced in the forma tion of colours to the laws of chemical at traction: and all the changes, which the colouring particles undergo, to the con junction of the elements of the new com bination. The first effect of the attrac tion he considers as analogous to the formation of neutral salts ; the second to combustion, putrefaction, and many other operations of nature.
Besides the authors mentioned, Chap tal, D'Apligny, D'Ambourney, and Hauff man, in. France, have published treatises on the art of dyeing, which have much contributed to its improvement. In Swe
den, Scheirer alone has written on the subject ; his work is accompanied with notes by the celebrated Bergman. In Germany, experiments in different pro cesses of dyeing have been published by Beckmann, Poerner,Vogler, and Franche ville. In England, two very valuable es says on dyeing, by Delaval and by Hen ry, have appeared ; to which may be added the excellent treatise on the phi losophy of permanent colours, by Dr. Bancroft.
Of the Attractions of Colouring Substances.
A variety of theories have been pro duced by ingenious men, to account for the effects of dyeing. Bergman seems to be the first who referred them entirely to chemical principles ; and this opinion is so consonant to, reason, that it is now universally adopted.
Dyeing, then, is to be considered mere. ly as a chemical process; but in order that it may succeed, it is necessary that the colouring matters should be dissolved in some fluid, (for in their solid state no at traction takes place between them and the stuff,) and that their attraction to the fluid should be less than that to the stuff. Besides, the colouring matters being brought within the proper distancelor attraction by this means, they are also causedto apply themselves more equally, as every part of the stuff has thus an op portunity of attracting to itself the proper quantity of colouring matter.
The stuff receives the dye better, in proportion to the degree of affinity which the colouring matter has to it and to the solvent relatively, for if its attraction to the stuff is much more than to the sol vent, the stuff receives the dye too ra pidly, and it will be scarcely possible to prevent its being unequal : but if, on the other hand, its attraction to the solvent is too great, the stuff will either not take the dye at all, or it will take it very slowly and faintly. Wool has a stronger attraction for colouring matters than silk, silk than cotton, and this latter a stronger than linen. Hence it is necessary to use solvents for the dyes of the stuffs last mentioned, which have a weaker affinity for them than for those used in dyeing wool.