54. In a practical point of view, such opinions arc of very little moment; and perhaps it may be more useful to attend to those distinctive characters of the colouring principles, which are founded on their chemical relations with the stuffs. Accordingly, Dr Bancroft has suggest ed the division of colouring matters into substantive and ic the first including those matters which, when put into a state of solution, may be fixed with all the per manency of which they are susceptible, awl made fully to exhibit their colours in or upon the dyed substance, without the interposition of any earthy or metallic basis; and the second comprehending all those matters which are incapable of being so fixed, and made to display their proper colours without the mediation of some such basis." (Philosonhy. of Permanent Colours, vol. i. p. 118.) These terms, though perhaps not altogether unexceptionable, must at least be admitted to have a reference to real dif ferences between the substances which they are intended to distinguish; and though the advantages which are de rived from this arrangement are indeed but slight, since very few colouring matters are contained under the first head, but most of them under the second, there can be no impropriety in adopting them, particularly as they are intended to express a fact, and not a theoretical opi nion. In treating of colouring matters, we shall there fore observe Dr Bancroft's distribution of them ; but in describing the processes by which these are applied to the stuffs, we conceive it will be more convenient, and not less systematic, to follow some arrangement which has a reference to the colours themselves, rather than to the colouring matters from which they are procured. We shall accordingly divide the colours, without any regard to the decompositions obtained by the prism, into simple and compound; meaning by the latter, such as can, and by the former, such as cannot, be produced by the intermixture of other colours. Under simple co lours, we shall include red, yellow, and blue; and under compound colours, the various modifications or thest which are obtained by mixture or superinduction Of Substantive Colouring Matters.
55. Dr Bancroft has subdivided this class of colouring matters into animal, vegetable, and mineral. The prin cipal dye belonging to the first head of arrangement, IS the celebrated Tyrian purple. The substance which yielded that beautiful colour, was a whitish half-Rudd matter, secreted by particular organs in certain unival vular shell fish, and retained by the animal in an appro priate receptacle. The accounts which have been trans mitted to us by the ancients respecting the shell-fish in question, are obscure and contradictory. Those of Pliny are the most explicit and intelligible, though they are sometimes inconsistent with each other. He mentions the shell-fish which afforded this dye, under the several names of conchvlium, murex, purpura, and huccinum; all of which are also used by other Roman writers. This curious subject has been so ably investigated by Dr Ban croft, that very little further elucidation can be thrown upon it; we shall therefore avail ourselves of his re -an I e-, ..net Lonniumeate the substaace ,.f them as
rullv as is consistent w ith the nature of our work.
56. It appears tl at Fabius Columna, a Neapolitan no denial), w s the earliest modern w riter st ho wrote a dis.c rt. lion on the Purpura, ++1 ieh he pu'ilished in 1616. After being at much panic to elucidate and reconcile the mite eta passages of ancient it r iters on this subject, he t Jule to the c onelusion that tht re wet L. two kinds or ge nera of shell-fish used for procuring the purple dy tz. the pm pura and buec'nuni ; that the term conehylium siznified generall, ali the species of purpurze, and some times the purple colour itself; and that the term murex was also used in the same generic sense. Pliny, indeed, xpressly mentions, that all the shell-fish y 'etching the urple and other lighter colours of the conchylia, are in matter the same, and differ only in that they arc of two kinds, one being called briceinum, from its resemblance to the instrument called the horn, and the other purtmra. The former he describes as being round at the aperture, and having a serrz.ted margin ; the latter as having a projecting pipe-shaped beak, with a lateral winding cavity, through which it puts forth its tongue. lie mentions also that the body of the shell of the purpura is muricated or armed with seven rows of spires, which are wanting in the buccinum. He adds, that the buccinum adheres to rocks and large stones, whence alone it can be collected.
57. The best purpunt found on the coasts of Asia were caught in the sea adjoining to Tyre; on thc Afri can coast, the best were those of Meninx and the Ge tulian shores; and the best on the coasts of Europe were found at Laconica. Pliny informs us, that the Tyrians took the finest out of the larger shells, in order to extract the purple more effectually, but obtained the colour from the smaller by grinding them in mills. He adds, that when the pitrpul a: were caught, the receptacle which contained the dyeing liquor was taken out and laid in salt for three days; and that after a sufficiency of the matter had been collected, it was boiled slowly in leaden vessels over a gentle fire, the workmen skimming off from time to time the fleshy impurities. This process lasted ten days, after which the liquor was tried by dipping wool into it, and if the colour produced by it was defective, the boil ing was renewed. Pliny afterwards erroneously repre sents the liquor of the buccinum as only yielding a fugi tive colour, and says that it was usually mixed with more than 1 alf as much of the liquor of the purpura, which of itself gave a very dark purple; and that the mixture produced a beautiful amethyst colour, the latter giving permanency to the former, and being in return brighten ed and enlivened by it. We learn on the same authority, that the Tyrians produced their purple by first dyeing the wool with the unprepared or greenish liquor of the pclagium, (another name which he applies to the pu• pura), and ancr•ards in the liquor or the buccinum, and that the resulting colour was deemed most perfect when it resembled that of coagulated blood.