The colouring matter contained in the _sac_ of the _Purpuræ_ is a liquid of a creamy consistency, and of a yellowish-white hue. On extraction, it is at first decidedly yellow; then after a little time it becomes green; and, finally, it settles into some shade of violet or purple. Chemical analysis has shown that in the case of the _Murex trunculus_ the liquid is composed of two elementary substances, one being cyanic acid, which is of a blue or azure colour, and the other being purpuric oxide, which is a bright red.[819] In the case of the _Murex brandaris_ one element only has been found: it is an oxide, which has received the name of _oxyde tyrien_.[820] No naturalist has as yet discovered what purpose the liquid serves in the economy, or in the preservation, of the animal; it is certainly not exuded, as sepia is by the cuttle-fish, to cloud the water in the neighbourhood, and enable the creature to conceal itself. Concerning the Phoenician process of dyeing, the accounts which have come down to us are at once confused and incomplete. Nothing is said with respect to their employment of mordants, either acid or alkali, and yet it is almost certain that they must have used one or the other, or both, to fix the colours, and render them permanent. The _gamins_ of Tyre employ to this day mordants of each sort;[821] and an alkali derived from seaweed is mentioned by Pliny as made use of for fixing some dyes,[822] though he does not distinctly tell us that it was known to the Phoenicians or employed in fixing the purple. What we chiefly learn from this writer as to the dyeing process is[823]--first, that sometimes the liquid derived from the _murex_ only, sometimes that of the _purpura_ or _buccinum_ only, was applied to the material which it was wished to colour, while the most approved hue was produced by an application of both dyes separately. Secondly, we are told that the material, whatever it might be, was steeped in the dye for a certain number of hours, then withdrawn for a while, and afterwards returned to the vat and steeped a second time. The best Tyrian cloths were called _Dibapha_, i.e. “twice dipped;” and for the production of the true “Tyrian purple” it was necessary that the dye obtained from the _Buccinum_ should be used after that from the _Murex_ had been applied. The _Murex_ alone gave a dye that was firm, and reckoned moderately good; but the _Buccinum_ alone was weak, and easily washed out.