Blue, is one of the seven colours of Nature, into which the rays of light divide themselves when refracted through a prism. The principal blues used in painting are, Prussian blue, bice, Saunders' blue, azure or smalt, verditer, etc.; for the preparation of which. Bee Colour-making. In dyeing, the principal ingredients which afford a blue colour, are indigo and woad. - See also Dyeing. The Dutch blue, commonly call-ed Turnsol, may be prepared by the following process : The kind of lichen called Arabic, or, in default of it, the large oak moss, being dried and cleansed, ought to be reduced to a powder, and by the assistance of a press, forced through a sieve, the holes of which should be small. This powder should be then mixed in a trough with an alkali called vetas, or the ashes of wine lees, in the proportion of one-third ashes, and two-thirds lees. This composition being moistened with human urine, a fermentation is excited, and a due degree of moisture preserved by the addition of the same liquor. When it assumes a red colour, it should be removed into another vessel, again moistened with urine, and stirred, to renew the fermentation. In a few days the blue colour will begin to appear, and it must then be carefully mixed with a third part of pure powder of pot-ash; after winch it should be removed into wooden pails, three feet high and six inches broad. As soon as the third fermentation begins, it ought to be mixed with pulverized chalk or marble. The last gives no addition but in weight.

A fine blue colour, equal to ultramarine, may be made by collecting the blue cern-bottle flower, or Centaurea cyanus, which abounds in almost every corn-field : it has two blue tints ; the one pale in the larger outward leaves, the other deeper, which lies in the middle of the flower; by rubbing the last, while fresh, so as to express the juice, it will yield a beautiful and unfading colour.

On the same day that the flower is gathered, the middle should be separated from the extremities, and when a quantity of the juice is obtained, a small addition of alum will produce a permanent, clear blue, which, in the opinion of many persons, is not inferior to ultramarine.

Blue John, among miners, is a kind of mineral which has lately been fabricated into vases and other ornamental articles. It is of the same quality as the cubical spar. At the foot of the high mountain called Mam-Tor, at Castleton, in perbyshire, it is still found in large pieces, which are sold for about nine pounds per ton.

Blue. - A fine colour has lately been discovered by Mr. Thomas WIllis, which promises to be useful in the art of painting. It is prepared by mixing a solution of alum and martial vitriol with the mother water, which remains after extracting the crystals of phosphorated soda, from a combination of the phosphoric acid with pure mineral alkali. - Our limits not permitting us to relate his various experiments, the curious reader will consult Mr. W.'s " Account of, and observations on, different blue colours produced from the mother-water of soda phosphorata, " etc. which is inserted in the 4th vol. of the " Memoirs of the Literary and Philosophical Society of Manchester".