COCHINEAL.

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print upon a woollen fabric a colour prepared with a known weight of pul-verised cochineal: in both cases standard samples of the ware are applied forthe sake of comparison. This method, however good, will give erroneousresults if the cochineal is mixed with Brazil wood ; but this sophisticationmay be detected, according to M. Persoz, by the following simple test:—When lime-water (not milk of lime) is added to a dilute solution (decortion) ofcochineal the colour is discharged forthwith, but if Brazil wood is presentalong with the cochineal a violet tinge remains. When apiece of calico isimpregnated with a decortion thus adulterated, and next steeped in a tepidsolution of bichromate of potassa, the tissue will assume a rather deep colour,Cochineal is sometimes mixed with artificially-moulded inserts, made up ofsome coloured paste ; but this fraud is readily deterted, even by steeping thesusperted substance in water, which has the effert of disintegrating themoulded material, and making it crumble to pieces.

In the selertion of cochineal considerable caution is needed. The grainsshould be full, plump, dark, and glossy. Silvery kinds and samples of areddish-grey mottled appearance are not to be trusted, as the former are oftenweighted, and the latter immature and deficient both in the quantity andquality of their colouring matter. Gritty samples, in which small stones,clay, and sand are perceived, should be rejerted. The specific gravity is animportant characteristic, and may often be taken when there is not time todye swatches, or to try the amount of colouring matter by any chemical pro-cedure. We may at least lay down the rule, that though a light sample maybe bad, yet a heavy one cannot be good. The specific gravity varies so muchthat of samples dried at 212 0 F. the superior kinds weighed 60 to 66 grs.,whilst the same measure of the inferior qualities weighed 80, go, andeven 100. A Rham’s specific-gravity bottle, or, in default, a very smalllight beaker, will answer the purpose. The vessel is accurately tared,and then filled exartly full of the cochineal, not by pressing, but by gentletapping and shaking. If the same vessel is always used the results will becomparable.

A good lac-dye should be soft enough to be broken by the fingers. Thefrarture should be deep in colour, but should not appear shining and resinous,and should, when breathed upon, give out a strong and peculiar odour. If thelac is hard and has a resinous frarture, it is poor in colour and rich in shellac.It should pulverise easily under the pestle. Excess of resin is especiallyobjertionable if the lac is intended for dyeing worsted goods which have to behot-pressed. If it is loaded with resinous matter the pressing-paper will stickto the surface in patches, and can only be removed at a great outlay of timeand labour, if at all. To ascertain roughly the amounts of resin, equal weightsof the samples, in powder, are placed in small flasks, with the addition ofequal measures of alcohol, and they are set to digest in a warm place, looselystoppered, and occasionally shaken. The alcoholic solutions are then allowedto settle in the cold, decanted off into light capsules, previously tared, evapo-rated to dryness, and weighed. Or excess of water may be added to the alco-holic extrarts, when the shellac will be precipitated, and its comparativeamount may be judged by the eye with sufficient accuracy.

Cochineal is employed in dyeing and printing in the following states:—Asrnefc with in trade, without having undergone any preparation at all; ground