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SECT. XI.— TLANTS USEVUL IN DYKINC.

stirring the whole up, when an effervescence will take place, the alumthat had united with the juice of the madder will be found to becomeneutralized by the pot-ash, and the result will be a precipitate of thered fecula. This may be washed over in different waters, and eitherl>ut by for use in a liquid state, or filtered and dried in powder or cakes.Most vegetable colours will not, however, admit of being extracted bywater, and it is necessary to use an acid for that purpose : vinegar isthe most common. But in making the extract from roots with acidsigreat care should be taken that they are sufficiently cleared frommould, sand, &c. ; for, if the same should contain either iron, or anymetallic substance, its union with the acid will cause a blackness, andof course spoil the tint. In a similar mode are all the different co-louring principles extracted, either from leaves, flowers, fruits, or woods.The preparation of woad is a curious process on similar principles;which see in page 31.

Weld, or dyers weed, is generally used after it is dried. Thewhole plant is ground in a mill, and the extract made by boiling it.It is then managed with alum and acids agreeably to the foregoing rules,which are necessary for throwing out the colour.

instructions how Substances way be tried, whether they are serviceable if 1Dyeing, from Hopson's Translation ofWeigleb's Chemistry.

" In order to discovei if any vegetable contains a colouring principlefit for dyeing, it should be bruised and boiled in water, and a bit ofcotton, linen, or woollen stuff, which has previously been well cleaned,boiled in this decoction for a certain time, and rinsed out and dried.If the stuff becomes coloured, it is a sign that the colour may be easilyextracted; but if little or no colour be perceived, we aie not immedi-ately to conclude that the body submitted to the irial has no colour atall, but must first try how it will turn out with the addi'ion of salinesubstances. It ought, therefore, to be boiled with pot-ash, commonsalt, sal ammoniac, tartar, vinegar, alum, or vitriol, and then triedwpon the stuff: if it then exhibit no colour, it may safely be pro-nounced to be unfit for dyeing with. But if it yields a dye or colourthe nature of this dye must then be more closely examined, whichmay be done in the following manner:—