5 8 Colouring Matters of Vegetables. [Book VIII, 



ftept in a warm temperature, it undergoes fermenta- 

 tion, attended with the extrication of air, by which 

 the particles are feparated from each other, and the 

 mafs fwells and becomes more porous. This diften- 

 fion is ftill further increafed by the rarefaction of the 

 air in baking, and thus is formed a fubftance much 

 more mjfcible with water than dough, and upon this 

 latter property feems to depend its greater wholefome- 

 nefs, as being more digeftible, 



9, COLOURING MATTERS of VEGETABLES. On a 

 knowledge of the colouring matters of vegetables, de- 

 pends the art of dying, which confifts in extracting 

 from various fubftances colouring particles, and apply- 

 ing them to {tuffs and other matters intended to be 

 dyed, fo that they fhall adhere as firmly and durably 

 as pofiible. Dyers enumerate five colours, which 

 they call primary, from the mixture of which other 

 colours are produced : thefe are blue, red, yellow, nut- 

 colour, and black. Good dyes arc thofe which can 

 refift the action of water, air, and of certain faline and 

 faponaceous liquors, which are ufed as the proofs of 

 the durability of colours. Falfe dyes are thofe which 

 cannot refift thefe proofs. 



A great number of vegetable colouring matters > 

 \vhich are of an extractive or faponaceous nature, are 

 readily difiblved in water. The colouring principle 

 of many other fubfiances refides in a purely refinous 

 matter, infoluble in water, and in fome inftances at- 

 tached to matters infoluble, even in fpirit of wine j but 

 they are all acted on by alkalies, which convert them 

 into a kind of foaps, mifcible with water. The prin- 

 cipal colours of this nature are the annotto, a kind of 

 fecuk, obtained by maceration of the feeds of the urucu 

 putrefied in water, and which dyes an orange yellow 



colour ; 



