154 On the Effvcl of Madder Boot 



ing of this property of madder, he says,* " Wc 

 ** have, in the fact before us, a beautiful ex- 

 " ample of a particular case of chemical at- 

 " traction ; such as in nmnberless instances, 

 " is observed to take place, between the co- 

 " louring particles of both animal and ve- 

 " getable substances, and various other bodies, 

 " especially earths and earthy salts, and oxydcs 

 " of metals. So strong is the affinity of the 

 *' colouring matter to- these bodies, that it is 

 " frequently observed to quk the menstruum, 

 *' in which it may chance to be dissolved, to 

 " unite with them : they, in consequence of 

 ^' its union, acquiring a particular tinge, 

 " whilst the menstruum is proportionably de- 



" prived of colour. From this principle, 



" this mutual attraction, is deduced the va- 

 " rious use of those bodies as mordents, as 

 " they are called, mtermedia, or means for 

 " fixing the colours in dying or staining thread 

 " or cloth, whether it be campos(!d of ani- 

 " mal or vegetable materials. Upon the 

 " same principle depends the preparation of 

 " those pigments, known to painters under 

 ** the name of lakes ^ these are truly precipi- 

 " tates of the colouring matter, in combina- 



* See Dr. Biakc's inaugural Disserlalion. Do dcatium 

 formalione. p. 119.— 179S. 



