404 Mr. Henry's Conftderations on different Materials^ 



wholly effefted : and it cannot be fuppofed 

 that the alkali with which the cotton has been 

 impregnated, in the previous operations, added to 

 that contained in the barilla liquor, mixed with 

 the alum can be fuch, even if it were in an un- 

 combined ftate, as to produce fuperfaturation. 



It fhould feem probable therefore that the 

 permanency of the Turkey red depends on the 

 caufes already afligned, and that its brightnefs 

 is produced by the adion of the mineral alkali 

 on the madder. This appears, at firft fight, 

 difcordant with the theory of Mr. Delaval that 

 alkalis reduce red colours to crimfon, and thefe 

 to purple ; whereas in the prefent cafe the red 

 is brightened, by boiling in a ftrong folution of 

 mineral alkali -, but the fad is perfeftly agreeable 

 with that theory -, for the madder colour is too 

 much inclining to a dufky orange, and this, by 

 means of the incraffating alkali, defcends to red. 



Before I conclude, permit me to advance a few 

 circumftances relative to the black dye. 



For the dying of black, the calx of iron 

 is the mordant employed, and this uniting with 

 the aftringent principle of the galls forms a 

 black pigment which is attrafted and adheres 

 to the material to be dyed. 



The late Dr. Lewis had afcertained, by a 

 number of well conduced experiments, that 

 the colouring matter of ink, confifted of a very 

 finely attenuated calx of iron, combined with 



this 



