as ObjeSis of the Art of Dyings lie. 307 



Again, let the fupernacant liquor, from which 

 all the earth has been precipitated, be examined, 

 to fee in what ftate of combination, the vitriolic 

 a-cid of the alum, remains. 



In the common dyes, then, with metallic or 

 earthy bales, the theory is pretty clearly as M. 

 Macquer has reprefented, only taking the aftrin- 

 gent principle into the account; and, with re- 

 fpeft to wool and filk, nothing more feems necef- 

 fary than the impregnation of the one, with 

 alum and tartar, and, of the other, with alum 

 alone, previous to their immerfion in the co- 

 loured liquor. 



But when Cotton is to be dyed, and fome 

 of thefe bafes are. requifite, not only the bafis 

 is to be precipitated by the aftringent colouring 

 principle, but the attraftion of the material 

 to the bafis is to be increafed by other inter- 

 media. The permanency of the extraftive 

 dyes, therefore, depends on the previous treat- 

 ment of the cotton, and, where alum is em- 

 ployed, of that fait, fo as to procure a more 

 copious precipitation of its earth, and to unite 

 it, by means of other fubftances, to the material. 



For this reafon, in the common dying of 

 cotton, the alum is previoufly neutralifed by 

 the addition of alkaline fait, whereby not only 

 the acid is prevented from injuring the cotton, 

 but the alum is put into a ftar^ more ready 10 



b? 



