as ObjeHs of the Art of Dyings (^c. 399 



and, while the vitriolic acid feizes on the alkali, 

 the earth of alum becomes intimately combined 

 with the animal acid. It appears to me highly 

 grobable, that this acid is fupplied to the cotton, 

 in the procefs for dying the Adrianople red. 

 That the attradlion between the cotton and acid 

 being ftrong, and that between the latter and 

 the earth of alum being likewife powerful, fuch 

 an union is effefted as aflTifts in rendering the 

 material capable of attrading and retaining the 

 colouring matter, in as forcible and permanent 

 a manner, as can be done either by wool or 

 filk. 



The ufe of the galls alfo in this and other pro- 

 cefles feems intended to promote a fimilar pur- 

 pofe. Cotton cither unbleached, or which has 

 undergone no procefs but that of bleachino-, 

 when immerfed in a folution of alum, produces 

 no change in the appearance of the folution j but 

 as has been already (hewn, cotton previoufly 

 fteeped in an infufion or decodion of galls, foon 

 renders the liquor turbid, occafioning a precipi- 

 tation of the earth of alum on the cotton. 



The imperfe<5t foap alfo, formed by the union 

 of the alkali and oil when mixed with the alum, 

 will both decompofe that fait, and be itfelf de- 

 compounded, and a foap of a different nature will 

 refult from the union of the oil, with the earth of 

 alum. M. Bertholet who has made feveral ex- 

 periments on earthy and metallic foaps, found 



this 



