60 Theory of Dying. [Book VIII, 



of galls, mixed with green vitriol, require no previous 

 preparation of the fluff. 



It is obfervable that wool takes the dye better than 

 filk, filk than cotton, and cotton than flax. Writers 

 on the art of dying hold different opinions refpet- 

 ing the manner in which colouring particles apply them- 

 felves to the fubftances expofed to their contact. Many 

 have fuppofed that this application takes place only in 

 proportion to the number and magnitude of the pores 

 in the various fubftances. Macquer, who has paid 

 great attention to this fubject, fuppofesthat the greater 

 or lefs facility with which the colour is applied, de- 

 pends on the refpective nature of the colouring parts, 

 and the fubftances propoled to be dyed : and that dying 

 is truly an external tinge or painting, which fucceeds 

 and lafts by virtue of an affinity and intimate union 

 between the colour and the dyed fubftance. This 

 ferves to explain the ufe of the matters, which it is on 

 many occafions neceffary that the fluffs fhould imbibe, 

 previous to immerfing them in the dying fubftance, 

 The fame thing may be illuftrattd by confidering the 

 procefs employed in the preparation of certain colours 

 called lakes. Vegetable colouring matters are diffolved, 

 and then precipitated by the addition of fome other fub- 

 ftance. Thus, for example, if madder is boiled in wa- 

 ter, together with an alkali, and alum is then added, 

 the earth of the alum will be precipitated, together 

 with the colouring matter, with which it will form an 

 infoluble pigment. A double decomposition here takes 

 place, the vitriolic acid quits the earth of alum to unite 

 with the fixed alkali, and the vegetable matter unites 

 itfelf with the earth, 



