3 94 ^f"' Henry's Conftderatiom on different Materials, 



ofthebafis, let us next inquire into the prin- 

 ciples on which this application is made, and 

 the modes by which it is more firmly attached 

 to the material. 



We have already remarked, that alum con- 

 tains an earth in its compofition, united to 

 vitriolic acid. This earth is purely argillaceous, 

 and may be feparated from the acid to which it 

 is united, by any fubftance to which either the 

 earth or the acid, has a ftronger attraflion than 

 they poffefs to each other. 



The fir(V> theory we meet with to account for 

 the acliony^ alum and other intermediate fub- 

 ftances ufed to furnifh bafes for colouring matter 

 in dying, is that of M. Hellot, who fuppofes 

 that thefe faline bodies form vitriolated tartar, 

 a fait difficult of folution in water; and, that 

 the minute cryftals of this fait, infinuate them- 

 felves into the pores of the material to be dyed j 

 that to thefe cryftals the colouring matter becomes 

 attached and firmly united, and they to the 

 material, fo as to refift the folvent power of 

 water. Thus, when tartar and alum are ufed, 

 he imagines the vitriolated tartar to be formed 

 from them ; and, in every other cafe of dying, 

 he accounts for the production of a vitriolated 

 tartar; but often in a manner by no means 

 fatisfaftory. This vitriolated tartar, he de- 

 fcribes, as cryftallizing in the dilated pores of the 

 cloth J attradling the colouring matter; and 



being 



