372 Mr. Henry's Conftderations on different Materials, 



principle ; and, in many cafes, that it fhould 

 poflefs perfed whitenefs, for the purpofe of re- 

 flefting the rays of light, fo as to enable the ting- 

 ing matter to exhibit its peculiar colour with 

 the greater brilliancy. — If to thefe properties, be 

 added, that, though foluble in acids, its folu- 

 bility fhould not be too eafy, and that it Ihould 

 even be capable of forming infoluble compounds 

 with fome other fubftances, which may be occa- 

 fionally added to it, for that purpofe, we have 

 perhaps a complete defcription of fuch a bafis, 

 or, as it is commonly called, a mordant. 



This is a term, that appears to have been firfl: 

 introduced by the French dyers j who, appre- 

 hending that the intention of pafling the fub- 

 ftances which were to be dyed, through certain 

 faline liquors, the nature of which they did not 

 underftand, was to corrode fomething that op- 

 pofed the entrance of the colouring principle, 

 and to enlarge the pores of the fubftances, gave 

 to the liquors, the appellation of mordants. — A 

 term which, as conveying a wrong idea, it is to 

 be wiftied were rejected. I ftiall therefore take 

 theliberty.to change theword mordant for basis, 

 adding an epithet occafionally, defcriptive of the 

 body, from which it is obtained. 



The fubftancesprincipallyufed to afford the white 

 bafes for colouring matter, are alum, and foluti- 

 ons of tin in different acids, but generally in ma- 

 rine acid, or in a mixture of marine and nitrous 

 acid, commonly known by the name of aqua regia. 



Alum, 



