374 ^1"' Henrfs Confiderations on different Materials, 



alkali be employed, the precipitation takes place 

 more flowly ; and if the pure alkali continue 

 to be added, after the precipitation is effefted, 

 the earth will be rediflblved. 



This earth has a ftrong attraftion for colour- 

 ing matter; particularly for fuch as forms the 

 red and yellow colours. Infomuch, that if a 

 folution of alum be poured into water, deeply 

 tinged with madder or weld, the earth will quit 

 its acid to unite with the colouring particles of 

 thefe fubftances, and form with them a precipi- 

 tate or laquej from which it cannot be feparated, 

 either by the adlion of water, or fpirit of wine, 

 and which is not even totally deftroyed by that 

 of fire. 



Marine fait of tin, and that formed by aqua 

 regia have, for their bafis, the white earth of that 

 metal, which has alfo a ilrong attraction for 

 colouring matter, and is, in fome cafes, pre* 

 ferred to that of alum. When united to the 

 colouring matter of cochineal, it forms a beauti- 

 ful pigment, well known by the name of car- 

 mine. If the addition of thefe faline fubftances 

 to the coloured liquors, be fufficiently long 

 continued and under proper circumftances, the 

 whole, of the colouring matter will be preci- 

 pitated, and the water be left colourlefs*. 



Tin is not the only metal which affords bafes 

 for colouring fubftances. Lead, bifmuth and 

 zinc alio afford earths or calces, which attraifl; 



* Macquer Diftionaire de Chymie. 



colouring 



