272 Chemistry. [Lecture 33. 



precipitated by it, and it acts upon those, because 

 the metals have a strong attraction for that sub- 

 stance which the prussian alkali contains ; at the 

 same time the acid acts upon the alkali, and pro- 

 duces a double exchange: and thus the metal 

 is precipitated. If iron, it is precipitated in a 

 prussian blue. If the prussian alkali is pro- 

 duced by calcination with blood, it will be very 

 difficult to saturate it entirely ; and if any part 

 is not saturated, it will precipitate a calcareous 

 earth. The only way of saturating it entirely is 

 by digesting it in prussian blue already pre- 

 pared, which tinges it yellow. The metallic sub- 

 stances found in mineral waters are seldom any 

 other than iron or copper, and we may at 

 once discover their precipitates by their colour. 

 When we desire to know more certainly if iron 

 is contained in water, and also the quantity of it, 

 we may use solution of galls, which is the most 

 exquisite test of the minutest quantity of iron. 

 Copper may be discovered by the addition of a 

 volatile alkali, which produces a blue colour, by 

 first precipitating the copper, and then re-dis- 

 solving it ; though the prussian alkali answers 

 better. The presence of copper, too, may be 

 discovered by throwing into the water a piece of 

 very bright iron ; very soon after (especially if 

 the liquor is warm) the copper is deposited on 

 the surface of the iron ; but this seldom happens, 

 unless there is a considerable quantity of copper ; 

 which is rarely the case in mineral waters. Zinc 



