226 METALLIC oxidation:, 



oxidation of iron ijientioned above are equally observable in tlie 



gallates and prussiates of this metal. 

 Gallates of The gall ate of whrte iron, which maybe obtained by decom- 



"°'^- posing the deep green sulphate of iron, is itself colourless : the 



gallate of green iron is blue : the gallate of red iron is black. 



They may a^lso be obtained by decomposing the acidulous or 



acid sulphate of n;reen or red iron by means of the gallic acid. 



Prusisates still The combinations of iron with the prussic acid exhibit much 



more nume- j^ore numerous varieties, which depend not only on the differ- 



rous, ' •' 



owing to the ent oxides of iron already mentioned,, but on the greater or- 



presence of smaller quantity of acid, and on the presence of the prussiate 



pruBsiate of n ^ ' r i 



potash. of potash that may remain combined with the prussiate of iron, 



Prussiate of The prussrate of white iron is that in which the iron; is in the 



white iron. state of white oxide, and the oxide is in excess, owing to the 



excess of alkali contained in the prussiate ofpotash. 



Green prussi- The prussiate of green iron is the same prussiate as the pre- 



ateofiron. exuding without excess of oxide.* They both contain too, as 



-^ M. Berthollet has shown, prussiate of pc?tash, which is strongly 



~ adherent to them. 



Prus^iates of From the ferruginous salts with bases of green and red oxides 



froii" ^" ^^ of iron, two prussiates each are equally obtainable, one with, 



and the other without excess of oxide. The prussiates obtained 



with the green oxide are blueish ; those with the red oxide are of 



a fine blue. The six prussiates above mentioned, are capable 



of exhibiting farther varieties in consequence of the prussia,te of 



potash they may contain. 



Improvements M. Thenard concludes his memoir with proposing some 



in manafactur- ^leans ol improving the fabrication of prussiate of iron, or 

 ing Prussian- i & i > 



blue. prussian-blue. These consist, 1st, in turning to account the 



great quantity of ammonia formed by the calcination: 2d, 

 in employing the most advantageous proportion of potash, 

 which appears to consist in equal parts of blood and alkali : 

 3d, in adding iron during the evaporation, which facilitates the 

 formation of the pinissiate of potasli : 4th, in causing the prussi- 

 ate of potash to crystallize. 



* Either M. Thenard here departs from the principles of his 

 nomenclature, and means to call this the green prussiate of iron; or 

 the abridger of his memoir has fallen into some mistake. T. 



VII. 



