232 On the Oxidation of Metals in general, 



•with the acidulated and acid sulphate of green iron it forma 

 a precipitate of a line blue ; and by repeating the experiment 

 with a highly oxygenated salt of iron, the matter deposited 

 is of a black inclining to grav. An excess of acid, however, 

 hinders the deposition from taking place. This is what we 

 remark in the acidulated sulphate of white iron, where the 

 addition of an alkali is indisp-msablc for the formation of the 

 gallate which then presents itself mider the form of violet- 

 coloured flakes J this is also what is observed in red acid sul- 

 phate of iron, where the saturation of the acid is necessary 

 for the precipitation of the composition ; this, on the con- 

 trary, does not take place in the red muriate of iron which 

 is only a little acid. 



The three oxides of iron of Vv'hich we have been speaking, 

 and all of which may be combined with acids, form com- 

 binations with the prussic acid much more multiplied than 

 those we are about to examine. Not only do prussiates of 

 iron exist, neutral and with an excess of oxide, but both the 

 one and the other are susceptible of uniting with the prus- 

 siate of potash and forming triple insoluble salts, if the me- 

 tallic prussiate predominates ; and, on the contrary, it will 

 form soluble salts when there is almost nothing in it but 

 alkaline prussiate. Such, in ^tw words, is the general his- 

 tory of Prussian blue ; but it is of too much importance 

 to science not to consider particularly every one of its com- 

 ponent parts. 



One department of this history, the most useful to studyj, 

 is the difference of colour in the precipitates obtained by de- 

 composing solutions of iron by the alkaline prussiates : the 

 shades of these precipitates are singularly various. Some- 

 times they are white, and sonietimes greenish; but oftenest 

 they are more or less blue, and the eye accustomed to judge 

 pf colours recognises many different shades in these prin- 

 cipal colours. These effects do not depend entirely upon the 

 state of the oxidation of the iron ; they depend also upon 

 the state of the alkaline prussiate and the metallic solution. 



If the iron is little oxidated, the solutiori but little acid, 

 and the prussiate has an excess of alkali, a white precipitate 

 will be obtained J it will be greenish white, if, while the 



other 



