and pnrtlcularhj the Oxidation of Iron. 235 



same time with Prussiaa blue, and consequently double the 

 products wiihout much augmenting the expense of the 

 manufacturer. The manufacturers of Prussian blue ought 

 not only to direct their attention to this point, but also to 

 several others which might, improve their difierent processes. 

 Some do not employ the most advantageous proportions of 

 potaih and blood, some do not add iron, and several main- 

 tain the heat too long : only a very few of them take the 

 trouble of crysialUzing the prussiate of potash ; many of 

 them do not know that in the calcination of substances the 

 sulphate of potash contained in the potash made use of is 

 transformed into sulphurets, and for a much stronger reason 

 they are ignorant of the means of preventing the bad effects 

 of it. In short, almost the whole of them consecrate too 

 much time in oxygenating Prussian blue; by that means 

 augment the manual labour, and deprive themselves of the 

 power of making more considerable quantities of it. The 

 prosperity of their manufactories depends, however, on all 

 these considerations united. 



The following is what I have remarked on this point : 

 Experience has proved to me, that a quantity of potash 

 equal to that of blood was preferable to every other propor- 

 tion ; we know that the addition of iron favours the forma- 

 tion of prussiate of potash, and fixes it. It is not less evi- 

 dent that the effect of fire is also as important to observe, 

 because, bv exceeding the point of fusion, we decompose 

 the prussiate of potash, or the matter which ought to form it. 

 It is equally necessary to crystallize the prussiate of potash, 

 particularly when we wish to obtain Prussian blue of a su- 

 perior quality. On the one hand, we thus transform the 

 sulphurated hydro-sulphuret of potash into sulphate of pot- 

 ash ; and, on the other hand, it is only necessary to add 

 very little alum in order to saturate the excess of alkali : we 

 might again decompose the sulphurated hydro-sulphuret of 

 potash by means of the sulphuric acid, because the acid docs 

 not alter the prussiate of potash combined with a certain 

 quantity of prussiate of iron. This crystallizaticm is not 

 indispensable in the fabrication of common Prussian blue, 

 into which much alumine enters, which cannot be separated 



but 



