ON PRUSSIAN BLUE 239 



iron, and of alkali, and is especially made use of for discover- 

 ing iron in mineral waters; but is not entirely to be de- 

 pended upon for this purpose, as long as chemistry is unable 

 to free it perfectly from iron ; and this cannot be effected 

 without decomposing this salt, of which the iron is constantly 

 a constituent part, and is the medium by which the colour- 

 ing matter is attached to the alkali (Sec. IT. (c)). The 

 lixivium sanguinis is more to be depended upon for this 

 purpose, though it likewise, as well as the lixivium of soot, 

 shows marks of the presence of iron. Though I have 

 mentioned (Sec. I.) that on supersaturating lixivium san- 

 guinis with an acid, and then adding vitriolic acid, no sign 

 of Prussian blue is perceived; but as blood and soot contain 

 a little iron, there is no reason to be surprised that lixivium 

 sanguinis should contain iron ; whence it happens that, in 

 such cases, there may really appear some mark of Prussian 

 blue, and this the more readily if the calcined mass be 

 boiled in an iron vessel. But if the vitriol be first put into 

 the lixivium, and then an acid be added to it, a great differ- 

 ence is found with regard to the quantity of Prussian blue 

 obtained. If a perfectly pure lixivium sanguinis be taken, 

 supersaturated with an acid, and vitriolated iron be added 

 to it, not the least mark of blue appears. Such a pure 

 lixivium is not obtained in the common way, but on mixing 

 the colouring matter in its purest state (Sec. x.) with alkali 

 of tartar. 



SECTION VI. 



To return to the above-mentioned neutral salt, I dis- 

 solved 1 oz. of it in a glass retort in 4 oz. of water, adding 

 3 clrms. of concentrated vitriolic acid (other acids produce 

 the same effect, but I prefer the vitriolic acid in this process), 

 and distilled this mixture into a luted receiver by a gentle 



