ON PRUSSIAN BLUE 281 



sanguinis does, (d) Magnesia, precipitated from Epsom salt 

 by caustic volatile alkali, was found dissolved in the 

 colouring matter, after they had been allowed to stand 

 together for several days in a well-closed bottle. When this 

 solution was exposed to the open air, the magnesia separated 

 from it, on account of its greater elective attraction for the 

 aerial acid, and formed on the surface of the water a pellicle 

 like that of cream of tartar. This solution was likewise 

 decomposed by alkalies and by lime-water, (e) This matter 

 dissolves but a very small quantity of terra ponderosa (barotes 

 of Mr. Morveau), which may be afterwards precipitated by 

 vitriolic acid, and even by aerial acid. (/) Pure clay, or the 

 basis of alum, is not attacked by it. (g) Lime is dissolved in 

 a certain quantity by this matter. The superabundant 

 portion should be separated by means of the filter ; and as 

 the liquor contains, besides the combined lime, the portion 

 which water itself is able to take up, in order to free it from 

 this, precisely the same quantity of water, impregnated with 

 aerial acid, is to be added as is requisite for precipitating an 

 equal quantity of lime-water. The colouring matter, thus 

 saturated with lime, is to be filtered again, and then to be 

 preserved in a well-closed bottle, in order to prevent the 

 access of the aerial acid. All acids, even the aerial, and the 

 fixed caustic or pure alkalies, decompose this solution. More- 

 over, if it be distilled to dryness, a portion of the colouring 

 matter is disengaged at the very beginning of the distillation 

 from the lime ; and there remains at the end of the 

 distillation nothing behind but pure lime. As it appeared to 

 me that the colouring matter was, if I may say so, completely 

 neutralised by the lime, in the manner just mentioned 

 (though this solution, as well as that of magnesia arid terra 

 ponderosa, still restores the blue colour of turnsol which has 

 been turned red), I resolved to employ it, in preference, for 



