242 ESSAY XXII. 



alkali. If afterwards the same paper be pencilled over with 

 muriatic acid, it grows blue. If the whole liquor of the 

 receiver be exposed to the open air, it all evaporates, leaving 

 behind* pure water. 



SECTION VIII. 



Among several other fruitless attempts to separate this 

 colouring matter from the Prussian blue in a more com- 

 modious way, I found that calcined quicksilver afforded 

 an excellent medium for accomplishing this purpose. I 

 observed that mercurius dulcis grew black in air impregnated 

 with this colouring matter. The same thing likewise 

 happened if it was put into the tinging water (Sec. iv. (c)) ; 

 and this water thence acquired an acid quicksilver taste. 

 Hence it was probable that this matter had united with the 

 sublimate in the mercurius dulcis, or at least with the calx, 

 which is its basis, and expelled the muriatic acid, which 

 would give rise to the acid taste ; and as quicksilver in its 

 metallic state is not soluble in muriatic acid, it must there- 

 fore separate from it; and this is the cause of its black 

 colour. I therefore boiled the liquor with corrosive subli- 

 mate, and the mixture hardly began to boil, when the blue 

 colour had already disappeared. I then boiled calcined 

 mercury or red precipitate with Prussian blue and water. 

 The colour disappeared entirely, and the filtered solution had 

 a strong taste of quicksilver ; it contained no iron, and, 

 what is remarkable, it could not be precipitated either by 

 acids, lime, or alkalies ; but, by a long digestion with metals, 

 the mercury was reduced to its metallic state by means of 

 a double elective attraction. If the union of the colouring 

 matter with metals be prevented, it becomes free and un- 

 combiiied, and may be easily separated by distillation. This 



