■' Chemical Science. 181 



blue colour, and insoluble. When heated in close vessels, the 

 prussic acid is given off as before, perfectly pure, and no other 

 eflect takes place, if the temperature be below that of boilin<j 

 mercury. The residue is yellowish brown, but becoming nearly 

 black in the air, it contains ammonia, and the iron is in such a 

 state of combination, that it is not affected either by sulphuric 

 acid or the magnet. If this residuum be heated still higher^ 

 then prussic acid in small quantities, and hydrogen and azote, 

 in the proportion of one to two, come oft, and charcoal and 

 metallic iron remain. No carbonic acid is found in this expe- 

 riment; hence the | iron is in the metallic state in the acid. M, 

 Robiquet concludes from this experiment, that the peculiar 

 acid is a combination of prussic acid and cyanuret of iron, 

 formed, by affinities, so powerful, that the poisonous properties 

 of the prussic acid are entirely neutralized and lost. 



" It results (says M. Robiquet,) from what has been said, 1. 

 That potash is an essential element in the white prussiate of 

 iron. 2. That the triple protoprussiate of iron is slightly so- 

 luble in water, capable of being crystallized, and of a yellow 

 colour. 3, That the acid of Prussian blue, and of triple prus- 

 siate in general, is a combination of iron, cyanogen, and prussic 

 acid. 4. That Prussian blue, and the triple prussiates in ge- 

 neral, are formed of a cyanuret and a hydrocyanate. 5. That 

 it is probable that Prussian blue owes its colour to a certain 

 quantity of water. — Annates de Chimie, xii. p. 277. 



3. Refractive Powers of Muriatic Acid. — Mr. H. Creighton 

 whilst making experiments on the formation of compound lenses, 

 to correct abberration, observed a remarkable law in the re- 

 fractive power of muriatic acid of different density, the refrac- 

 tive power being nearly as the density of the acid employed. 

 The table below exhibits the focal distances with acids of dif- 

 ferent strengths. The two double convex lenses were of crown 

 glass, the focus of one about 24, and of the other 27 inches ; 

 when placed together, the focus was about 13 inches, and when 

 the space between them was filled with water and acid, the 

 focal distances were as in the second column of figures ; the 



