.150 POTASSIUM. 



so much as two equivalents of iodine, but allows one equivalent 

 to precipitate when diluted. Iodide of potassium is much used 

 in medicine; it is not poisonous in doses of one or two 

 drachms. Its solution is also employed as a vehicle for iodine 

 itself, 20 grains of iodine, and 30 grains of iodide of potassium 

 being usually dissolved in 1 ounce of water. The bromide of 

 potassium is capable of dissolving bromine, but the solution of 

 chloride of potassium has no affinity for chlorine. 



Ferrocyanide of potassium, Yellow prussiate of potash; 

 K 2 ,FeCy 3 + 3HO; 2303.7 +337.5 or 185 +27- This important 

 salt is formed when carbonate of potash is fused in an iron pot 

 with animal matter, such as dried blood, hoofs, clippings of 

 hides, &c., and is the product of a reaction to be hereafter de- 

 scribed. This salt occurs in a state of great purity in com- 

 merce. It is of a lemon yellow colour, and crystallized in large 

 quadrangular tables, with truncated angles and edges, belonging 

 to the square prismatic system. The crystals contain 3 equiva- 

 lents of water, which they lose at 2 1 2, are soluble in 4 parts 

 of cold and 2 parts of boiling water, and are insoluble in alco- 

 hol. The taste of this salt is saline, and it is not poisonous. 

 By a red heat it is converted, with escape of nitrogen gas, into 

 carburet of iron and cyanide of potassium ; but with exposure 

 to air the latter salt absorbs oxygen, and becomes cyanate of 

 potash. This salt is represented by Liebig as containing a salt- 

 radical, Ferrocyanogen, composed of 1 eq. of iron and 3 eq. of 

 cyanogen, or Fe Cy 3 . This imaginary radical is bibasic, and is 

 in combination with 2 eq. potassium in the salt, as will be 

 seen by reference to its formula. The same salt has been re- 

 presented by myself as a compound of a tribasic salt-radical 

 prussine (3 Cy) with Fe-f 2 K. But its reactions with other 

 salts are most easily stated on the former view of its constitu- 

 tion. The iron in this salt is not precipitated by alkalies. 

 When ferrocyanide of potassium is added to salts of lead and 

 various other metallic solutions, it produces precipitates, in 

 which two atoms of the lead or other metal are substituted, in 

 combination with ferrocyanogen, for the two atoms of potas- 

 sium. In salts of peroxide of iron, ferrocyanide of potassium 

 produces the well known precipitate prussian blue. 



Ferricyanide of potassium, Red prussiate of potash ; 

 3K, Fe 2 Cy 6 ; 4127.6 or 331.74. -This salt, which like the last, 

 is a valuable reagent, is formed by transmitting chlorine gas 



