178 APPENDIX. [No. VII. 



throw down the phosphate, and a drop of dilute sulphuric acid may then 

 be added to remove any excess of baryta. 



The solution will now be found not to precipitate persalts of iron, 

 but, on the contrary, a large quantity of Prussian blue is produced with 

 the protosalts. The actual combustion of the phosphorus seems essential 

 to this change ; for if the water in which phosphorus has been allowed to 

 burn be added to the solution of the ferrocyanate, a similar change will 

 not be produced. The cause of this change appears paradoxical, for 

 phosphorus has in other instances a deoxidizing agency, so that a piece 

 placed in a solution of either gold, silver, platinum, or copper, has the 

 metal precipitated upon it. Perhaps it depends upon decomposition of 

 water and the formation of phosphuretted hydrogen ; for a narrow bottle, 

 to which air has but limited access, is more favourable to the change than 

 a wide vessel. If this explanation is correct, the action of phosphorus 

 must be classed with the other oxygenating substances, for oxygen, and 

 not phosphorus, removes the potassium.* 



No mode of abstracting the half equivalent of potassium by sulphur 

 is known ; for if half an equivalent of sulphur be heated with powdered 

 ferrocyauuret, the ferrosesquicyanuret is not produced, and the alcoholic 

 or terebinthine solution of sulphur, added to a solution of the ferro- 

 cyanuret, also failed to produce this change. Even nascent sulphur arising 

 from the decomposition of sulphured of potash by an acid did not produce 

 any effect .f 



A current of cyanogen gas passed through a solution of the salt is 

 gradually absorbed, and it becomes of a very dark colour, but red ferro- 

 cyanate is not formed. 



Doubtless many may be surprised that the action of iodine has not 

 been adverted to before, and more especially that it should not have been 

 mentioned with chlorine and bromine, as to these it has a striking analogy 

 in most of its properties ; but in reality little resemblance exists between 

 the action of iodine on the ferrocyanate of potassa, and that of chlorine 

 and bromine, as we shall immediately see. If iodine is added to a solution 

 of the salt, it speedily becomes dissolved, the solution turning to a dark 

 red, and gives a blue precipitate with salts of either oxide of iron. One 

 equivalent of ferrocyanate of potash dissolves about one equivalent of 

 iodine, which remains in great part uncombined in solution. If the 

 solution is allowed spontaneously to evaporate, the free iodine passes off, 

 and a whitish uncrystallized mass is obtained which has no free iodine, 

 but hydriodate of potassa in its composition. This gives a precipitate 

 with both oxides of iron. Now there is a ready method of ascertaining 

 how much iodine the ferrocyanate will not only dissolve, but combine 

 with, and for this purpose a definite quantity of the salt is to be dissolved 

 in a small quantity of water, and then placed in a phial. Upon the 

 solution ether is to be poured, then the iodine is to be added gradually, 

 when as soon as the ether is discoloured the saturation is known to be 

 effected. Brisk and continued agitation must follow each addition of the 



* No change takes place if the phosphorus is completely under the solution 

 of the salt. 



f It is foreign to this paper to describe the sulphocyanuret of potassium. 



