No. VII.] APPENDIX. 179 



iodine, in order that the ether may part with any iodine previously to the 

 point of saturation. When evaporated to dryness, more of the iodine is 

 evolved, but still hydriodate of potash may be abstracted from the mass by 

 alcohol. When all the iodine is removed from the mass, a result which is 

 known by its not discolouring starch upon the addition of nitric acid, it 

 still retains its power of forming Prussian blue with salts of either oxide 

 of iron, and still presents the same indisposition to crystallize, for it 

 neither shows itself as the yellow nor the red ferrocyanate of potash, but 

 as a compound having properties intermediate with both. 



When iodide of potassium is added to the ferrosesquicyanuret, iodine 

 is evolved, the solution loses its red colour, and the salt possesses the 

 characters similar to the mass obtained by the action of iodine on the 

 ferrocyanate of potash. Thus it is evident that if a solution of persulphate 

 of iron be treated with the red ferrocyanate whilst an iodide is present, 

 Prussian blue will be formed. 



Whether this is really a mixture of the fcrrocyanuret and ferro- 

 sesquicyanuret or a distinct compound, it is difficult to determine, but 

 the latter is rendered probable from its generally presenting itself as an 

 amorphous mass; yet, however, when the purified mixture is dissolved 

 two or three times in water, a dark mass is deposited, and at last crystals 

 of the yellow salt are formed. 



Every method which has been discovered of converting the ferro- 

 cyanate of potassa into the ferrosesquicyanuret has now been detailed, 

 and we have seen that they may each be referred to the class of anions, 

 for of the cathions the powerful agency of potassium was unable to effect 

 this change. 



Upon the first formation of the ferrosesquicyanuret the colour will 

 occasionally be a very dark red, but this is an adventitious, not a necessary 

 property ; for when prepared by peroxide of manganese or chloride of soda, 

 it does not possess this dark colour. If the red crystals be carefully 

 picked and re-dissolved, in no instance is this seen, and in every case where 

 the dark red exists it yields to liquor ammonia or potassse, with the pro- 

 duction of a small quantity of the ferrocyanate. 



The ferrosesquicyanuret, however prepared, has the same peculiar 

 properties. It has been already mentioned that the protosalts are preci- 

 pitated blue, whilst the persalts are not effected by this agent ; however, 

 the solution in the latter case is always much darkened, and after a time 

 a small quantity of dark-coloured substance is deposited. The mode of 

 preparation of the ferrosesquicyanuret does not influence this result. 



With almost every acid, especially if heat be applied, Prussian blue is 

 formed and hydrocyanic acid is given off ; and thus upon testing for minute 

 quantities of metal, care must be taken to prevent any excess of acid, as in 

 that case the chemist would find iron in everything he examines. With 

 excess of alkali, on the contrary, no precipitate of Prussian blue is pro- 

 duced ; and therefore if search be made for that most useful of all metals, 

 the experiment would declare that iron had no real existence : but if the 

 golden mean be employed, or the solution be but very slightly acid, the 

 ferrosesquicyamiret, as well as the ferrocyanuret, become most valuable 

 and delicate tests, the one for the peroxide, the other for the protoxide of 

 that metal. 



N 2 



