Composition of an Acid Oxide of Iron. 219 



suspension. The German chemists prefer deflagrating dry 

 nitrate of potash with half its weight of iron-filings at a tem- 

 perature approaching to visible redness, and also direct the 

 employment of chlorine gas, potash and precipitated oxide 

 of iron, as mentioned by M. Fremy, to procure it. M. Pog- 

 gendorff found that when a current from Grove's battery is 

 passed through a solution of one part of hydrate of potash in 

 four of water, using cast iron for the positive pole plunged in 

 the potash, and wrought iron or any other convenient metal 

 as the negative pole, ferrate of potash was formed in the so- 

 lution, which immediately becomes dark red and opake. 

 Wrought iron and steel do not produce this compound, but 

 evolve oxygen gas ; the red solution soon decomposes either 

 with or without the continuance of the passage of the electri- 

 cal current, oxygen gas being liberated and sesquioxide of 

 iron precipitated. 



To prepare this compound I have found the subjoined process 

 attended with very uniform success : — Wash the ferri sesqui- 

 oxydum of the shops with boiling water until free from sul- 

 phate of soda, dry and ignite at a low red heat; this fur- 

 nishes a very pure oxide of iron and in a state of minute 

 division ; one part of this is to be intimately mixed with four 

 of dried nitre, reduced to fine powder; place this mixture 

 in a crucible of about twice the capacity of the bulk of the 

 mixture, lute a well-fitting cover on, making a few small 

 holes in the lute to allow the escape of gas, and ignite at a 

 full red heat for about an hour, if six or eight ounces are 

 made: the time of ignition depends much on the quantity 

 prepared, and the temperature should never be raised above 

 a full red. When well prepared it presents the appearance 

 of a dark reddish-brown porous mass, rapidly deliquescing 

 on exposure to the air, so that I have found it advantageous 

 to powder it whilst still warm, when it may be preserved for 

 use in a well-stoppered bottle, apparently for any length of 

 time. 



To examine the solution of this substance it is most con- 

 venient to employ ice-cold water, as much heat is evolved 

 when it is thrown into water, and the decomposition of the 

 solution is augmented or retarded by the elevation or depres- 

 sion of temperature. On the addition of water it evolves much 

 oxygen gas with effervescence, probably owing to the decom- 

 position of peroxide of potassium ; if the water employed be 

 ice-cold and the vessel containing it plunged in ice, the de- 

 composition of the solution will be retarded. By allowing the 

 mixture to subside for a few minutes, a solution is obtained 

 almost free from oxide of iron in suspension, of so deep an 



