IRON RUST. 313 



off "even at the common temperature of the air, but the 

 oxyd will then be in the form of a black powder, and will 

 not exhibit that glossy appearance which it does in the former 

 method," i.e., when steam is passed over red hot iron. "The 

 preparation called Martial iEthiops is iron oxidated by water, 

 and was first made by Lemery the younger," by covering a 

 quantity of clean iron filings with water and stirring from 

 time to time. " After a while bubbles of hydrogen con- 

 stantly rise from the mass, and the vessel becomes full of a 

 very fine black powder. This black powder is iron in the 

 first or lowest state of oxydation or sub-oxyd ; it is strongly 

 magnetic, but has a constant tendency to absorb an addi- 

 tional quantity of oxygen from the air or any other substance 

 by which it loses its magnetic property, changes from black 

 to yellow or red, and acquires very different chemical 

 characters." 



Further, " Von Mons, having prepared a quantity of 

 this black sub-oxyd, heated it in a retort to drive off the 

 superfluous moisture, but on taking it out " a spontaneous 

 motion or heaving took place through the mass ; it became 

 so hot as to burn a thick double paper in contact with it, and 

 the whole in a few minutes was converted into the red or 

 perfect oxyd or saffron of Mars " 



Leopold Gmelin, in his " Handbook of Chemistry," vol. v., 

 p. 185, Cavendish Society, 1851, says : — "Iron covered by a 

 thin layer of water and exposed to the air is converted into 

 the hydrated sesquioxide ; ammonia is also formed. 



" If the iron be covered by a deeper stratum of water so 

 that the transference of the oxygen of the air through the 

 water to the iron may take place more slowly, a formation of 

 black hydrated flerroso-ferric oxide takes place, because 

 hydrated ferric oxide, as it slowly forms, induces the iron to 

 decompose the water and form ferrous oxide, with which the 

 ferric oxide then unites. — (Wohler.) 



p. 186: "Beneath the flocculent hydrated sesquioxide of 

 iron there is formed a thin black crust (of ferroso-ferric 

 oxide?) which adheres firmly to the iron. 



" When cast iron of this description (i.e., mixed grey and 

 white) is exposed to the action of a mixture of 75 measures 

 of aerated water and one measure of a saturated solution of 

 chloride of sodium and carbonate of sodium, oxidation begins 

 in a minute, and thus is formed — first, whitish hydrated 

 ferrous oxide and hydrated ferroso-ferric oxide, which at some 



