980 IRON 



and alkaline earths, arc occasionally found in crude iron, but very rarely in quantities 

 that can at all affect the qualities of the product. The elements, the quantitative 

 estimation of which has been given in the above analyses, do, however, materially 

 modify the physical qualities of cast iron. We shall, therefore, offer a few obser- 

 vations on each. 



Carbon. Iron can take up any quantity of carbon up to a little over 5 per cent., 

 at which point it becomes saturated : the compound thus formed is the white crystal- 

 line pig or specular iron (i) (r) (s) (t) ; when absolutely pure, its composition is 94*88 

 iron, and 6*12 carbon: it is a tetra-carburet, Fe'C. The most highly carburettod iron 

 which Faraday and Stodart could produce consisted of 92*36 iron, 5*64 carbon. There 

 seems no reason for admitting, as some metallurgists have done, the existence of a 

 polycarburet of iron, containing 18*3 per cent, of carbon, inasmuch as iron containing 

 under 6 per. cent, appears to be completely saturated. The specific gravity of pure 

 tetra-carburet of iron is 7*66 ; it is the most fusible of all the carburets of iron, its 

 melting-point being 1600 Cent. ; it is brittle and silver-white, and crystallises in 

 oblique prisms, which are frequently tabular. According to Gurlt the carburet of 

 iron existing in grey pig is the octo-carburet, Fe 8 C, the crystals of which belong to 

 the regular or cubic system, but almost always appears in grey iron in the form of 

 confused octohedral groups. The specific gravity of pure octo-carburet of iron, ac- 

 cording to the same authority, is 7'15, and its composition 97'33 iron, and 2*63 

 carbon ; its colour is iron-grey, its hardness is inferior, and its fusibility less than 

 that of specular iron ; the groups of crystals often found in cavities in largo castings 

 are composed of this peculiar carburet. Gurlt very ingeniously endeavours to show 

 that in grey pig-iron the carbon of the octo-carburet is partially replaced by silicon, 

 sulphur, and phosphorus, and the iron by manganese and other metals. In like 

 manner the carbon of the tetra-carburet may be partially replaced by silicon, phos- 

 phorus, or sulphur, the eliminated carbon appearing in the form of graphite : the 

 same decomposition is effected by heat, and specular iron, if exposed to a temperature 

 considerably above its fusing-point, becomes grey ; if cooled slowly, the graphite 

 separates in large flakes, if rapidly, in minute particles. Some metallurgists suppose 

 that in grey cast iron a portion only of the iron is chemically united with carbon, 

 the rest of the metal being dissolved in the carburetted compound in the form of 

 malleable iron. Dr. Noad inclines to the opinion of Gurlt, that the whole mass of 

 the iron is in a state of combination with the electro-negative constituents, such as 

 carbon, sulphur, phosphorus, and silicon. Thus in the white pig-iron of heavy 

 burden (u), there is a deficiency of carbon, that element being replaced by sulphur 

 and phosphorus. 



Karsten gives as the mean of several analyses, 3*5865 per cent, as the quantity of 

 carbon in cast iron smelted with charcoal from spathic ore. He states that iron 

 containing as little as 2*3 per cent, of carbon still retains the properties of cast iron, 

 particularly the faculty of separating graphite when allowed to cool slowly. With 

 2 per cent, of carbon iron is not forgeable, and scarcely so if it contain only 1*9 per 

 cent. With this quantity of carbon it is steel, though not of the weldablo land (cast 

 steel) ; even with so small a proportion of carbon as 1*75 per cent, it is weldable only 

 in a slight degree ; the latter property increases as the hardness of the iron decrcax s. 

 An amount of from 1*4 to 1*5 per cent, of carbon in iron denotes the maximum of both 

 hardness and strength. Iron containing 0'5 per cent, of carbon is a very soft steel, 

 and forms the boundary between the steel (i.e. iron which may yet be hardened) and 

 malleable or bar-iron. These limits lie perceptibly higher if the iron be pure ; and 

 lower if it contain silicon, sulphur, and phosphorus. 



The composition of the various carbides of iron, according to Berthior, is as under : 



FeC 3 FeC" FeC Fe'C Fn'C Fe n C 



Iron . . 0-600 0*690 0-819 0'899 0*947 0-9643 

 Carbon . 0*400 0*310 0*183 0*101 0*053 0-0357 



In the blast-furnace, the reduced iron may take up carbon in two different ways : 

 1. By immediate contact with the incandescent fuel ; and 2. By taking carbon from 

 carbonic oxide ; thus Fe + 2CO = FeC -f CO 2 . That iron decomposes carbonic oxide 

 is considered by Le Play and Laurent to bo proved by the following experiment : 

 pure oxide of iron and charcoal were heated in two separate porcelain boats, plarnl in 

 a glass tube ; the air in the tube furnished oxygen to the carbon ; carbonic oxidr was 

 formed, which was converted into carbonic acid, at the expense of the oxygen of the 

 oxide of iron ; the carbonic acid was again transformed into carbonic oxide, by taking 

 Tip a fresh quantity of carbon, which was again converted into carbonic oxido l>y 

 taking oxygen from the oxide of iron, and this went on until the whole of the oxide 

 of iron was reduced; the metallic iron then decomposed carbonic oxide, producing 

 carbonic acid and carbide of iron : and this went on till a certain quantity of carbon 



