544 IRON. 



which accompanies the Uralian platinum, and a thin vein about 

 two inches in thickness, observed in chlorite slate, near Canaan 

 in the United States. In a state of combination iron is exten- 

 sively diffused, being found in small quantity in the soil, and 



characteristic attractive power of hydrogen, or of any other basyle, is a con- 

 stant quantity. When the hydrogen is in combination with chlorine, as hy- 

 drochloric acid, that affinity is entirely engrossed by the chlorine. The che- 

 mical affinity of the chlorine, on the other hand, which is also a constant 

 quantity, is then entirely engrossed by the hydrogen. But if an atom of zinc 

 Zn, be brought in contact with a particular molecule of hydrochloric acLl 

 Cl + H, then a portion of the affinity of Cl is engaged by Zn, and diverte 1 

 from H, which is proportionally relieved from that affinity. The unoccupied 

 affinity of H can act upon the Ci> of an adjoining particle of hydrochloric 

 acid ; of which the H', in so far as it is relieved from its own Cl', can attract 

 the Cl" of a third particle of hydrochloric acid, and the hydrogen H", of this 

 third, the chlorine of a fourth, and thus an action be propagated in a recti- 

 linear direction through the acid to a considerable distance from Zn, where it 

 originated. The unoccupied affinity of the first H, instead of acting upon 

 a single line of particles of hydrochloric acid, as above supposed, may be di- 

 vided among several lines of particles ; these lines will radiate from a common 

 centre Zn, being mutually repulsive of each other, for the same reason as the 

 threads of iron filings attached to the pole of a magnet are so, (page 204). As 

 the number of lines and of particles of acid affected at any particular distance 

 from Zn, will increase with that distance, the action upon any one particle will 

 necessarily diminish with its distance from the disturbing centre Zn, indeed it 

 will be in the inverse ratio of the square of the particle's distance from Zn. 



A class of phenomena depending immediately upon the propagation of che- 

 mical affinity to a distance are those of cementation. When a compact mass 

 of pure iron (a bar of the metal) is exposed to carbonic oxide gas, at a red 

 heat, the superficial particles of iron decompose that gas, by the exertion of a 

 zincous affinity, taking carbon from the oxygen with which it is united, and 

 becoming carburet of iron. But if exposure to the carbonic oxide be conti- 

 nued, the combined carbon does not remain at the surface of the iron, but 

 travels inwards, diffusing itself through the metallic mass. It thus appears 

 that when the iron Fe, of the superficial carburet, which we may represent by 

 Fe + C is in contact with a second atom of carbon C', it attracts C', and C 

 being proportionally relieved from the affinity of Fe, may att upon the ad- 

 joining and interior atom of iron, Fe', and indeed combine with it, while the 

 external atom Fe combines at the same time with C/. The original atom of 

 carbon C may thus combine in succession with a series of atoms of iron, Fe, 

 Fe', Fe'', &c., extending into the interior of the metallic mass, provided 

 always that carbon be constantly supplied to the external atom of iron Fe. 

 Again, the steel may be decarbonised, by exposing it to a source of oxygen, as 

 by heating it in contact with oxide of iron, when the converse of what has 

 been described occurs. The superficial particles of iron being deprived of their 

 carbon, the balance of the attractive forces soliciting that element is turned, 

 and it now travels in an outward direction, and abandons the iron entirely, 



