IRON. 54.) 



in most minerals, and as sulphuret, oxide and carbonate in 

 quantities which afford an inexhaustible supply of the metal 

 and its preparations, for economical purposes. 



Iron differs from any other metal in two points, which 



if the external oxidating action is supported for a sufficient length of time. It 

 is very obvious, from the phenomena of cementation, which are exhibited by 

 a great variety of solid bodies besides iron, that a particle of carbon, when in 

 combination with a particle of iron, may still attract and be attracted by the 

 surrounding particles of that metal, and thus exercise an influence at a distance. 

 The action of chemical affinity described in the preceding cases, as in direc- 

 tion rectilineal, may very readily assume a circular direction or return upon 

 itself. Thus, if two particles of hydrochloric acid, A and B, be disposed to- 

 wards each other, with their unlike atoms together, as in 

 figure 79, it is obvious that, by an inconceivably minute 

 expenditure of force, the h of A may be made to unite 

 with the cl of B, while the h of B combines, at the same 

 time, with the cl of A, or the combinations take place 

 indicated by the brackets, and the two new molecules of 

 hydrochloric acid C and D are produced. It is impossible 

 to prove the occurrence of such a decomposition in 

 molecules of the same kind, but we have it constantly 

 illustrated in double decompositions where the molecules are different as in 

 hydrochloric acid and cyanide of silver, when the new products, hydrocyanic 

 acid and chloride of silver are formed, and demonstrate its occurrence by a 

 sensible change. Now, instead of a pair of molecules of hydrochloric acid, 

 we may have a circle composed of any number thus in contact, and under- 

 FlG. 80. going decomposition, as in the figure. For 



when the affinity of the cl of any acid mole- 

 cule A(fig. 80) is engaged by hot the adjoining 

 molecule B, to its left, the h of A is propor- 

 tionally relieved from the affinity of its own 

 cl. The h of A is thus free to act upon the 

 cl of the acid molecule C to its right ; and 

 the relieved hydrogen of that upon the 

 chlorine of a third molecule to the right, 

 and so on round the circle, as indicated by 

 the brackets. When this action reaches B, 

 the h of that molecule is thereby relieved 

 from the attraction of its own cl, and on that account can the more readily 

 combine with the cl of A. 



We pass at once from this to the voltaic circle, by supposing that part of 

 these molecules are acid (A), part zinc (B), and part copper (C), but all 

 having the same binary or saline organization, and symmetrically placed in 

 regard to each other. This, which I previously described as the inductive ac- 

 tion of affinity from its analogy to magnetic induction, I now think may, with 

 more propriety, be distinguished as the roial action of affinity, and founded 

 upon as a fundamental law of chemical affinity. Other applications will bo 

 found, for the molecular theory which it involves, in the sequel. 



N N 2 



