IRON. 543 



widely although thinly scattered over the earth's surface, and 

 probably first attracted the attention of mankind to this metal. 

 Of the occurrence of metallic iron as a terrestrial mineral in 

 situ, the best established instances are the species of native iron 



mony and arsenic is equivalent to three magnesian atoms, and yet it is the 

 least combining proportion of the elements enumerated. This view, which 

 was always probable, seems now rendered necessary by the observation of 

 MM. Liebig and Dumas, that in the potash-tartrate of antimony strongly dried, 

 T eq. of antimony replaces 3 eq. of hydrogen. Yet the elements of this triple 

 molecule are not separable, fn their individual action, however, we appear 

 to have the cause of the singular tendency of the members of the phosphorus 

 family to combine with three equivalents of other bodies, as with 3H, 3O, 

 3Ni, 3Cu, 3Co, 3Hg, &c., and of the tribasic character of phosphoric, arsenic, 

 and phosphorous acids. These elements, then, have an indissoluble sali- 

 molecule. Metallic antimony also is isomorphous with tellurium, and con- 

 nected, therefore, through sulphur, with the magnesian family. 



5. Of the formation of molecular groups of atoms of the same element, 

 apparently united by chemical affinity, it would not be difficult to multiply 

 instances. Thus the atoms of sulphur appear to be associated in a molecular 

 group composed of 12 atoms, when it possesses the crystalline form of bisul- 

 phate of potash ; for the integrant particle of the salt contains not less than 

 that number of atoms. Supposing also sulphur, in the state of vapour, to be 

 similarly constituted, then, instead of one-third of a volume, its molecule will 

 give four volumes of vapour, the most usual of all proportions. In crystal- 

 lized sulphur, then, there may exist the same arrangement and aggregation of 

 atoms as in bisulphate of potash, resulting from the action of similar affinities. 

 M. Liebig has represented KS 5 , the pentasulphuret of potassium, by KS,SS 3 , 

 or as a sort of sulphate of the sulphuret of potassium, which is quite in ac- 

 cordance with these molecular views. 



A change in the number of atoms forming the sulphur molecule, or in their 

 arrangement, will account for the dimorphism of that body; indeed, incon- 

 stancy of molecular structure may be the general origin of dimorphism. In 

 compound bodies, such as the acids, we have often illustrations of a similar 

 association of several atoms. It appears, in the proportions in which they 

 occasionally unite with bases, as in the terchromate of potash, the teriodate 

 of soda, and maybe inferred from the products of their decomposition in 

 other cases. Thus, when chlorate of potash is decomposed by sulphuric acid, 

 three equivalents of that salt are decomposed together (page 371), which is 

 certainly a strong presumption that these three equivalents were previously 

 associated in some way, forming one whole. On a similar presumption, Mits- 

 cherlich triples the equivalent of white precipitate, and makes it (3Hg Ad + 

 3Hg Cl), because that compound affords N Hg 3 as one of the products of its 

 decomposition. It is certainly curious that the aggregation so indicated is 

 very often that of three atoms, as if the atoms of compound bodies affected a 

 salimolecular arrangement, similar to that assumed by the atoms of elements. 



III. The Rotal action of chemical affinity. Chemical affinity is certainly ca- 

 pable of acting at a distance in a particular manner. The chemical affinity or 



N N 



