116 Prof. Odling on the Hex atomicity 



atoms of ferricum and aluminium are considered as hexatomic. It 

 seems to me, however, that, in the present state of knowledge, 

 the arguments for and against this hexatomicity are pretty evenly 

 balanced, or at any rate that those in its favour do not preponde- 

 rate so decidedly as to warrant us in hastily giving up our old 

 established notions. 



As I understand them, the chief arguments for the hexatomi- 

 city of ferricum and aluminium are deducible from the vapour- 

 densities of the two chlorides (a), from the existence of certain 

 basic and complex salts of ferricum (/3), and from the properties 

 of aluminium-ethyl, &c. (7). 



a. If we regard the chlorides of ferricum and aluminium as 

 trichlorides, their respective molecules will have only half the 

 volume of the molecule of hydrogen, H 2 , chlorhydric acid, H CI, 

 and 99 per cent, of all volatile bodies ; whereas if we regard them 

 as hexachlorides, the gaseous volumes of their molecules will be 

 perfectly normal. On the other hand, it may be argued that the 

 molecules of arsenious anhydride, As 2 O 3 , arsenicum, As 2 , and 

 phosphorus, P 2 , are deliberately represented by formulae corre- 

 sponding to volumes only half as great as those of the molecules of 

 hydrogen and chlorhydric acid, and that possibly the same cause 

 or want of knowledge which renders the vapour-densities of these 

 last bodies anomalous may render those of the trichlorides of 

 ferricum and aluminium similarly anomalous also. 



ft. Several complex and basic salts of ferricum have been de- 

 scribed particularly by M. Scheurer-Kestner, whose respective 

 constitutions are in harmony with the formula Fe 2 CI 6 ' or Ffe CI 6 ; 

 and certainly cannot be brought into harmony with the formula 

 Fe CI 3 . On the other hand, it may be urged that some, and ap- 

 parently the best-defined, of M. Scheurer-Kestner's new bodies 

 are in harmony with the formula Fe Ci 3 , and that several very 

 definite compounds are known (related to fluorhydric acid, to chlo- 

 ride of sodium, to nitrate of silver, and to the acetic, oxalic, and 

 sulphuric acids for instance) whose respective constitutions are 

 not in harmony with the recognized formulae of the several 

 types, but only with the doubles of those formulae — such com- 

 pounds, for example, as the following: 



£} p2 > lt}ci% ^ Q3 y Ag 2 , £{c 2 h 3 0) 2 , g 3 {c*oy, 



Na 3 \ S04 ) 2 ' (NK 4 ) 3 \ S ° 4 ) 2 ' aud man y others - 



7. What little is known of aluminium-ethyl and aluminium - 

 methyl seems certainly to be in favour of the formula Al 2 CI 6 , or 

 All CI 6 . But, on the other hand, these bodies have been too im- 

 perfectly examined to warrant the deduction of any weighty argu- 



