884 Dr. E. Frankland on a New Series of 



but no compound corresponding to arsenic acid can be formed, 

 and yet it cannot be urged that cacodylic acid is decomposed by 

 the powerful reagents requisite to procure further oxidation, for 

 concentrated nitric acid may be distilled from cacodylic acid 

 without decomposition or oxidation in the slightest degree ; the 

 same anomaly presents itself even more strikingly in the case of 

 stansethylium, which, if we are to regard it as a conjugate radical, 

 ought to combine with oxygen in two proportions at least, to 

 form compounds corresponding to protoxide and peroxide of tin ; 

 now stansethylium rapidly oxidizes when exposed to the air and 

 is converted into pure protoxide, but this compound exhibits 

 none of that powerful tendency to combine with an additional 

 equivalent of oxygen, which is so characteristic of protoxide of 

 tin ; nay, it may even be boiled with dilute nitric acid without 

 evincing any signs of oxidation : I have been quite unable to 

 form any higher oxide than that described ; it is only when the 

 group is entirely broken up and the sethyle separated, that the 

 tin can be induced to unite with another equivalent of oxygen. 

 Stibsethyle also refuses to unite with more or less than two equi- 

 valents of oxygen, sulphur, iodine, &c., and thus forms com- 

 pounds, which are not at all represented amongst the combina- 

 tions of the simple metal antimony. 



When the formulse of inorganic chemical compounds are con- 

 sidered, even a superficial observer is struck with the general 

 symmetry of their construction; the compounds of nitrogen, 

 phosphorus, antimony and arsenic especially exhibit the ten- 

 dency of these elements to form compounds containing 3 or 5 

 equivs. of other elements, and it is in these proportions that their 

 affinities are best satisfied ; thus in the ternal group we have 

 NO^ NH3, NP, NS3, P03, PH^ PCl^, SbO^, SbH^, SbCF, AsO^, 

 AsH3, AsCF, &c. ; and in the five-atom group N0^ NH^O, 

 NH^I, PO^ PH'*I, &c. Without offering any hypothesis regard- 

 ing the cause of this symmetrical grouping of atoms, it is suffi- 

 ciently evident, from the examples just given, that such a ten- 

 dency or law prevails, and that, no matter what the character of 

 the uniting atoms may be, the combining power of the attracting 

 element, if I may be allowed the term, is always satisfied by the 

 same number of these atoms. It was probably a glimpse of the 

 operation of this law amongst the more complex organic groups 

 which led Laurent and Dumas to the enunciation of the theory 

 of types ; and had not those distinguished chemists extended 

 their views beyond the point to which they were well supported 

 by then existing facts, — had they not assumed, that the proper- 

 ties of an organic compound are dependent upon the position 

 and not upon the nature of its single atoms, that theoiy would 

 undoubtedly have contributed to the development of the science 



