440 
DR. E. FRANKLAND ON A NEW SERIES OF 
conjugated with various hydrocarbons, we might reasonably expect, that the che- 
mical relations of the metal to oxygen, chlorine, sulphur, &c. would remain un- 
changed ; a glance at the formulee of these compounds will however suffice to 
show us that this is far from being the case ; it is true that cacodyl forms prot- 
oxide of cacodyl and cacodylic acid, corresponding the one to a somewhat hypo- 
thetical protoxide of arsenic, which, if it exist, does not seem to possess any well- 
defined basic character, and the other to arsenious acid ; but no compound corre- 
sponding 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 decompo- 
sition or oxidation in the slightest degree ; the same anomaly presents itself even 
more strikingly in the case' of stanethylium, which, if we are to regard it as a con- 
jugate radical, ought to combine with oxygen in two proportions at least, to form 
compounds corresponding to protoxide and peroxide of tin ; now stanethylium 
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 ethyl separated, that the tin can be induced to unite 
with another equivalent of oxygen. Stibethyl also refuses to unite with more or less 
than two equivalents of oxygen, sulphur, iodine, &c., and thus forms compounds, 
which are not at all represented amongst the combinations of the simple metal anti- 
mony. 
When the formulae of inorganic chemical compounds are considered, even a super- 
ficial observer is struck with the general symmetry of their construction ; the com- 
pounds of nitrogen, phosphorus, antimony and arsenic especially exhibit the tendency 
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 NO3, NH3, NI3, NS3, PO3, PII35 PCI3, Sb 03 , SbI- 13 , SbCl 3 , AsOg, ASH3, 
ASCI 3 , &c. ; and in the five-atom group NO5, NH4O, NII4I, PO5, PH4I, &c. Without 
olfering any hypothesis regarding the cause of this symmetrical grouping of atoms, it 
is sufficiently evident, from the examples just given, that such a tendency 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 ope- 
ration 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 properties of an organic 
compound are dependent upon the position and not upon the nature of its single 
