618 Scientific Proceedings, Royal Dublin Society. 
a group of similar identically-situated balls substitution for one only 
will produce identically the same effect whichever of the similar 
balls is selected for removal,’ we have the fact of the presence in 
some combinations of similarly situated atoms, é.e., of atoms whose — 
ties are similar; the proof of this being that there is but one mono- 
substitution product of such combinations, e.g., of methane, and 
that this is the case notwithstanding the employment of methods 
which ensure that different atoms and not the same atom are dis- 
placed in a series of displacements of a single atom.” 
That the atoms have distinct spheres of influence, and, although 
similarly related to the combination in which they occur, are not 
without relative arrangement of some kind is in evidence. Thus 
we have two and not merely one tetra-substitution derivative ab cd 
of Methane.° 
Corresponding to the limited number of ways in which double 
substit ution can be made in the simpler groups, as shown by the 
tables given above,* we have the facts referred to by Lothar Meyer 
as follows :— 
“<The circumstance which has especially conduced to the wide 
recognition of Kekule’s hypothesis is the fact that hitherto but 
three mutually isomeric di-substitution products of the benzenes 
have been obtained, e.g., but three dichlorobenzenes, and that this 
continues to be the case in spite of the great efforts made by many 
investigators to find a fourth.° 
The formation employed by Kekulé for the cases just referred 
to is the hexagonal one marked 6a in the list just given. One of 
the other simple arrangements 6d or 6e would, however, give the 
1 See p. 609. 
2 Comp. Lothar Meyer’s ‘‘Grundztige der theoretischen Chemie.’’ Note, p. 96. 
In speaking of the ‘‘Schablone ” of Kekulé, Lothar Meyer says: “ By the ring form 
here referred to it is not intended to be conveyed that the atoms probably lie in a plane 
ring, but merely that they form a chain whose ends are connected, i.e., a ‘closed’ 
chain.’’ 
3 Comp. the law of le Bel and van ’t Hoff. ‘‘ Atoms or groups connected by four 
valencies are unable, wanting some other condition, to exchange places with one 
another.” Proof—There are but two tetra-substitution products of methane a dc d. 
See Satz IV. Bischoff’s ‘‘ Handbuch der Stereochemie,”’ p. 50. 
# See in particular pp. 611-613 above. 
* Lothar Meyer’s “ Grundziige der theoretischen Chemie,”’ p. 87. 
