32 PRESIDENT’S ADDRESS—SECTION A. 
ment of the elementary atoms (molécules) out of which the mole- 
cules (particules) are built up. The latter were assumed to be 
the smallest quantities of any substance which could maintain 
a stable existence in the free state. That endeavour has not yet 
been crowned with complete success, more especially in the realm 
of inorganic chemistry. 
In 1819 Dulong (0) and Petit (py) enunciated the view that the 
atoms of simple substances have equal capacities for heat, or that 
specific heats are inversely proportional to the atomic weights (q). 
Although this doctrine is of great practical value in considering 
determinations of molecular weight, yet the frequency of excep- 
tions discloses the impossibility of retaining so simple a view of 
the constitution of material bodies. 
28. Lsomorphism and Isomerism.—The discovery by E. 
Mitscherlich (7) in 1819 of the relation of isomorphism to simi- 
larity: of chemical composition, the recognition of isomerism 
through the identity of the composition of silver cyanate inyesti- 
gated by Wohler (s) in 1822, and silver fulminate by Liebig (¢) 
in 1823 (wz), confirmed if needs be by the transformation, in 1828, 
by the former chemist, of ammonium cyanate into urea (v), 
showed with increasing significance the value of atomism to che- 
mistry, if supplemented by the theory of definite molecular 
structure. The simplicity of the original atomism is, of course, 
clearly given up, and Lucretius’ idea of conciliwm comes into 
striking prominence. Atoms tend to associate according to 
definite laws, which as yet are only empirically known, and their 
relations can be changed in general only by the introduction of 
energy in some form or other. Ordinarily their concilium is 
well established, though in the case of the so-called unstable com- 
pounds this is not so. The molecules of cyanic acid, for example, 
enter into closer relations in a most energetic manner at ordi- 
nary temperatures; it polymerises with explosive violence into 
cyamelide (w). 
29. Valency and Atomic Linking.—Anythmg like detailed 
reference to the discussion of the general theory of chemical con- 
stitution is here impossible: Suffice it to say that the evidence 
of definite arrangement of the atoms in a molecule is continually 
augmenting. The singular nature of this, and of the atomic 
and molecular affinities to which this arrangement must be as- 
cribed, was brought into prominence by the founding in 1852 
(m) [1778—1829. ] (q) Annal. d. Chim., &., X., pp. 395-413. 
(n) [1775 —1836.] (7) [1794—1863. ] 
(0) [1785—1838. | (s) [1800—1882. ] 
(p) [1791—1820. ] (t) [1803—1873.] 
(u) Silver cyanate Silver fulminate 
2[Ag—N:C:0] Agz=CK< s Be 
(v) Ammonium Cyanate Urea 
H4: N-N:C:0 H2:N—[C:0]—N:H2 
(w) GE. N 3G =O)n 
