56 PRESIDENTS ADDRESS—SECTION B. 
nection that existed between the atomic weights of the elements 
and their physical and chemical properties, and this was elabo- 
rated and systematised by Mendeleef in 1869, and by Lothar 
Meyer into what is known as the periodic law of the elements. 
The immense importance of this far-reaching and suggestive 
generalisation is hardly to be over-estimated. It may be said 
that its influence has revolutionised our conceptions of the 
material universe. It is true that some modification of its pre- 
sent representation has become necessary, particularly since the 
discovery by Rayleigh and Ramsay of the atmospheric elements, 
‘argon, metargon, krypton, and of helium. Amongst suggestions 
for such a rearrangement the most satisfactory is that recently 
proposed by Crookes. 
CONSTITUTION OF CHEMICAL COMPOUNDS. 
In 1828 the German chemist, Wohler, made the remarkable 
discovery that by simply heatimg a solution of ammonium 
cyanate it was converted into urea. This introduced to chemists 
two distinct substances of different properties, but of identical 
composition. 
It became, then, of the first importance to ascertain not alone 
the composition of chemical substances, not only the number of 
atoms of the elements which went to form a compound; it 
became important to find out in what way the separate atoms 
were combined amongst each other, and to discover the laws 
which guided such combination. 
At that time the electro-chemical theory of Berzelius explained 
the affinity of the atoms of different atoms for each other, those 
which were electro-positively polarised having the strongest 
affinity for the electro-negative ones. 
In 1834 the discovery was made by Dumas that chlorine (—) 
could replace hydrogen (+) in certain compounds without 
materially affecting the nature of the compound, thus the dif- 
ferent chloracetic acids all partake of the nature of acetic acid. 
Dumas was hence led to enunciate the doctrine of types, which 
assumed that compounds containing the same number of atoms, 
similarly united, were of the same type, though these atoms 
were not necessarily of the same elements. 
In 1832 the discoveries of Liebig and Wohler introduced the 
idea of radicles or residues, groups of atoms comon to a series 
of compounds. These radicles are not necessarily capable of being 
isolated. Such are ethyl, methyl, benzoyl. Several systems of 
classification of organic compounds on these or similar bases are 
now to be noted. Laurent introduced the idea of nuclei, which 
was an extension of the theory of radicles. Gerhardt introduced 
a classification of compounds on the system of types represented 
by He O, N Hs, H Cl., HH. 
