108 TRANSACTIONS OF THE 
tals from coarse ones. At first sight thisseemed quite prob- 
able, for was not river water different from spring water? 
Closer examination, however, showed that neither is pure 
and that both contain other things than water. When 
these other things are removed, river water and spring 
water give samples of water which cannot be distin- 
guished each from the other. Dalton concluded, then, 
that the atoms of any pure substance are all exactly alike. 
He assumed, further, that compound substances are made 
up of atoms of their component elements in fixed numbers, 
and arranged in a perfectly definite manner. 
In 1808, Gay-Lussac published his researches on the 
combining volumes of gases, and stated what we now 
know as the law of Combining Volumes, or the Law of 
Definite Proportion by Volume. He did not, however, 
state the very obvious corollary, viz:—that the number 
of particles in equal volumes of different gases, under 
like conditions, must be the same, or at least proportional. 
This was done three years later, by Avogadro, who also 
pointed out the necessity of a new conception, viz:—that 
the ultimate particle of a compound gas must be different 
from that of a gas not compound. He used the terms 
“integrated molecule” for the former, and “elementary 
molecule” for the later, they being the equivalents of the 
molecule and atom of the present nomenclature. 
The atomic theory, then, as stated by Dalton and 
modified by subsequent workers, is that all substances 
are composed of discrete particles called molecules, and 
that these molecules are in turn made up of one or more 
particles called atoms. These atoms are of a limited 
number of kinds, but atoms of any given kind are all 
exactly alike, and these atoms represent the limit of 
divisibility of matter. 
About 80 different kinds of atoms are known, and 
their relative weights have been worked out most care- 
fully, over a long period of years. In fact, our table of 
atomic weights is probably the memorial of the greatest 
amount of the most exacting scientific work of one kind 
ever attempted. A large amount of this work had been 
accomplished by the middle of the 19th century, though a 
great deal of correction, revision and refining has gone 
on since. 
