SECTION B. 
54 PRESIDENTS ADDRESS 
multiple proportions, which asserts that the elements only enter 
into combination in definite and fixed proportions by weight, and 
that if two elements combine to form more than one compound, 
the proportions by weight of the element are always multiples 
of each other. 
The same relationship was also shown by Gay-Lussac to exist 
between the volumes of interacting gases. The discovery of 
these fundamental laws was the immediate outcome of the intro- 
duction of quantitative methods of investigation. 
They are, of course, empirical, and independent of hypothesis. 
The explanation of their cause was afforded by the atomic 
hypothesis of John Dalton (1808), which states in effect that 
matter is composed of aggregations of minute indivisible 
particles, and that chemical union takes place between the atoms 
of matter. Further, that these atoms are possessed of definite 
weight differing with the nature of the element, and that these 
weights are proportional to the combining weights. 
This hypothesis, known as the Atomic Theory, provided a 
satisfactory explanation of the laws then known governing the 
constitution of matter. The further development of this theory 
is the characteristic feature of modern chemistry, and all the 
subsequent generalisations and discoveries are either directly 
due to its recognition, and are corollaries from it, or have 
received their interpretation by its means. 
It is often insisted that the atomic theory is essentially a 
legacy from the Greek philosophers, and that it is to them we 
owe the conception that matter is built up of minute indivisible 
particles. There is, however, a fundamental difference between 
the e-nceptions of the Greeks and of Lucretius, and the concep- 
tion of Dalton, and one that is sufficiently important to make 
all the difference between a fanciful guess and a well-considered 
theory. The distinction les in the conception of Dalton, that 
these atoms are possessed of weight differing with the nature of 
the element, and that they are separated from each other by 
space. 
The weights of the atoms are, of course, relative, and not 
actual. 
As soon as the fact was recognised that the elements consisted 
of a number of minute and indivisible particles, and that the 
elements combined together in definite proportions by weight, 
the problem of determining the relative weights of the atoms 
of the different elements engaged the attention of chemists. The 
equivalent weight is not, as we know, in all cases, the atomic 
weight. 
The equivalent weights are capable of direct determination, 
but as we have no direct method of ascertaining the number of 
atoms of the elements entering into any compound, the actual 
weight of the atoms must be determined by indirect means. 
