34 
POPULAR SCIENCE REVIEW. 
Why is there so intimate a connection between the numbers 
expressing the proportions in which the elements combine, and 
the general chemical and physical properties of these elements ? 
It seems hard to point out so many generalizations which may 
be made from facts, but to forbid us to generalize these generaliza- 
tions. The molecular theory of matter — aided by the atomic 
theory of the chemist — professes to give some kind of explana- 
tion of the generalizations I have alluded to. And it appears to 
me that the rejection or acceptance of this theory must be based 
upon the success which attends its profession to give this ex- 
planation. 
Matter, it is said, is possessed of a grained structure of some 
kind. A mass of matter is not a homogeneous whole, but is built 
up of parts, just as a brick wall is built up of individual bricks. 
These little parts are called molecules : when a body is heated 
the parts of the molecules do not part company. In chemical 
actions the molecules are split up ; the smaller parts of matter then 
produced are termed atoms. Whether these are susceptible of 
further subdivision, or not, we do not know. From this theory 
certain deductions are made ; very many of these deductions have 
been experimentally shown to be true. Among these deductions 
is the statement, generally known as Avogadro’s law, that, “under 
the same conditions of temperature and pressure, equal volumes 
of gases contain equal numbers of molecules.” Assuming the 
truth of this statement, it becomes very easy to determine the 
relative molecular weights of gaseous bodies. The weight of two 
volumes of hydrogen is the standard : this weight is called 
2. The weight of two volumes of any other gas expresses the 
(relative) molecular weight of that gas. The smallest (relative) 
amount of an element occurring in one molecule — i.e. in 2 
volumes — of any of its compounds is the maximum atomic weight 
of that element. In practice we generally assume this number 
to represent the true atomic weight. 
The modern system of notation may therefore be regarded as 
an atomic notation. The two-volume formulae become molecular 
formulae ; the combining numbers of the elements become atomic 
weights. 
The constant number found by multiplying combining number 
into specific heat represents the atomic heat of the elements. 
The crystal is a molecular structure, each compound molecule 
being built up of elementary atoms ; the amount of one element 
which can replace an atom of another without altering the 
general form of the structure represents the weight of an atom 
of the replacing element. The fact that there exist intricate 
relations between the combining numbers and general proper- 
ties of the elements receives some kind of explanation, when 
these numbers are regarded as the relative weights of the atoms- 
of the elements. 
