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POPULAR SCIENCE REVIEW. 
sometimes suffices to prevent the second or third change from 
occurring ; and, in place thereof, we are presented with a new 
chemical change traceable to the action of the dissociated hy- 
drogen or oxygen atoms upon the new substance introduced. 
Now we call the atoms of the two elements which, by their union, 
produce water, atoms of hydrogen and oxygen ; and, in a sense, 
this is correct. These atoms, by their union together, produce 
hydrogen and oxygen respectively ; they are therefore atoms of 
hydrogen and oxygen. But we know very little concerning the 
exact nature of these atoms per se , and all we do know points 
unmistakably to the conclusion that the properties of these 
atoms are very different from those which are characteristic of 
the bodies known to us as hydrogen and oxygen. 
I think it would scarcely be too much to say that the whole 
edifice of modern chemical theory rests upon the hypothesis — 
may we not say upon the fact? — that the properties of the 
elementary atoms are markedly different from the properties of 
the elementary molecules. But, at the same time, facts do not 
allow us to regard the elementary molecules as composed of 
atoms of two or more kinds of matter ; in other words, all the 
atoms of hydrogen are certainly possessed of the same properties. 
The hydrogen molecule may then be regarded as A + A, not 
A + B. We know the properties of the body A + A ; we also know 
the properties of many other bodies into the composition of 
which A enters ( A + C, A + D, A -f- E, &c., &c.) ; but we know very 
little regarding the properties of A, or B, or C, &c., as distinct 
from the properties of the compounds of those bodies. We 
can, it is true, trace modifications in the properties of a com- 
pound brought about by the entrance of A, or B, or C into that 
compound ; but we are unable to isolate A, or B, or C, and study 
its properties when in the free state. 
The decomposition of an elementary molecule into its com- 
ponent atoms would thus be a phenomenon not altogether 
analogous to the decomposition of a compound molecule into 
its components. 
Until recent years we have been content to decide the ques- 
tion of the decomposition of a given substance in a certain 
reaction by examining the supposed products of decomposition 
by the ordinary physical or chemical tests. Chemists found no 
difficulty in distinguishing sodium from soda, iron from rust, 
mercury from its compound with oxygen ; but now we learn that 
more searching examination must be made before an answer can 
be given to the question, Are the elements themselves capable 
of decomposition ? 
If our elements are non-elementary, they are at any rate 
much more stable, they are in a sense more definitely marked 
chemical entities, than our compounds. 
