28 The Past and Present of Chemistry. [Jan., 
metal ; this very element, when in the condition in which it is called 
ozone, possesses properties as utterly different as we should expect 
to find only in a totally distinct substance. It is now a powerfully 
pungent gas, violently attacking at the common temperature the 
very same substances upon which ordinary oxygen has no action, 
and causing moist silver rapidly to become black with rust; in both 
these cases we perceive the existence of what appear to be different 
substances of undoubtedly similar composition. 
Do not such facts appear to contradict the thesis advanced 
above as the acquisition of modern chemistry? They certainly do 
contradict the earlier and narrower conception of it; but they only 
serve to confirm the wider and more modern view, and to maintain 
it as the fundamental doctrine of chemistry. A few words of expla- 
nation will serve to render this clear. Science can no longer dis- 
pense with the hypothesis, that bodies consist of very small ultimate 
particles, which cannot be further divided without the production of 
something totally different from the matter subjected to this divi- 
sion. These homogeneous particles, of which a substance is built 
up, are called the physical atoms or molecules of that substance. A 
piece of copper is composed of copper molecules, the smallest per- 
ceptible quantity of oxygen or alcohol, of oxygen or alcohol 
molecules. A distinction is made between the molecules of bodies, 
ze. the smallest particles which can be conceived as capable of 
independent existence, and atoms, i.e. the smallest particles which 
can enter into a chemical compound or contribute to the formation 
of a molecule. The molecules are composed of atoms. The mole- 
cules of compound bodies are composed of dissimilar atoms, atoms 
of different elements, whilst we assume that the molecules of un- 
decomposable bodies—of elements—-are built up entirely of the 
same kind of atoms. We have no knowledge of the absolute 
number of atoms which unite to form the molecules of different 
substances, whether a copper molecule, for instance, consists of 2 or 
10 or 100 atoms of copper. But we do know or at least we can form 
very probable conjectures concerning the proportion in which the 
atoms unite to form a molecule; for instance, in what proportion 
the carbon, hydrogen, and oxygen atoms are combined in a molecule 
of alcohol, or what is the proportion between the number of oxygen 
atoms contained on the one hand im a molecule of alcohol, and on 
the other in a molecule of common oxygen. 
It will now be apparent how the different conditions of the 
same elementary substance may depend upon differences in its 
composition. The same elementary atoms may obviously, by com- 
bining in different numbers to form molecules, give rise to dis- 
similar molecules. We know with almost absolute certainty that a 
greater number of oxygen atoms is contained in a molecule of ozone 
than in a molecule of ordinary oxygen. We can assume with great 
