THE EVOLUTION OF THE ELEMENTS. 
131 
The evidence in proof of the second and third propositions 
laid down on p. 1 29 — viz. that molecules are very stable systems, 
but that they are nevertheless decomposable — may be said to be 
furnished by the whole science of Natural Philosophy. 
Physical research has determined series of constants for the 
elementary and compound molecules ; it has shown us how stable 
these systems are, what definite and clearly marked properties may 
be predicated of each of them, and how the properties of masses 
of matter are to be traced to the properties of the molecules 
of which these masses are built up. 
Chemistry, on the other hand, teaches, without doubt or 
hesitation, that, stable as those molecules are, they are neverthe- 
less decomposed when acted upon by chemical force. To enumerate 
the leading instances of such decomposition would be to write a 
treatise on Descriptive Chemistry. 
The fourth proposition — viz. that changes may occur in the 
molecular weights of elements or compounds, which changes are 
correlated with considerable modifications of the properties of 
those elements or compounds — demands illustration in some 
detail. 
Apart from the evidence furnished by chemistry in support 
of this proposition, we know of facts which can scarce find an 
explanation if this statement be denied. Schuster’s recent work 
on the Spectra of Oxygen obliges us to regard the existence of 
varying molecular groupings of that element as extremely 
probable, and tends to discredit the idea that mutual inter- 
ferences among the vibrations of the same molecules may be a 
vera causa of the totally different spectra of oxygen which he 
has observed. (“Nature,” xviii. 269.) Certain of the changes 
observed by Dr. Schuster to occur in the spectrum of oxygen 
are most abrupt and complete, and seem only to find an ex- 
planation in the hypothesis that distinct molecular groupings 
of that element exist even at high temperatures. 
But let us look at the chemical evidence in support of our fourth 
proposition. I suppose everyone regards oxygen and ozone as forms 
of one and the same elementary body. The marked differences in 
the behaviour of these bodies are traceable to the fact that the 
molecule of the latter contains half as many more atoms as the 
molecule of the former. The molecular weights of oxygen and 
ozone stand to one another in the proportion of 2 to 3 ; their 
molecules are composed of similar atoms, nevertheless the pro- 
perties of these molecules are by no means identical. Again, 
the molecular weight of sulphur vapour at temperatures a little 
above the boiling point of that substance is three times as great 
as the molecular weight of the same element at considerably 
higher temperatures. Little is known of the properties of the 
molecules of sulphur as they exist at either of these temper- 
