72 
PROFESSOR W. RAMSAY AND DR. S. YOUNG 
compounds, and the fact that heat is evolved or absorbed during passage 
from the one form to the other. 
(c“.) The abnormal vapour-density of sulphur and selenium among elements, and of 
the compound nitric peroxide appear to be undoubtedly due to the existence 
of complex molecules. It is to be remarked that in these cases molecular 
grouping occurs before the liquid condition is attained. 
(cl.) The abnormal vapour-densities of all substances at temperatures near their 
boiling-points, shown especially by acetic and homologous acids. Here also 
the grouping, if any, must occur in the gaseous state. 
§ 3. The phenomena stated in («) and ( b ) are explained equally well, it may be 
remarked, by a systematic arrangement of gaseous molecules, and do not necessarily 
imply union of those molecules to form more complex molecular groups ; while as 
regards ( d ) it appears almost certain that the abnormality common to non-dissociable 
vapours near their condensing-points at high temperatures and corresponding high 
pressures, is due simply to a deviation from Avogadro’s law, for it disappears, at 
least in the case of alcohol, on lowering temperature and pressure, so as to keep the 
vapour nearly saturated. With acetic acid vapour, on the contrary, the abnormality, 
which is considerable at the ordinary boiling-point, increases greatly with reduction 
of temperature and pressure. It might be expected that a decrease of temperature 
would tend towards the formation of complex molecular groups ; while it is difficult 
to imagine a rise of temperature, even if accompanied by a rise of pressure, bringing 
about this result. 
§ 4. If it be true that liquids consist of more complex molecular groups than gases, 
that each such complex molecule is composed of a definite number of gaseous mole¬ 
cules, and that the molecule of a liquid may be regarded as a chemical compound of 
like gaseous molecules, then the phenomena attending the change from the solid or 
liquid to the gaseous state should be similar both with stable and dissociable bodies. 
§ 5. Certain analogies are evident on first inspection. 
(1.) To convert a liquid into gas, work must be done 
(«.) in overcoming molecular cohesion ; 
( b .) in expansion against pressure, which may be that of the atmosphere. 
If liquids consist simply of gaseous molecules in such proximity that they exercise 
mutual attraction on each other, the work is wholly expended in these two directions; 
if, on the contrary, molecules of a liquid are more complex than those of its gas, work 
is also done 
(c.) in decomposing the complex molecules ; 
(cZ.) in imparting increased velocity to the resulting gaseous molecules. 
The last assertion (d) will hold if the mean energy of each of the liquid molecules 
is equal to the mean energy of each of the gaseous molecules in contact with it. It 
is known, at any rate, that no heat change occurs when a saturated vapour is brought 
in contact with its liquid at the same temperature and pressure. Since the energy 
