82 THE ROYAL SOCIETY OF CANADA 
measurements. The formula connecting them contains only one ap- 
proximation of importance theoretically—the value of the density of 
the liquid is used instead of the difference in density of the liquid and 
vapour; but except at high pressures and near the critical state this 
approximation leads to no appreciable numerical error. It cannot 
affect the significant figures for ordinary temperatures and pressures 
of the great majority of liquids and vapours. It will be unnecessary 
to repeat here the proof of the formula which they give. It may be 
emphasized, however, that the formula merely represents the functional 
relation of the various quantities deduced directly from experimental 
observations in accordance with the principles of energy. It is, there- 
fore, undoubtedly entitled to as much credence as other similarly de- 
duced functions; but a mere statement of the functional relation of 
physical quantities is not sufficient to satisfy the mind. Some think- 
able causal relation is demanded. Is it quite satisfactory to know, 
Merely, that, in order to maintain equilibrium, the vapour-density close 
to a convex liquid surface must be denser than is necessary when the 
surface is plane? This mental hiatus is sometimes bridged by express- 
ing the relations in terms of pressure. But is not pressure Just as much 
a non sequitur as curvature? How can a pressure exerted on a liquid 
conceived of as made up of discrete particles held together by attractive 
forces, in any thinkable way, be connected causally with a greater vapour 
density, especially when the pressure due to the surface-tension is in- 
finitesimal as compared to the interior pressure in liquids? If the 
pressure, supposed to be due to internal attractions, should be sufficient 
to increase the density, how could this increase in density due to the 
molecular attractions aid in freeing the molecules from the very attrac- 
tions which, when sufficient, prevents the formation of any vapour 
whatever? 
These considerations forcibly presented themselves to my mind 
while studying the supersaturation of the atmosphere with moisture. 
A large increase in pressure of saturated air does not cause a deposit of 
moisture unless the temperature is lowered at the same time. It is but 
recently that hygrometrie tables have been prepared that take into 
account the effect of barometric pressure as well as temperature, in 
determining the moisture content of the air. It is sometimes stated 
as one of Daiton’s laws that the quantity of moisture in saturated air de- 
pends only on the temperature. This is a mistaken notion; for it can 
be shown that air under pressure in contact with liquid water will take 
up more moisture with increasing pressure.! But, as previously in- 
timated, increased pressure except as measured by the actual increase 
‘Poynting, Phil. Mag. XII p, 39. Lewis, Proc. Am. Acad. of Arts & Sci. XXXII 
9 Oct. 1900. Zeitsch, f. phys. Chemie 35, 343 (1900). 

