540 On the Constitution of mixed Gases, Gc. 
what the above ratio assigns; because aqueous 
vapour, one of the elements of atmospheric air, 
loses its form, or becomes partly converted into 
water by pressure. If air confined by sulphuric 
acid be tried, it accords very exactly with the 
abeve law. If air confined by water be tried, 
the condensed air is always something /ess and 
the rarefied air something more than what the 
theory assigns, which is entirely owing to the 
destruction or formation of a quantity of aque- 
ous vapour. My method of experimenting is 
very simple ; it consists in condensing or rarefy- 
ing the air by a column of mercury in a long 
straight tube, divided into. equal portions; the 
tube must be +z or #5 of an inch internal diameter, 
and then it may be inverted without losing its 
contents, if the mercurial column be less than 
39 inches, when the rarefaction of air is the 
object. | 
Prop. 2. Homogeneous elastic fluids are 
constituted of particles that repel one another 
with a force decreasing directly as the distance of 
their centres from each other. 
This proposition is a mathematical one, and 
its demonstration founded upon the fact of the 
density being as the pressure. The proof 
may be seen in the Principia, B. 2. Prop. 25. 
It follows too that the distances of the centres of 
the particles, or which is the same thing, the - 
