72 Intelligence and Miscellaneous Articles. 
rapidly ; its density increases, in the same ratio as its weight, 
without any known limit. On the other hand, the portion which 
has remained liquid undergoes an expansion that increases until at 
last it exceeds that of gases (Thilorier). It is clear that, by the 
effect of these reverse changes, a limitiug temperature is finally 
attained, at which liquid and vapour have the same weight under 
the same volume. 
At that moment they are no longer separate : the vapour does 
not escape to the top ; the liquid does not sink to the bottom. 
First of all the meniscus is seen to disappear, the surface of 
separation ceases to be distinct (Drion) ; then the entire mass is 
mingled, with undulating and moving streaks giving evidence of a 
mixture of different densities ; and finally the whole assumes a 
homogeneous state which is supposed to be gaseous. The critical 
point is reached, which may be defined as the temperature at which 
a liquid and Its saturated vapour have the same density. But, for 
all that, the general law of vaporization is not suddenly inter- 
rupted : the liquid continues to be at its boiling-point and maxi- 
mum tension. If it is no longer visible, that is because it is 
mixed with the gas, in which it floats on account of the equa- 
lization of the densities ; and when the temperature continues to 
be augmented, the tension continues to increase, remaiuing a 
maximum, until the liquid is entirely volatilized ; after which, but 
not till then, the space ceases to be saturated and the pressure to 
be limited : there is no longer any thing but dry vapour, a gas far 
from its Kqueiying-poiut. 
It is necessary to rightly account for this very peculiar state of 
the liquid while it is thus invisible and floats in its vapour. In 
general a saturated vapour is distinguished from the generating 
liquid by two conditions — its less density, and its latent heat ; 
here, as we have seen, the densities are equal, and there is no 
latent heat, since the volume is unchanged and there is no work of 
expansion. Hence comes it that in Cagniard-Latour's experi- 
ment the liquid is seen so suddenly to reappear or disappear 
through the least depression or elevation of temperature. In 
short, at its critical point nothing differentiates the liquid from its 
vapour — neither tension, nor density, nor heat of constitution, nor 
aspect, nor any property that could distinguish them. 
If instead of Cagnaird-Latour's experiment we follow in detail 
those of Andrews, they can be summed up as follows :- -WTien a 
gas confined in a closed space is gradually compressed, 
(1) Its pressure increases up to a fixed limit, the maximum 
tension : if we try to go beyond that, the gas condenses : it is the 
liquefaction-point; it is the boiling-point under that pressure. 
(2) The liquefaction-pressure augments rapidly with th 
perature, without any known limit, but without ceasing to exist 
and without changing its character at any critical point whatever. 
(3) At low temperatures the density of saturated vapour is less 
than that of the generating liquid. Starting from a determined 
