which we call Heat. 115 
The pressure of the gas on the liquid arises solely from the 
fact, that here and there single molecules of gas strike against 
the surface of the liquid. In other respects, however, inasmuch 
as the molecules of gas themselves actually fill but a very small 
part of the superincumbent space, the latter must be considered 
as empty, and as offering a free passage to the molecules of the 
liquid. In general these molecules will only come into collision 
with those of the gas at comparatively great distances from the 
surface, and the former will then deport themselves towards the 
latter as would the molecules of any other admixed gas. We 
must conclude, therefore, that the liquid also expels its molecules 
into the space filled with gas; and that in this case also the 
quantity of vapour thus mixed with the gas continues to increase 
until, on the whole, as many molecules of vapour strike against 
and are absorbed by the surface of the liquid as the latter itself 
expels ; and the number of molecules of vapour to the unit of 
volume requisite hereto, is the same whether the space does or 
does not contain additional molecules of gas, 
The pressure of the gas, however, exercises a different influ- 
ence on the interior of the liquid. Here also, or at places where 
the mass of liquid is bounded by a side of the vessel, it may 
happen that the molecules separate from each other with such 
force that for a moment the continuity of the mass is broken, 
The small vacuum thus produced, however, is surrounded on all 
sides by masses which do not admit of the passage of the moved 
molecules; and hence this vacuum will only then become mag- 
nified into a bubble of vapour, and be able to continue as such, 
when the number of molecules expelled from its enclosing liquid 
walls is sufficient to produce an internal vapour-pressure capable 
of holding in equilibrium the pressure which acts externally and 
tends to compress the bubble again. Hence the expansive force 
of the enclosed vapour must be greater, the greater the pressure 
to which the liquid is exposed, and thus is explained the relation 
which exists between the pressure and the temperature of the 
boiling-point. 
The relations will be more complicated when the gas above 
the liquid is itself condensable, and forms a liquid which mixes 
with the given one, for then of course the tendency of the two 
kinds of matter to mix enters as a new force. I shall not here 
enter into these phenomena, 
As in liquids, so also in solids the possibility of an evaporation 
may be comprehended ; nevertheless it does not follow from this 
that, on the contrary, an evaporation must take place on the sur- 
face of all bodies. It is, in fact, readily conceivable that the 
mutual cohesion of the molecules of a body may be so great, 
that, so long as the temperature does not exceed a certain limut, 
