HEAT. 39 - 
noticed under Density (page 5). There is thus a contest going on between 
the binding power of cohesion and the repelling power of heat. At first, 
with a small amount of heat, the cohesion holds its own; but as the 
heat increases, the vibrations become more violent, and the atoms are 
strongly pushed apart. Cohesion, then, has less power, because it has to 
act at a greater distance ; therefore, as the repulsion of the heat increases, 
the attraction of cohesion diminishes, till the atoms gain sufficient freedom 
to be able to slide or roll upon one another. The body is then said to be 
an a liquid state. 
In the liquid state, the power of cohesion has not been altogether 
conquered ; the atoms, although they are movable on one another, still 
‘resist being torn asunder. But if the heat be still further increased, 
the last feeble efforts of cohesion are overcome, and the atoms fly apart 
in the form of vapour. When a liquid has assumed the gaseous form, 
it is clear that the space it occupies is very much increased ; thus, 
water converted into steam occupies a space about 1700 times greater than 
it did before—that is, a cubic inch of water becomes a cubic foot of steam. 
3. Vaporisation.— When sufficient heat has been applied to a liquid to 
make it assume the form of visible vapour, the first particles fly off from the 
‘surface, as is seen in the vapour that rises from all water when it becomes 
‘heated at all. Let us see what is going on meantime within the liquid 
mass. At the bottom of the liquid, where the heat is generally applied, 
the particles are being more and more repelled 
from one another, the liquid becomes lighter than 
that above it, and rises, while the liquid above it 
sinks down, as seen in the figure, which represents 
a vessel of water with a lamp under it. While 
this is going on, small bubbles of vapour rise 
from the bottom; but ‘as they rise near the 
surface, where the temperature is lower, they are 
‘condensed again to water. The formation and 
condensation of these first bubbles give rise to 
the swmging sound heard coming from water just 
before it boils. When, however, the whole of 
the water has been raised to a certain temperature, 
the bubbles of vapour that are formed at the 
bottom rise to the surface, and the water is then 
said to boil. Sometimes the bubbles are seen to 
rest on the surface; that is, there is a small 
quantity of vapour enclosed in a thin coating of 3 
the liquid. The repelling power of the heat in Fig. 40. 
the steam of course tends to make it burst the 
bubble ; but it is prevented for a short time from doing so by the pressure 

