AO PHYSICS. 
of the air, which amounts to 15 pounds on every square inch. But take 
a bubble when it is first formed at the bottom of the water; there the 
pressure on it from without is the weight of all the water above it, as well 
as that of the air; so that the heat necessary to raise a quantity of water 
to the botling-point is exactly the quantity of heat that will introduce 
among its particles a force of repulsion sufficient to overcome the pressure 
arising from the weight of the liquid above it and the weight of the 
atmosphere. Hence, to boil a large quantity of water, its temperature 
must be higher than in a smaller quantity, because the pressure to be 
overcome by the steam in the bubbles is greater ; hence too, water will 
boil at a much lower temperature if the pressure of the atmosphere be 
diminished, as is the case on high mountains. 
4, Latent Heat.—When cold water is placed in a vessel over the fire, 
heat from the fire is communicated to the water, which gradually becomes — 
hotter till it reaches the boiling-point ; but, after the water boils, the 
temperature of the water does not rise. What, then, becomes of all the 
heat that continues to be communicated to it? We noticed in Section 1 
the manner in which different modes of energy could be converted into 
heat: this is a case of the reverse process ; here heat is converted into 
motion. Fora certain amount of heat a certain amount of work is done 
in pulling the particles of the liquid asunder. The heat which is con- | 
sumed in this way after a liquid has been raised to the boiling-point— 
that is, heat which goes to form vapour without raising the temperature 
of the liquid or of the steam—has been called latent, from the notion, 
at one time entertained, that heat was a fluid, and, consequently, that 
the heat which seemed to be lost in this way concealed itself in the 
vapour. The same thing takes place when a solid is being reduced 
to a liquid. When heat is applied to a piece of ice, its temperature 
does. not rise above the point at which it began to melt till every 
bit of it is melted. The heat thus absorbed in the melting of ice, is 
called the latent heat of water; and that absorbed in converting water 
into steam, the latent heat of steam. When this latent heat is lost in any 
way, the repulsion existing among the particles diminishes, cohesion 
regains the mastery, and the steam returns to the form of water, or water 
to that of ice. It is on this principle that distillation is accomplished.. 
The sted usually consists of a copper boiler, in which the fermented 
liquor is converted into vapour; of a pipe, which conveys the vapour 
from the top of the boiler ; and of the worm, a coiled metal tube packed 
in a vessel through which there is a constant flow of cold water. The 
vapour arising from the boiling liquor in the copper is deprived of its 
heat in passing through the tube in the cold water; in consequence of | 
this, it assumes again the liquid form, and drops or runs in a small 
stream from the end of the worm into a vessel placed to receive it. 
