96 PHYSICS. 
takes place either by boiling, in which case vapor is formed throughout the 
whole mass of the liquid, or by evaporation, where the surface only is affected. 
In the first case, two conditions must be fulfilled : firstly, the heat must be 
sufficient to enable the tension of the vapor to resist the pressure of the 
liquid on the vesicles of vapor, on which account the boiling point depends 
upon the amount of this pressure; secondly, there must be enough heat to 
admit of a sufficiency being absorbed in the formation of steam. For this 
reason the rapidity of boiling will depend upon the amount of heat applied 
within a given time. Under the receiver of the air-pump, water of mode- 
rate warmth, as at 86° F., will begin to boil as soon as the air is sufficiently 
rarefied. 
A curious experiment, relating to this subject, may be performed by means 
of the apparatus represented in fig. 24. A glass balloon, a, with a long neck, 
is half filled with water, and this is made to boil: when, by the ascending 
steam, all the air is expelled, the mouth is closed by a cock, b, and: the bal- 
loon inverted as in the figure. Now, if cold water be poured on the upper 
part of the balloon, the water in this vessel will begin to boil violently, owing 
to the condensation of the vapor above the water, and the consequent dimi- 
nution of pressure. 
Since the height of the boiling point of any liquid depends upon the 
atmospheric pressure, the boiling will not only vary under different pressures 
at one and the same point, but the boiling point itself will be different in 
different countries, and at different heights above the level of the sea. 
‘ Boiling water will therefore not be equally hot everywhere, as at Quito 
water boils at 194° F., while in the latitude and level of New York, 212° F. 
are required. 
As by diminishing the pressure, the boiling of a liquid may be accelerated, 
so, also, by increasing this pressure, it may be retarded. Papin’s digester 
(pl. 19, fig. 25) depends upon this principle, and is an instrument in which 
water may be heated far above the usual boiling point without boiling. It 
consists of a cylindrical vessel, abcd, of metal—best of brass or copper—whose 
sides can sustain a very great pressure, and which, after being filled, may 
be closed by a cover, pressed down firmly by the screw passing through the 
bow, m. The single opening in the cover is closed by a safety-valve, which 
may be loaded so heavily as to require a very great pressure to elevate it. 
If this vessel be filled with water and strongly heated, the water cannot boil, 
on account of the pressure exerted by the vapor which forms, and is pre- 
vented from escaping. 
The lower layers of fluid, as is well known to our readers. have to 
sustain the pressure of all the superincumbent ones, in addition to the entire 
weight of the atmosphere ; for this reason boiling should commence later at 
the bottom than at the top of the liquid. Nevertheless, the lower layers, 
expanded by heat, and becoming consequently specifically lighter, rise 
continually through those above them; the bubbles or vesicles of vapor 
which are formed, increase in size as they approach the surface, that is, as 
the pressure becomes less. This arrival at the surface takes place, however, 
only when the upper strata have attained the same temperature as the 
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